Lgr6 marks nail stem cells and is required for digit tip regeneration

Abstract

The tips of the digits of some mammals, including human infants and mice, are capable of complete regeneration after injury. This process is reliant on the presence of the overlaying nail organ and is mediated by a proliferative blastema. Epithelial Wnt/β-catenin signaling has been shown to be necessary for mouse digit tip regeneration. Here, we report on Lgr5 and Lgr6 (leucine-rich repeat-containing G protein-coupled receptor 5 and 6), two important agonists of the Wnt pathway that are known to be markers of several epithelial stem cell populations. We find that Lgr5 is expressed in a dermal population of cells adjacent to the specialized epithelia surrounding the keratinized nail plate. Moreover, Lgr5-expressing cells contribute to this dermis, but not the blastema, during digit tip regeneration. In contrast, we find that Lgr6 is expressed within cells of the nail matrix portion of the nail epithelium, as well as in a subset of cells in the bone and eccrine sweat glands. Genetic lineage analysis reveals that Lgr6-expressing cells give rise to the nail during homeostatic growth, demonstrating that Lgr6 is a marker of nail stem cells. Moreover, Lgr6-expressing cells contribute to the blastema, suggesting a potential direct role for Lgr6-expressing cells during digit tip regeneration. This role is confirmed by analysis of Lgr6-deficient mice, which have both a nail and bone regeneration defect.

Getting nervous about regeneration

“…Studies in the salamander limb and mouse digit tip used lineage tracing techniques to demonstrate that regenerating cells retain their lineage identity, evidence that dedifferentiation of remaining cells to a pluripotent state does not occur (10,11). Wnt-agonist Lgr6 has recently been shown to be a marker of progenitor populations of nail matrix, bone, and sweat glands that contribute to digit tip regeneration in rodents (12). A FGF8-SHH circuit encoding positional information was identified in the regenerating salamander limb (13).…”

Evidence for Lgr6 as a Novel Marker of Osteoblastic Progenitors in Mice

“…(28) To further explore the expression of Lgr6 in calvaria, we undertook mouse primary calvarial cell differentiation assays to determine whether the expression is specific to preosteoblasts, or whether it is also expressed during differentiation of these cells. For these experiments, we harvested cells on days 3, 7, and 21 of osteogenic culture.…”

LGRs receptors as peculiar GPCRs involved in cancer

“…In taste buds, Lgr6 marks stem/ progenitor cells in both the anterior and posterior tongue whereas Lgr5-expressing cells are only detected in the posterior part [53]. In digit tips, Lgr6 expression is detected in nail stem cells and bone, in a pattern correlating also with Lgr4 expression [52]. Regarding Lgr6 function in vivo, Lgr6 knockout mice are fertile and absence of the receptor in the skin does not impact on cell proliferation, differentiation in sebaceous glands or even on wound healing and cell migration [76].…”

“…The Lgr5 progeny population switches from the luminal to the myoepithelial compartments during postnatal development of this gland [51]. In digits, Lgr5 expression is detected in the dermis [52]. Of relevance, Lgr5 is detected in cells also expressing the paralogue receptors Lgr4 and/or Lgr6, thus rendering the analysis of individual receptor function quite complex [7,28,30,35,53].…”

“…Regarding Lgr6 function in vivo, Lgr6 knockout mice are fertile and absence of the receptor in the skin does not impact on cell proliferation, differentiation in sebaceous glands or even on wound healing and cell migration [76]. In contrast, although nails develop normally in absence of Lgr6, they fail to regenerate in some Lgr6-deficient mice after amputation, indicating a contribution of this receptor during digit tip regeneration [52].…”