Reintegration of the regenerated and the remaining tissues during joint regeneration in the newt Cynops pyrrhogaster

FGF-stimulated tendon cells embrace a chondrogenic fate with BMP7 in newt tissue culture

Newts can regenerate functional elbow joints after amputation at the joint level. Previous studies have suggested the potential contribution of cells from residual tendon tissues to joint cartilage regeneration. A serum-free tissue culture system for tendons was established to explore cell dynamics during joint regeneration. Culturing isolated tendons in this system, stimulated by regeneration-related factors, such as fibroblast growth factor (FGF) and platelet-derived growth factor, led to robust cell migration and proliferation. Moreover, cells proliferating in an FGF-rich environment differentiated into Sox9-positive chondrocytes upon BMP7 introduction. These findings suggest that FGF-stimulated cells from tendons may aid in joint cartilage regeneration during functional elbow joint regeneration in newts.

The salamander limb: a perfect model to understand imperfect integration during skeletal regeneration

Limb regeneration in salamanders is achieved by a complex coordination of various biological processes and requires the proper integration of new tissue with old. Among the tissues found inside the limb, the skeleton is the most prominent component, which serves as a scaffold and provides support for locomotion in the animal. Throughout the years, researchers have studied the regeneration of the appendicular skeleton in salamanders both after limb amputation and as a result of fracture healing. The final outcome has been widely seen as a faithful re-establishment of the skeletal elements, characterised by a seamless integration into the mature tissue. The process of skeletal integration, however, is not well understood, and several works have recently provided evidence of commonly occurring flawed regenerates. In this Review, we take the reader on a journey through the course of bone formation and regeneration in salamanders, laying down a foundation for critically examining the mechanisms behind skeletal integration. Integration is a phenomenon that could be influenced at various steps of regeneration, and hence, we assess the current knowledge in the field and discuss how early events, such as tissue histolysis and patterning, influence the faithful regeneration of the appendicular skeleton.

Building a Co-ordinated Musculoskeletal System: The Plasticity of the Developing Skeleton in Response to Muscle Contractions

The skeletal musculature and the cartilage, bone and other connective tissues of the skeleton are intimately co-ordinated. The shape, size and structure of each bone in the body is sculpted through dynamic physical stimuli generated by muscle contraction, from early development, with onset of the first embryo movements, and through repair and remodelling in later life. The importance of muscle movement during development is shown by congenital abnormalities where infants that experience reduced movement in the uterus present a sequence of skeletal issues including temporary brittle bones and joint dysplasia. A variety of animal models, utilising different immobilisation scenarios, have demonstrated the precise timing and events that are dependent on mechanical stimulation from movement. This chapter lays out the evidence for skeletal system dependence on muscle movement, gleaned largely from mouse and chick immobilised embryos, showing the many aspects of skeletal development affected. Effects are seen in joint development, ossification, the size and shape of skeletal rudiments and tendons, including compromised mechanical function. The enormous plasticity of the skeletal system in response to muscle contraction is a key factor in building a responsive, functional system. Insights from this work have implications for our understanding of morphological evolution, particularly the challenging concept of emergence of new structures. It is also providing insight for the potential of physical therapy for infants suffering the effects of reduced uterine movement and is enhancing our understanding of the cellular and molecular mechanisms involved in skeletal tissue differentiation, with potential for informing regenerative therapies.

Photoperiod-independent testicular development in the model newt Pleurodeles waltl

Urodele amphibian newts have unique biological properties in male gametogenesis, in addition to their extreme regenerative capacity. Male newts are able to regenerate new testes even after reaching sexual maturity and can possess multiple testes. Notably, these animals maintain primordial germ cell-like cells in a tissue adjacent to the testis. Spermatogenesis proceeds while synchronizing in a region-specific manner in the testis. However, the newt species that have been used most commonly require 2-3 years to achieve sexual maturity, and spermatogenesis in these species shows seasonality. These traits have restricted the use of newts for studies on testicular development and spermatogenesis, and testis development in newts remains poorly characterized. Recently, the Iberian ribbed newt Pleurodeles waltl has been established as an emerging model organism. P. waltl reaches sexual maturity more quick after birth than do other newts and is capable of breeding year-round. Thus, P. waltl is expected to serve as an appealing experimental model for studying the mechanisms of male gametogenesis in the urodeles. In the present study, we use P. waltl to describe the entire developmental process of the newt testis from primordial gonad to maturity. Notably, the mature testes show synchronized progression of spermatogenesis along the anteroposterior axis. Additionally, we demonstrate that the process of spermatogenesis in P. waltl proceeds irrespective of day length. Our results show that P. waltl newts are a suitable model for investigating the process of testicular development. We also expect that these results will be useful for the maintenance of P. waltl bioresources.

Salamanders: The molecular basis of tissue regeneration and its relevance to human disease

Salamanders are recognized for their ability to regenerate a broad range of tissues. They have also have been used for hundreds of years for classical developmental biology studies because of their large accessible embryos. The range of tissues these animals can regenerate is fascinating, from full limbs to parts of the brain or heart, a potential that is missing in humans. Many promising research efforts are working to decipher the molecular blueprints shared across the organisms that naturally have the capacity to regenerate different tissues and organs. Salamanders are an excellent example of a vertebrate that can functionally regenerate a wide range of tissue types. In this review, we outline some of the significant insights that have been made that are aiding in understanding the cellular and molecular mechanisms of tissue regeneration in salamanders and discuss why salamanders are a worthy model in which to study regenerative biology and how this may benefit research fields like regenerative medicine to develop therapies for humans in the future.

Maturating Articular Cartilage Can Induce Ectopic Joint-Like Structures in Neonatal Mice

Abstract

Osteoarthritis is a huge health burden to our society. Seeking for potential ways to induce regeneration of articular cartilage (AC) that is intrinsically limited, we focused on the interaction between two opposing joints. To evaluate the role of the interaction of opposing regions of AC for joint maturation, we amputated digits at the distal interphalangeal level without injuring the articular surface of the intermediate phalanx (P2) and observed that the zonal organization of AC was defective. We then removed the P2 bone without injuring the articular surface of the proximal phalanx (P1), and the remaining part of the digit was amputated near the distal interphalangeal level. The distribution pattern of type II collagen and proteoglycan 4 (PRG4) suggested that maturation of AC in P1 was delayed. These two experiments suggested that an interaction between the opposing AC in a joint is necessary for maturation of the zonal organization of AC in neonatal digits. To test if an interaction of the joints is sufficient to induce articular cartilage, a proximal fragment of P2 was resected, inverted, and put back into the original location. Newly formed cartilage was induced at the interface region between the AC of the inverted graft and the cut edge of the distal part of P2. Type II collagen and PRG4 were expressed in the ectopic cartilage in a similar manner to normal AC, indicating that neonatal AC can induce ectopic joint-like structures in mice comparable with what has been reported in newts and frogs. These results suggest that the neonatal joint could be a source of inductive signals for regeneration of AC.

Lay Summary

In this study, we experimentally show that neonatal mice appear to have the capacity to regenerate articular cartilage (AC) in digits. It is already known that mice can regenerate a digit tip after amputation, but do not regenerate in response to amputations at more proximal levels. Therefore, it has been thought that mammalian joint structures are non-regenerative. However, we found that normal digit AC can induce AC-like structures in a non-joint region when it is placed next to the cut edge of a bone, suggesting that the normal AC has regenerative capacity in certain situations in neonatal mice.

Future Works

Joint disorders are a huge health problem of our society. The results of this study suggest that neonatal AC could be a potential source of inductive signals for regeneration of AC. The discovery of these inductive signals will aid in developing regenerative therapies of a joint in human.

Brief Local Application of Progesterone via a Wearable Bioreactor Induces Long-Term Regenerative Response in Adult Xenopus Hindlimb

The induction of limb repair in adult vertebrates is a pressing, unsolved problem. Here, we characterize the effects of an integrated device that delivers drugs to severed hindlimbs of adult Xenopus laevis, which normally regenerate cartilaginous spikes after amputation. A wearable bioreactor containing a silk protein-based hydrogel that delivered progesterone to the wound site immediately after hindlimb amputation for only 24 hr induced the regeneration of paddle-like structures in adult frogs. Molecular markers, morphometric analysis, X-ray imaging, immunofluorescence, and behavioral assays were used to characterize the differences between the paddle-like structures of successful regenerates and hypomorphic spikes that grew in untreated animals. Our experiments establish a model for testing therapeutic cocktails in vertebrate hindlimb regeneration, identify pro-regenerative activities of progesterone-containing bioreactors, and provide proof of principle of brief use of integrated device-based delivery of small-molecule drugs as a viable strategy to induce and maintain a long-term regenerative response.

The complexity of vertebrate limbs drives the search for regenerative treatments that trigger endogenous processes of repair. Herrera-Rincon et al. show that a wearable bioreactor containing progesterone, applied for only 24 hr, induces months of regenerative growth and patterning of amputated hindlimbs in the frog Xenopus laevis.

A comprehensive reference transcriptome resource for the Iberian ribbed newt Pleurodeles waltl, an emerging model for developmental and regeneration biology

Urodele newts have unique biological properties, notably including prominent regeneration ability. The Iberian ribbed newt, Pleurodeles waltl, is a promising model amphibian distinguished by ease of breeding and efficient transgenic and genome editing methods. However, limited genetic information is available for P. waltl. We conducted an intensive transcriptome analysis of P. waltl using RNA-sequencing to build and annotate gene models. We generated 1.2 billion Illumina reads from a wide variety of samples across 12 different tissues/organs, unfertilized egg, and embryos at eight different developmental stages. These reads were assembled into 1,395,387 contigs, from which 202,788 non-redundant ORF models were constructed. The set is expected to cover a large fraction of P. waltl protein-coding genes, as confirmed by BUSCO analysis, where 98% of universal single-copy orthologs were identified. Ortholog analyses revealed the gene repertoire evolution of urodele amphibians. Using the gene set as a reference, gene network analysis identified regeneration-, developmental-stage-, and tissue-specific co-expressed gene modules. Our transcriptome resource is expected to enhance future research employing this emerging model animal for regeneration research as well as for investigations in other areas including developmental biology, stem cell biology, and cancer research. These data are available via our portal website, iNewt (http://www.nibb.ac.jp/imori/main/).

Changing While Staying the Same: Preservation of Structural Continuity During Limb Evolution by Developmental Integration

More than 150 years since Charles Darwin published "On the Origin of Species", gradual evolution by natural selection is still not fully reconciled with the apparent sudden appearance of complex structures, such as the bat wing, with highly derived functions. This is in part because developmental genetics has not yet identified the number and types of mutations that accumulated to drive complex morphological evolution. Here, we consider the experimental manipulations in laboratory model systems that suggest tissue interdependence and mechanical responsiveness during limb development conceptually reduce the genetic complexity required to reshape the structure as a whole. It is an exciting time in the field of evolutionary developmental biology as emerging technical approaches in a variety of non-traditional laboratory species are on the verge of filling the gaps between theory and evidence to resolve this sesquicentennial debate.

Functional joint regeneration is achieved using reintegration mechanism in Xenopus laevis

A functional joint requires integration of multiple tissues: the apposing skeletal elements should form an interlocking structure, and muscles should insert into skeletal tissues via tendons across the joint. Whereas newts can regenerate functional joints after amputation, Xenopus laevis regenerates a cartilaginous rod without joints, a "spike." Previously we reported that the reintegration mechanism between the remaining and regenerated tissues has a significant effect on regenerating joint morphogenesis during elbow joint regeneration in newt. Based on this insight into the importance of reintegration, we amputated frogs' limbs at the elbow joint and found that frogs could regenerate a functional elbow joint between the remaining tissues and regenerated spike. During regeneration, the regenerating cartilage was partially connected to the remaining articular cartilage to reform the interlocking structure of the elbow joint at the proximal end of the spike. Furthermore, the muscles of the remaining part inserted into the regenerated spike cartilage via tendons. This study might open up an avenue for analyzing molecular and cellular mechanisms of joint regeneration using Xenopus.