Postembryonic development and aging of the appendicular skeleton in Ambystoma mexicanum.

BACKGROUND: The axolotl is a key model to study appendicular regeneration. The limb complexity resembles that of humans in structure and tissue components; however, axolotl limbs develop postembryonically. In this work, we evaluated the postembryonic development of the appendicular skeleton and its changes with aging. RESULTS: The juvenile limb skeleton is formed mostly by Sox9/Col1a2 cartilage cells. Ossification of the appendicular skeleton starts when animals reach a length of 10 cm, and cartilage cells are replaced by a primary ossification center, consisting of cortical bone and an adipocyte-filled marrow cavity. Vascularization is associated with the ossification center and the marrow cavity formation. We identified the contribution of Col1a2-descendants to bone and adipocytes. Moreover, ossification progresses with age toward the epiphyses of long bones. Axolotls are neotenic salamanders, and still ossification remains responsive to l-thyroxine, increasing the rate of bone formation. CONCLUSIONS: In axolotls, bone maturation is a continuous process that extends throughout their life. Ossification of the appendicular bones is slow and continues until the complete element is ossified. The cellular components of the appendicular skeleton change accordingly during ossification, creating a heterogenous landscape in each element. The continuous maturation of the bone is accompanied by a continuous body growth.

Efficient gene knockin in axolotl and its use to test the role of satellite cells in limb regeneration.

Salamanders exhibit extensive regenerative capacities and serve as a unique model in regeneration research. However, due to the lack of targeted gene knockin approaches, it has been difficult to label and manipulate some of the cell populations that are crucial for understanding the mechanisms underlying regeneration. Here we have established highly efficient gene knockin approaches in the axolotl (Ambystoma mexicanum) based on the CRISPR/Cas9 technology. Using a homology-independent method, we successfully inserted both the Cherry reporter gene and a larger membrane-tagged Cherry-ERT2-Cre-ERT2 (∼5-kb) cassette into axolotl Sox2 and Pax7 genomic loci. Depending on the size of the DNA fragments for integration, 5-15% of the F0 transgenic axolotl are positive for the transgene. Using these techniques, we have labeled and traced the PAX7-positive satellite cells as a major source contributing to myogenesis during axolotl limb regeneration. Our work brings a key genetic tool to molecular and cellular studies of axolotl regeneration.

Connective tissue cells, but not muscle cells, are involved in establishing the proximo-distal outcome of limb regeneration in the axolotl.

During salamander limb regeneration, only the structures distal to the amputation plane are regenerated, a property known as the rule of distal transformation. Multiple cell types are involved in limb regeneration; therefore, determining which cell types participate in distal transformation is important for understanding how the proximo-distal outcome of regeneration is achieved. We show that connective tissue-derived blastema cells obey the rule of distal transformation. They also have nuclear MEIS, which can act as an upper arm identity regulator, only upon upper arm amputation. By contrast, myogenic cells do not obey the rule of distal transformation and display nuclear MEIS upon amputation at any proximo-distal level. These results indicate that connective tissue cells, but not myogenic cells, are involved in establishing the proximo-distal outcome of regeneration and are likely to guide muscle patterning. Moreover, we show that, similarly to limb development, muscle patterning in regeneration is influenced by ß-catenin signalling.

Axolotl Transgenesis via Injection of I-SceI Meganuclease or Tol2 Transposon System.

The axolotl (Ambystoma mexicanum ) has been widely used as an animal model for studying development and regeneration. In recent decades, the use of genetic engineering to alter gene expression has advanced our knowledge on the fundamental molecular and cellular mechanisms, pointing us to potential therapeutic targets. We present a detailed, step-by-step protocol for axolotl transgenesis using either I-SceI meganuclease or the mini Tol2 transposon system, by injection of purified DNA into one-cell stage eggs. We add useful tips on the site of injection and the viability of the eggs.

Embryonic Tissue and Blastema Transplantations.

Embryo grafts have been an experimental pillar in developmental biology, and particularly, in amphibian biology. Grafts have been essential in constructing fate maps of different cell populations and migratory patterns. Likewise, autografts and allografts in older larvae or adult salamanders have been widely used to disentangle mechanisms of regeneration. The combination of transgenesis and grafting has widened even more the application of this technique.In this chapter, we provide a detailed protocol for embryo transplants in the axolotl (Ambystoma mexicanum ). The location and stages to label connective tissue, muscle, or blood vessels in the limb and blood cells in the whole animal. However, the potential of embryo transplants is enormous and impossible to cover in one chapter. Furthermore, we provide a protocol for blastema transplantation as an example of allograft in older larvae.

Osteoclast-mediated resorption primes the skeleton for successful integration during axolotl limb regeneration.

Early events during axolotl limb regeneration include an immune response and the formation of a wound epithelium. These events are linked to a clearance of damaged tissue prior to blastema formation and regeneration of the missing structures. Here, we report the resorption of calcified skeletal tissue as an active, cell-driven, and highly regulated event. This process, carried out by osteoclasts, is essential for a successful integration of the newly formed skeleton. Indeed, the extent of resorption is directly correlated with the integration efficiency, and treatment with zoledronic acid resulted in osteoclast function inhibition and failed tissue integration. Moreover, we identified the wound epithelium as a regulator of skeletal resorption, likely releasing signals involved in recruitment/differentiation of osteoclasts. Finally, we reported a correlation between resorption and blastema formation, particularly, a coordination of resorption with cartilage condensation. In sum, our results identify resorption as a major event upon amputation, playing a critical role in the overall process of skeletal regeneration.

The Prrx1 limb enhancer marks an adult subpopulation of injury-responsive dermal fibroblasts.

The heterogeneous properties of dermal cell populations have been posited to contribute toward fibrotic, imperfect wound healing in mammals. Here we characterize an adult population of dermal fibroblasts that maintain an active Prrx1 enhancer which originally marked mesenchymal limb progenitors. In contrast to their abundance in limb development, postnatal Prrx1 enhancer-positive cells (Prrx1enh+) make up a small subset of adult dermal cells (∼0.2%) and reside mainly within dermal perivascular and hair follicle niches. Lineage tracing of adult Prrx1enh+ cells shows that they remain in their niches and in small numbers over a long period of time. Upon injury however, Prrx1enh+ cells readily migrate into the wound bed and amplify, on average, 16-fold beyond their uninjured numbers. Additionally, following wounding dermal Prrx1enh+ cells are found out of their dermal niches and contribute to subcutaneous tissue. Postnatal Prrx1enh+ cells are uniquely injury-responsive despite being a meager minority in the adult skin.

Single-cell analysis uncovers convergence of cell identities during axolotl limb regeneration.

Amputation of the axolotl forelimb results in the formation of a blastema, a transient tissue where progenitor cells accumulate prior to limb regeneration. However, the molecular understanding of blastema formation had previously been hampered by the inability to identify and isolate blastema precursor cells in the adult tissue. We have used a combination of Cre-loxP reporter lineage tracking and single-cell messenger RNA sequencing (scRNA-seq) to molecularly track mature connective tissue (CT) cell heterogeneity and its transition to a limb blastema state. We have uncovered a multiphasic molecular program where CT cell types found in the uninjured adult limb revert to a relatively homogenous progenitor state that recapitulates an embryonic limb bud-like phenotype including multipotency within the CT lineage. Together, our data illuminate molecular and cellular reprogramming during complex organ regeneration in a vertebrate.

Live Imaging of Axolotl Digit Regeneration Reveals Spatiotemporal Choreography of Diverse Connective Tissue Progenitor Pools.

Connective tissues-skeleton, dermis, pericytes, fascia-are a key cell source for regenerating the patterned skeleton during axolotl appendage regeneration. This complexity has made it difficult to identify the cells that regenerate skeletal tissue. Inability to identify these cells has impeded a mechanistic understanding of blastema formation. By tracing cells during digit tip regeneration using brainbow transgenic axolotls, we show that cells from each connective tissue compartment have distinct spatial and temporal profiles of proliferation, migration, and differentiation. Chondrocytes proliferate but do not migrate into the regenerate. In contrast, pericytes proliferate, then migrate into the blastema and give rise solely to pericytes. Periskeletal cells and fibroblasts contribute the bulk of digit blastema cells and acquire diverse fates according to successive waves of migration that choreograph their proximal-distal and tissue contributions. We further show that platelet-derived growth factor signaling is a potent inducer of fibroblast migration, which is required to form the blastema.

Tissue- and time-directed electroporation of CAS9 protein-gRNA complexes in vivo yields efficient multigene knockout for studying gene function in regeneration.

A rapid method for temporally and spatially controlled CRISPR-mediated gene knockout in vertebrates will be an important tool to screen for genes involved in complex biological phenomena like regeneration. Here we show that in vivo injection of CAS9 protein-guide RNA (gRNA) complexes into the spinal cord lumen of the axolotl and subsequent electroporation leads to comprehensive knockout of Sox2 gene expression in SOX2+ neural stem cells with corresponding functional phenotypes from the gene knockout. This is particularly surprising considering the known prevalence of RNase activity in cerebral spinal fluid, which apparently the CAS9 protein protects against. The penetrance/efficiency of gene knockout in the protein-based system is far higher than corresponding electroporation of plasmid-based CRISPR systems. We further show that simultaneous delivery of CAS9-gRNA complexes directed against Sox2 and GFP yields efficient knockout of both genes in GFP-reporter animals. Finally, we show that this method can also be applied to other tissues such as skin and limb mesenchyme. This efficient delivery method opens up the possibility for rapid in vivo genetic screens during axolotl regeneration and can in principle be applied to other vertebrate tissue systems.

CRISPR-mediated genomic deletion of Sox2 in the axolotl shows a requirement in spinal cord neural stem cell amplification during tail regeneration.

The salamander is the only tetrapod that functionally regenerates all cell types of the limb and spinal cord (SC) and thus represents an important regeneration model, but the lack of gene-knockout technology has limited molecular analysis. We compared transcriptional activator-like effector nucleases (TALENs) and clustered regularly interspaced short palindromic repeats (CRISPRs) in the knockout of three loci in the axolotl and find that CRISPRs show highly penetrant knockout with less toxic effects compared to TALENs. Deletion of Sox2 in up to 100% of cells yielded viable F0 larvae with normal SC organization and ependymoglial cell marker expression such as GFAP and ZO-1. However, upon tail amputation, neural stem cell proliferation was inhibited, resulting in spinal-cord-specific regeneration failure. In contrast, the mesodermal blastema formed normally. Sox3 expression during development, but not regeneration, most likely allowed embryonic survival and the regeneration-specific phenotype. This analysis represents the first tissue-specific regeneration phenotype from the genomic deletion of a gene in the axolotl.

Fundamental differences in dedifferentiation and stem cell recruitment during skeletal muscle regeneration in two salamander species.

Salamanders regenerate appendages via a progenitor pool called the blastema. The cellular mechanisms underlying regeneration of muscle have been much debated but have remained unclear. Here we applied Cre-loxP genetic fate mapping to skeletal muscle during limb regeneration in two salamander species, Notophthalmus viridescens (newt) and Ambystoma mexicanum (axolotl). Remarkably, we found that myofiber dedifferentiation is an integral part of limb regeneration in the newt, but not in axolotl. In the newt, myofiber fragmentation results in proliferating, PAX7(-) mononuclear cells in the blastema that give rise to the skeletal muscle in the new limb. In contrast, myofibers in axolotl do not generate proliferating cells, and do not contribute to newly regenerated muscle; instead, resident PAX7(+) cells provide the regeneration activity. Our results therefore show significant diversity in limb muscle regeneration mechanisms among salamanders and suggest that multiple strategies may be feasible for inducing regeneration in other species, including mammals.

Optimized axolotl (Ambystoma mexicanum) husbandry, breeding, metamorphosis, transgenesis and tamoxifen-mediated recombination.

The axolotl (Mexican salamander, Ambystoma mexicanum) has become a very useful model organism for studying limb and spinal cord regeneration because of its high regenerative capacity. Here we present a protocol for successfully mating and breeding axolotls in the laboratory throughout the year, for metamorphosing axolotls by a single i.p. injection and for axolotl transgenesis using I-SceI meganuclease and the mini Tol2 transposon system. Tol2-mediated transgenesis provides different features and advantages compared with I-SceI-mediated transgenesis, and it can result in more than 30% of animals expressing the transgene throughout their bodies so that they can be directly used for experimentation. By using Tol2-mediated transgenesis, experiments can be performed within weeks (e.g., 5-6 weeks for obtaining 2-3-cm-long larvae) without the need to establish germline transgenic lines (which take 12-18 months). In addition, we describe here tamoxifen-induced Cre-mediated recombination in transgenic axolotls.

Germline transgenic methods for tracking cells and testing gene function during regeneration in the axolotl.

The salamander is the only tetrapod that regenerates complex body structures throughout life. Deciphering the underlying molecular processes of regeneration is fundamental for regenerative medicine and developmental biology, but the model organism had limited tools for molecular analysis. We describe a comprehensive set of germline transgenic strains in the laboratory-bred salamander Ambystoma mexicanum (axolotl) that open up the cellular and molecular genetic dissection of regeneration. We demonstrate tissue-dependent control of gene expression in nerve, Schwann cells, oligodendrocytes, muscle, epidermis, and cartilage. Furthermore, we demonstrate the use of tamoxifen-induced Cre/loxP-mediated recombination to indelibly mark different cell types. Finally, we inducibly overexpress the cell-cycle inhibitor p16 (INK4a) , which negatively regulates spinal cord regeneration. These tissue-specific germline axolotl lines and tightly inducible Cre drivers and LoxP reporter lines render this classical regeneration model molecularly accessible.