Regeneration-specific promoter switching facilitates Mest expression in the mouse digit tip to modulate neutrophil response.

The mouse digit tip regenerates following amputation, a process mediated by a cellularly heterogeneous blastema. We previously found the gene Mest to be highly expressed in mesenchymal cells of the blastema and a strong candidate pro-regenerative gene. We now show Mest digit expression is regeneration-specific and not upregulated in post-amputation fibrosing proximal digits.

Regeneration-specific promoter switching facilitates Mest expression in the mouse digit tip to modulate neutrophil response.

The mouse digit tip regenerates following amputation, a process mediated by a cellularly heterogeneous blastema. We previously found the gene Mest to be highly expressed in mesenchymal cells of the blastema and a strong candidate pro-regenerative gene. We now show Mest digit expression is regeneration-specific and not upregulated in post-amputation fibrosing proximal digits.

Skin-type-dependent development of murine mechanosensory neurons.

Mechanosensory neurons innervating the skin underlie our sense of touch. Fast-conducting, rapidly adapting mechanoreceptors innervating glabrous (non-hairy) skin form Meissner corpuscles, while in hairy skin, they associate with hair follicles, forming longitudinal lanceolate endings. How mechanoreceptors develop axonal endings appropriate for their skin targets is unknown.

Toeing the line between regeneration and fibrosis.

Understanding the remarkable capacity of vertebrates to naturally regenerate injured body parts has great importance for potential translation into human therapeutic applications. As compared to other vertebrates, mammals have low regenerative capacity for composite tissues like the limb. However, some primates and rodents can regenerate the distal tips of their digits following amputation, indicating that at least very distal mammalian limb tissues are competent for innate regeneration.

The Maresin 1-LGR6 axis decreases respiratory syncytial virus-induced lung inflammation.

The resolution of infection is an active process with specific molecular and cellular mechanisms that temper inflammation and enhance pathogen clearance. Here, the specialized pro-resolving mediator (SPM) Maresin 1 (MaR1) inhibited respiratory syncytial virus (RSV)-induced inflammation. inlerleukin-13 production from type 2 innate lymphoid cells (ILC) and CD4 T helper type 2 cells was decreased by exogenous MaR1.

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

The mouse digit tip regenerates following amputation. How the regenerate is patterned is unknown, but a long-standing hypothesis proposes developmental patterning mechanisms are re-used during regeneration. The digit tip bone exhibits dorsal-ventral (DV) polarity, so we focus on En1 and Lmx1b, two factors necessary for DV patterning during limb development.

Mouse LGR6 regulates osteogenesis in vitro and in vivo through differential ligand use.

Leucine-rich repeat containing G-protein-coupled receptor 6 (LGR6) is a marker of osteoprogenitor cells and is dynamically expressed during in vitro osteodifferentation of mouse and human mesenchymal stem cells (MSCs). While the Lgr6 genomic locus has been associated with osteoporosis in human cohorts, the precise molecular function of LGR6 in osteogenesis and maintenance of bone mass are not yet known. In this study, we performed in vitro Lgr6 knockdown and overexpression experiments in murine osteoblastic cells and find decreased Lgr6 levels results in reduced osteoblast proliferation, differentiation, and mineralization.

Mammalian Digit Tip Regeneration: Moving from Phenomenon to Molecular Mechanism.

In this review, we present the current state of knowledge surrounding mammalian digit tip regeneration. We discuss the origin and formation of the blastema, a structure integral to digit tip regeneration, as well as recent insights driven by single-cell RNA sequencing into the molecular markers and cellular composition of the blastema. The digit tip is a composite of many different tissue types and we address what is known about the role of these separate tissues in regeneration of the whole digit tip.

Cellular Heterogeneity and Lineage Restriction during Mouse Digit Tip Regeneration at Single-Cell Resolution.

Innate regeneration following digit tip amputation is one of the few examples of epimorphic regeneration in mammals. Digit tip regeneration is mediated by the blastema, the same structure invoked during limb regeneration in some lower vertebrates. By genetic lineage analyses, the digit tip blastema has been defined as a population of heterogeneous, lineage-restricted progenitor cells.

Cellular Plasticity in Musculoskeletal Development, Regeneration, and Disease.

In this review, we highlight themes from a recent workshop focused on "Plasticity of Cell Fate in Musculoskeletal Tissues" held at the Orthopaedic Research Society's 2019 annual meeting. Experts in the field provided examples of mesenchymal cell plasticity during normal musculoskeletal development, regeneration, and disease. A thorough understanding of the biology underpinning mesenchymal cell plasticity may offer a roadmap for promoting regeneration while attenuating pathologic differentiation.

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

Bone marrow-derived mesenchymal stem cells are an important source of osteoblasts critical for both bone homeostasis and repair. The ability to isolate, or specifically target, mesenchymal stem cells committed to the osteogenic lineage is necessary for orthopedic translational therapy efforts; however the precise molecular signature of these cells remains elusive. Previously, we identified a population of osteoprogenitor cells expressing the Wnt signaling agonist Lgr6, which contributes to the development and regeneration of the mouse digit tip bone.

Are fingernails a key to unlocking the puzzle of mammalian limb regeneration?

Some mammalian digit tips, including those of mice and human children, can regenerate following amputation, whereas mammalian limb regeneration does not occur. One major difference between the digit tip and the rest of the limb is the presence of the nail, which is necessary for this type of regeneration. This couples well with the finding that canonical Wnt signalling and Lgr6, an agonist of Wnt signalling that marks nail stem cells, are necessary for digit tip regeneration.

Digit Tip Regeneration and Beyond: Schwann Cell Progenitors to the Rescue.

Digit tip regeneration following amputation is an innate response in some mammals, including mice. In this issue of Cell Stem Cell, Johnston et al. (2016) show that Schwann cell precursors are necessary for this process and can rescue regeneration in denervated digit tips through secretion of pro-regenerative factors including OSM and PDGF-AA.

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

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.

A novel intergenic ETnII-β insertion mutation causes multiple malformations in polypodia mice.

Mouse early transposon insertions are responsible for ~10% of spontaneous mutant phenotypes. We previously reported the phenotypes and genetic mapping of Polypodia, (Ppd), a spontaneous, X-linked dominant mutation with profound effects on body plan morphogenesis. Our new data shows that mutant mice are not born in expected Mendelian ratios secondary to loss after E9.

Inducible genetic system for the axolotl.

Transgenesis promises a powerful means for assessing gene function during amphibian limb regeneration. This approach is complicated, however, by the need for embryonic appendage development to proceed unimpeded despite the genetic alterations one wishes to test later in the context of regeneration. Achieving conditional gene regulation in this amphibian has not proved to be as straightforward as in many other systems.

Mouse digit tip regeneration is mediated by fate-restricted progenitor cells.

Regeneration of appendages is frequent among invertebrates as well as some vertebrates. However, in mammals this has been largely relegated to digit tip regeneration, as found in mice and humans. The regenerated structures are formed from a mound of undifferentiated cells called a blastema, found just below the site of amputation.

Dynamic expression of two thrombospondins during axolotl limb regeneration.

The molecular processes underlying regeneration remain largely unknown. Several potential factors have been elucidated by focusing on the regenerative function of genes originally identified in a developmental context. A complementary approach is to consider the roles of factors involved in wound healing.

Expanded HOXA13 polyalanine tracts in a monotreme.

The N-terminal region of human HOXA13 has seven discrete polyalanine tracts. Our previous analysis of these tracts in multiple major vertebrate clades suggested that three are mammal-specific. We now report the N-terminal HOXA13 repetitive tract structures in the monotreme Tachyglossus aculeatus (echidna).

BAC transgenic analysis reveals enhancers sufficient for Hoxa13 and neighborhood gene expression in mouse embryonic distal limbs and genital bud.

We previously demonstrated that a approximately 1 Mb domain of genes upstream of and including Hoxa13 is co-expressed in the developing mouse limbs and genitalia. A highly conserved non-coding sequence, mmA13CNS, was shown to be insufficient in transgenic mice to direct precise Hoxa13-like expression in the limb buds or genital bud, although some LacZ expression from the transgene was reproducibly found in these tissues. In this report, we used beta-globin minimal promoter LacZ recombinant BAC transgenes encompassing mmA13CNS to identify a single critical region involved in mouse Hoxa13-like embryonic genital bud expression.

A mouse transgene drives embryonic dorsal posterior commissure expression.

In this report we generated mice co-transgenic for a minimal promoter LacZ construct and a mouse BAC from the gene poor region upstream of the Hoxd cluster. In addition to expression in the distal limb, genital bud, and spinal cord, we show that this BAC transgene also reproducibly drives unique bilateral, dorsal posterior commissure expression. The ability of this BAC to direct posterior commissure expression makes it worthy of further study as a valuable tool in transgenic/targeting experiments.

Description and genetic mapping of Polypodia: an X-linked dominant mouse mutant with ectopic caudal limbs and other malformations.

In this report we present a spontaneous mouse mutant, named Polypodia (Ppd), that primarily exhibits ectopic, ventral/caudal limbs and associated pelvic girdle malformation or duplication. Less penetrant features include diphallia, microphthalmia, small kidney, curled or kinked tail, forelimb anomaly, and skin papillae. Ppd mice have a normal karyotype and no large-scale genomic deletions or insertions by BAC-based array comparative genomic hybridization (CGH).

A group 13 homeodomain is neither necessary nor sufficient for posterior prevalence in the mouse limb.

Posterior prevalence is the general property attributed to HOX proteins describing the dominant effect of more posterior HOX proteins over the function of anterior orthologs in common areas of expression. To explore the HOX group 13 protein domains required for this property, we used the mouse Prx-1 promoter to drive transgenic expression of Hox constructs throughout the entire limb bud during development. This system allowed us to conclusively demonstrate a hierarchy of Hox function in developing limbs.

Conserved expression domains for genes upstream and within the HoxA and HoxD clusters suggests a long-range enhancer existed before cluster duplication.

The posterior HoxA and HoxD genes are essential in appendicular development. Studies have demonstrated that a "distal limb enhancer," remotely located upstream of the HoxD complex, is required to drive embryonic autopod expression of the posterior Hox genes as well as the two additional non-Hox genes in the region: Evx2 and Lnp. Our work demonstrates a similar mode of regulation for Hoxa13 and four upstream genes: Evx1, Hibadh, Tax1bp, and Jaz1.