Aberrant wound healing and TGF-beta production in the autoimmune-prone MRL/+ mouse

Epithelial-mesenchymal transition: an organizing principle of mammalian regeneration

Introduction: The MRL mouse strain is one of the few examples of a mammal capable of healing appendage wounds by regeneration, a process that begins with the formation of a blastema, a structure containing de-differentiating mesenchymal cells. HIF-1α expression in the nascent MRL wound site blastema is one of the earliest identified events and is sufficient to initiate the complete regenerative program. However, HIF-1α regulates many cellular processes modulating the expression of hundreds of genes. A later signal event is the absence of a functional G1 checkpoint, leading to G2 cell cycle arrest with increased cellular DNA but little cell division observed in the blastema. This lack of mitosis in MRL blastema cells is also a hallmark of regeneration in classical invertebrate and vertebrate regenerators such as planaria, hydra, and newt. Results and discussion: Here, we explore the cellular events occurring between HIF-1α upregulation and its regulation of the genes involved in G2 arrest (EVI-5, γH3, Wnt5a, and ROR2), and identify epithelial-mesenchymal transition (EMT) (Twist and Slug) and chromatin remodeling (EZH-2 and H3K27me3) as key intermediary processes. The locus of these cellular events is highly regionalized within the blastema, occurring in the same cells as determined by double staining by immunohistochemistry and FACS analysis, and appears as EMT and chromatin remodeling, followed by G2 arrest determined by kinetic expression studies.

The Extracellular Matrix: Its Composition, Function, Remodeling, and Role in Tumorigenesis

The extracellular matrix (ECM) is a ubiquitous member of the body and is key to the maintenance of tissue and organ integrity. Initially thought to be a bystander in many cellular processes, the extracellular matrix has been shown to have diverse components that regulate and activate many cellular processes and ultimately influence cell phenotype. Importantly, the ECM's composition, architecture, and stiffness/elasticity influence cellular phenotypes. Under normal conditions and during development, the synthesized ECM constantly undergoes degradation and remodeling processes via the action of matrix proteases that maintain tissue homeostasis. In many pathological conditions including fibrosis and cancer, ECM synthesis, remodeling, and degradation is dysregulated, causing its integrity to be altered. Both physical and chemical cues from the ECM are sensed via receptors including integrins and play key roles in driving cellular proliferation and differentiation and in the progression of various diseases such as cancers. Advances in 'omics' technologies have seen an increase in studies focusing on bidirectional cell-matrix interactions, and here, we highlight the emerging knowledge on the role played by the ECM during normal development and in pathological conditions. This review summarizes current ECM-targeted therapies that can modify ECM tumors to overcome drug resistance and better cancer treatment.

A remarkable rodent: Regeneration and reproduction in spiny mice (Acomys)

Although certain organisms are chosen and employed to better understand a specific problem in biology (so-called model organisms), sometimes an animal model reveals its' biomedical importance by happenstance. In many ways, the advent of spiny mice (Acomys) as an emerging model to study regeneration and menstruation stands as a case study in scientific pseudoserendipity (Diaz de Chumaceiro, 1995). As we recount in this chapter, the discovery of these phenotypes, while not entirely accidental, was nonetheless unexpected. In addition to recounting how we uncovered these unusual mammalian traits, we outline recent work by our groups and others that has begun to outline the cellular and genetic mechanisms underlying bonafide mammalian tissue regeneration and a human-like mode of reproduction in spiny mice.

Spiny mice (Acomys) exhibit attenuated hallmarks of aging and rapid cell turnover after UV exposure in the skin epidermis

The study of long-lived and regenerative animal models has revealed diverse protective responses to stressors such as aging and tissue injury. Spiny mice (Acomys) are a unique mammalian model of skin wound regeneration, but their response to other types of physiological skin damage has not been investigated. In this study, we examine how spiny mouse skin responds to acute UVB damage or chronological aging compared to non-regenerative C57Bl/6 mice (M. musculus). We find that, compared to M. musculus, the skin epidermis in A. cahirinus experiences a similar UVB-induced increase in basal cell proliferation but exhibits increased epidermal turnover. Notably, A. cahirinus uniquely form a suprabasal layer co-expressing Keratin 14 and Keratin 10 after UVB exposure concomitant with reduced epidermal inflammatory signaling and reduced markers of DNA damage. In the context of aging, old M. musculus animals exhibit typical hallmarks including epidermal thinning, increased inflammatory signaling and senescence. However, these age-related changes are absent in old A. cahirinus skin. Overall, we find that A. cahirinus have evolved novel responses to skin damage that reveals new aspects of its regenerative phenotype.

Intra-articular delivery of extracellular vesicles secreted by chondrogenic progenitor cells from MRL/MpJ superhealer mice enhances articular cartilage repair in a mouse injury model

Background

Chondrogenic progenitor cells (CPCs) have high self-renewal capacity and chondrogenic potential. Intra-articular delivery of purified mesenchymal stem cells (MSCs) from MRL/MpJ “superhealer” mice increased bone volume during repair and prevents post-traumatic arthritis. Recently, although extracellular vesicles released from MSCs have been used widely for treating OA, the application of extracellular vesicles secreted by CPCs from MRL/MpJ mice in OA therapy has never been reported. In this study, we evaluated the effects of extracellular vesicles secreted by CPCs from control CBA (CBA-EVs) and MRL/MpJ mice (MRL-EVs) on proliferation and migration of murine chondrocytes. We also determined here if weekly intra-articular injections of CBA-EVs and MRL-EVs would repair and regenerate surgically induced model in mice.

Methods

CPC surface markers were detected by flow cytometry. CBA-EVs and MRL-EVs were isolated using an ultrafiltration method. Nanoparticle tracking analysis, transmission electron microscopy, and western blots were used to identify extracellular vesicles. CBA-EVs and MRL-EVs were injected intra-articularly in a mouse model of surgical destabilization of the medial meniscus (DMM)-induced OA, and histological and immunohistochemistry analyses were used to assess the efficacy of exosome injections. We used miRNA-seq analysis to analyze the expression profiles of exosomal miRNAs derived from CBA-EVs as well as MRL-EVs. Cell-counting and scratch assays were used to evaluate the effects of CBA-EVs and MRL-EVs on proliferation and migration of murine chondrocytes, respectively. Meanwhile, a specific RNA inhibitor assesses the roles of the candidate miRNAs in CPC-EV-induced regulation of function of chondrocytes.

Results

Both CBA-EVs and MRL-EVs stimulated chondrocyte proliferation and migration, but MRL-EVs exerted a stronger effect than CBA-EVs. The similar result was also observed in in vivo study, which indicated that injecting either CBA-EVs or MRL-EVs attenuated OA, but MRL-EVs showed a superior therapeutic effect in comparison with CBA-EVs. The results of bioinformatics analyses revealed that the differentially expressed exosomal miRNAs participated in multiple biological processes. We identified 80 significantly upregulated and 100 downregulated miRNAs. Moreover, we found that the top 20 differentially expressed exosomal miRNAs connected OA repair to processes such as AMPK signaling, regulation of autophagy, and insulin signaling. Notably, miRNA 221-3p were highly enriched in MRL-Exos and treatment with miR 221-3p inhibitor markedly decreased chondrocyte proliferation and migration induced by CBA-EVs or MRL-EVs in vitro.

Conclusions

This is the first study to demonstrate MRL-EVs had a greater therapeutic effect on the treatment of OA than CBA-EVs. This study will hopefully provide new insight into the pathogenesis, prevention, and treatment of OA.

Enhanced proliferation of pancreatic acinar cells in MRL/MpJ mice is driven by severe acinar injury but independent of inflammation

Adult pancreatic acinar cells have the ability to re-enter the cell cycle and proliferate upon injury or tissue loss. Despite this mitotic ability, the extent of acinar proliferation is often limited and unable to completely regenerate the injured tissue or restore the initial volume of the organ, thus leading to pancreatic dysfunction. Identifying molecular determinants of enhanced proliferation is critical to overcome this issue. In this study, we discovered that Murphy Roths Large (MRL/MpJ) mice can be exploited to identify molecular effectors promoting acinar proliferation upon injury, with the ultimate goal to develop therapeutic regimens to boost pancreatic regeneration. Our results show that, upon cerulein-induced acinar injury, cell proliferation was enhanced and cell cycle components up-regulated in the pancreas of MRL/MpJ mice compared to the control strain C57BL/6. Initial damage of acinar cells was exacerbated in these mice, manifested by increased serum levels of pancreatic enzymes, intra-pancreatic trypsinogen activation and acinar cell apoptosis. In addition, MRL/MpJ pancreata presented enhanced inflammation, de-differentiation of acinar cells and acinar-to-ductal metaplasia. Manipulation of inflammatory levels and mitogenic stimulation with the thyroid hormone 5,3-L-tri-iodothyronine revealed that factors derived from initial acinar injury rather than inflammatory injury promote the replicative advantage in MRL/MpJ mice.

Drug delivery and epimorphic salamander-type mouse regeneration: A full parts and labor plan

The capacity to regenerate entire body parts, tissues, and organs had generally been thought to be lost in evolution with very few exceptions (e.g. the liver) surviving in mammals. The discovery of the MRL mouse and the elucidation of the underlying molecular pathway centering around hypoxia inducible factor, HIF-1α, has allowed a drug and materials approach to regeneration in mice and hopefully humans. The HIF-1α pathway is ancient and permitted the transition from unicellular to multicellular organisms. Furthermore, HIF-1α and its regulation by PHDs, important oxygen sensors in the cell, provides a perfect drug target. We review the historical background of regeneration biology, the discovery of the MRL mouse, and its underlying biology, and novel approaches to drugs, targets, and delivery systems (see Fig. 1).

Inhibition of apoptosis signal-regulating kinase 1 alters the wound epidermis and enhances auricular cartilage regeneration

Why regeneration does not occur in mammals remains elusive. In lower vertebrates, epimorphic regeneration of the limb is directed by the wound epidermis, which controls blastema formation to promote regrowth of the appendage. Herein, we report that knockout (KO) or inhibition of Apoptosis Signal-regulated Kinase-1 (ASK1), also known as mitogen-activated protein kinase kinase kinase 5 (MAP3K5), after full thickness ear punch in mice prolongs keratinocyte activation within the wound epidermis and promotes regeneration of auricular cartilage. Histological analysis showed the ASK1 KO ears displayed enhanced protein markers associated with blastema formation, hole closure and regeneration of auricular cartilage. At seven days after punch, the wound epidermis morphology was markedly different in the KO, showing a thickened stratum corneum with rounded cell morphology and a reduction of both the granular cell layer and decreased expression of filament aggregating protein. In addition, cytokeratin 6 was expressed in the stratum spinosum and granulosum. Topical application of inhibitors of ASK1 (NQDI-1), the upstream ASK1 activator, calcium activated mitogen kinase 2 (KN93), or the downstream target, c-Jun N-terminal kinase (SP600125) also resulted in enhanced regeneration; whereas inhibition of the other downstream target, the p38 α/β isoforms, (SB203580) had no effect. The results of this investigation indicate ASK1 inhibition prolongs keratinocyte and blastemal cell activation leading to ear regeneration.

Oxygen, Metabolism, and Regeneration: Lessons from Mice

The discovery that the Murphy Roths Large (MRL) mouse strain is a fully competent, epimorphic tissue regenerator, proved that the machinery of regeneration was preserved through evolution from hydra, to salamanders, to mammals. Such concepts have allowed translation of the biology of amphibians, and their ability to regenerate, to a mammalian context. We identified the ancient hypoxia-inducible factor (HIF)-1α pathway, operating through prolyl hydroxylase domain proteins (PHDs), as a central player in mouse regeneration. Thus, the possibility of targeting PHDs or other HIF-1α modifiers to effectively recreate the amphibian regenerative state has emerged. We posit that these regenerative pathways are critical in mammals. Moreover, the current approved use of PHD inhibitors in the clinic should allow fast-track translation from mouse studies to drug-based regenerative therapy in humans.

Genome-wide DNA methylation profiling of the regenerative MRL/MpJ mouse and two normal strains

Aim: We aimed to identify the pivotal differences in the DNA methylation profiles between the regeneration capable MRL/MpJ mouse and reference mouse strains. Materials & methods: Global DNA methylation profiling was performed in ear pinnae, bone marrow, spleen, liver and heart from uninjured adult females of the MRL/MpJ and C57BL/6J and BALB/c. Results & conclusion: A number of differentially methylated regions (DMRs) distinguishing between the MRL/MpJ mouse and both references were identified. In the ear pinnae, the DMRs were enriched in genes associated with development, inflammation and apoptosis, and in binding sites of transcriptional modulator Smad1. Several DMRs overlapped previously mapped quantitative trait loci of regenerative capability. The results suggest potential epigenetic determinants of regenerative phenomenon.

Macrophages are necessary for epimorphic regeneration in African spiny mice

How the immune system affects tissue regeneration is not well understood. In this study, we used an emerging mammalian model of epimorphic regeneration, the African spiny mouse, to examine cell-based inflammation and tested the hypothesis that macrophages are necessary for regeneration. By directly comparing inflammatory cell activation in a 4 mm ear injury during regeneration (Acomys cahirinus) and scarring (Mus musculus), we found that both species exhibited an acute inflammatory response, with scarring characterized by stronger myeloperoxidase activity. In contrast, ROS production was stronger and more persistent during regeneration. By depleting macrophages during injury, we demonstrate a functional requirement for these cells to stimulate regeneration. Importantly, the spatial distribution of activated macrophage subtypes was unique during regeneration with pro-inflammatory macrophages failing to infiltrate the regeneration blastema. Together, our results demonstrate an essential role for inflammatory cells to regulate a regenerative response.

The contribution of interleukin-2 to effective wound healing

Ineffective skin wound healing is a significant source of morbidity and mortality. Roughly 6.5 million Americans experience chronically open wounds and the cost of treating these wounds numbers in the billions of dollars annually. In contrast, robust wound healing can lead to the development of either hypertrophic scarring or keloidosis, both of which can cause discomfort and can be cosmetically undesirable. Appropriate wound healing requires the interplay of a variety of factors, including the skin, the local microenvironment, the immune system, and the external environment. When these interactions are perturbed, wounds can be a nidus for infection, which can cause them to remain open an extended period of time, or can scar excessively. Interleukin-2, a cytokine that directs T-cell expansion and phenotypic development, appears to play an important role in wound healing. The best-studied role for Interleukin-2 is in influencing T-cell development. However, other cell types, including fibroblasts, the skin cells responsible for closing wounds, express the Interleukin-2 receptor, and therefore may respond to Interleukin-2. Studies have shown that treatment with Interleukin-2 can improve the strength of healed skin, which implicates Interleukin-2 in the wound healing process. Furthermore, diseases that involve impaired wound healing, such as diabetes and systemic lupus erythematosus, have been linked to deficiencies in Interleukin-2 or defects Interleukin-2-receptor signaling. The focus of this review is to summarize the current understanding of the role of Interleukin-2 in wound healing, to highlight diseases in which Interleukin-2 and its receptor may contribute to impaired wound healing, and to assess Interleukin-2-modulating approaches as potential therapies to improve wound healing.

Chasing the recipe for a pro-regenerative immune system

Identification of the key ingredients and essential processes required to achieve perfect tissue regeneration in humans has so far remained elusive. Injury in vertebrates induces an obligatory wound response that will precede or overlap any regeneration specific program or scarring outcome. This process shapes the cellular and molecular landscape of the tissue, influencing the success of endogenous repair pathways or for potential clinical intervention. The involvement of immune cells is also required for aspects of development extending beyond the initial inflammatory phase of wounding. It has now become clear from amphibian, fish and mammalian models of tissue injury that the type of immune response and the profile of immune cells attending the site of injury can act as the gatekeepers that determine wound repair quality. The heterogeneity among innate and adaptive immune cell populations, along with the developmental origin of these cells, form key ingredients affecting the potential for downstream repair and the suppression of fibrosis. Cell-to-cell interactions between immune cells, such as macrophages and T cells, with stem cells and mesenchymal cells are critically important for shaping this process and these exchanges, are in turn influenced by the type of injury, tissue location and developmental stage of the organism. Developmentally, mouse cardiac regeneration is restricted to early stages of postnatal life where the balance of innate to adaptive immune cells may be poised towards regeneration. In the injured adult mouse liver, specific macrophage subsets improve repair while other bone marrow derived cells can exacerbate injury. Other studies using genetically diverse mice have shown enhanced regeneration in certain strains, restricted to specific tissues. This enhanced repair is linked with expression of genes such as Insulin-like Growth Factor- 1 (IGF-1) and activin (Act 1), that both play important roles in shaping the immune system. Immune cells are now appreciated to have powerful influences on critical cell types required for regeneration success. The winning recipe for tissue regeneration is likely to be found ultimately by identifying the genetic elements and specific cell populations that limit or allow intrinsic potential. This will be essential for developing therapeutic strategies for tissue regeneration in humans.

Signalling by Transforming Growth Factor Beta Isoforms in Wound Healing and Tissue Regeneration

Transforming growth factor beta (TGFβ) signalling is essential for wound healing, including both non-specific scar formation and tissue-specific regeneration. Specific TGFβ isoforms and downstream mediators of canonical and non-canonical signalling play different roles in each of these processes. Here we review the role of TGFβ signalling during tissue repair, with a particular focus on the prototypic isoforms TGFβ1, TGFβ2, and TGFβ3. We begin by introducing TGFβ signalling and then discuss the role of these growth factors and their key downstream signalling mediators in determining the balance between scar formation and tissue regeneration. Next we discuss examples of the pleiotropic roles of TGFβ ligands during cutaneous wound healing and blastema-mediated regeneration, and how inhibition of the canonical signalling pathway (using small molecule inhibitors) blocks regeneration. Finally, we review various TGFβ-targeting therapeutic strategies that hold promise for enhancing tissue repair.

Enhanced cartilage repair in 'healer' mice-New leads in the search for better clinical options for cartilage repair

Adult articular cartilage has a poor capacity to undergo intrinsic repair. Current strategies for the repair of large cartilage defects are generally unsatisfactory because the restored cartilage does not have the same resistance to biomechanical loading as authentic articular cartilage and degrades over time. Recently, an exciting new research direction, focused on intrinsic cartilage regeneration rather than fibrous repair by external means, has emerged. This review explores the new findings in this rapidly moving field as they relate to the clinical goal of restoration of structurally robust, stable and non-fibrous articular cartilage following injury.

Ear wound regeneration in the African spiny mouse Acomys cahirinus

While regeneration occurs in a number of taxonomic groups across the Metazoa, there are very few reports of regeneration in mammals, which generally respond to wounding with fibrotic scarring rather than regeneration. A recent report described skin shedding, skin regeneration and extensive ear punch closure in two rodent species, Acomys kempi and Acomys percivali. We examined these striking results by testing the capacity for regeneration of a third species, Acomys cahirinus, and found a remarkable capacity to repair full thickness circular punches in the ear pinna. Four‐millimeter‐diameter wounds closed completely in 2 months in 100% of ear punches tested. Histology showed extensive formation of elastic cartilage, adipose tissue, dermis, epidermis and abundant hair follicles in the repaired region. Furthermore, we demonstrated abundant angiogenesis and unequivocal presence of both muscle and nerve fibers in the reconstituted region; in contrast, similar wounds in C57BL/6 mice simply healed the borders of the cut by fibrotic scarring. Our results confirm the regenerative capabilities of Acomys, and suggest this model merits further attention.

Drug-induced regeneration in adult mice

Whereas amphibians regenerate lost appendages spontaneously, mammals generally form scars over the injury site through the process of wound repair. The MRL mouse strain is an exception among mammals because it shows a spontaneous regenerative healing trait and so can be used to investigate proregenerative interventions in mammals. We report that hypoxia-inducible factor 1α (HIF-1α) is a central molecule in the process of regeneration in adult MRL mice. The degradation of HIF-1α protein, which occurs under normoxic conditions, is mediated by prolyl hydroxylases (PHDs). We used the drug 1,4-dihydrophenonthrolin-4-one-3-carboxylic acid (1,4-DPCA), a PHD inhibitor, to stabilize constitutive expression of HIF-1α protein. A locally injectable hydrogel containing 1,4-DPCA was designed to achieve controlled delivery of the drug over 4 to 10 days. Subcutaneous injection of the 1,4-DPCA/hydrogel into Swiss Webster mice that do not show a regenerative phenotype increased stable expression of HIF-1α protein over 5 days, providing a functional measure of drug release in vivo. Multiple peripheral subcutaneous injections of the 1,4-DPCA/hydrogel over a 10-day period led to regenerative wound healing in Swiss Webster mice after ear hole punch injury. Increased expression of the HIF-1α protein may provide a starting point for future studies on regeneration in mammals.

Allogeneic Bone Marrow Transplant from MRL/MpJ Super-Healer Mice Does Not Improve Articular Cartilage Repair in the C57Bl/6 Strain

Background

Articular cartilage has been the focus of multiple strategies to improve its regenerative/ repair capacity. The Murphy Roths Large (MRL/MpJ) “super-healer” mouse demonstrates an unusual enhanced regenerative capacity in many tissues and provides an opportunity to further study endogenous cartilage repair. The objective of this study was to test whether the super-healer phenotype could be transferred from MRL/MpJ to non-healer C57Bl/6 mice by allogeneic bone marrow transplant.

Methodology

The healing of 2mm ear punches and full thickness cartilage defects was measured 4 and 8 weeks after injury in control C57Bl/6 and MRL/MpJ “super-healer” mice, and in radiation chimeras reconstituted with bone marrow from the other mouse strain. Healing was assessed using ear hole diameter measurement, a 14 point histological scoring scale for the cartilage defect and an adapted version of the Osteoarthritis Research Society International scale for assessment of osteoarthritis in mouse knee joints.

Principal Findings

Normal and chimeric MRL mice showed significantly better healing of articular cartilage and ear wounds along with less severe signs of osteoarthritis after cartilage injury than the control strain. Contrary to our hypothesis, however, bone marrow transplant from MRL mice did not confer improved healing on the C57Bl/6 chimeras, either in regards to ear wound healing or cartilage repair.

Conclusion and Significance

The elusive cellular basis for the MRL regenerative phenotype still requires additional study and may possibly be dependent on additional cell types external to the bone marrow.

Tendon regeneration and scar formation: The concept of scarless healing

Tendon healing is characterized by the formation of fibrovascular scar tissue, as tendon has very little intrinsic regenerative capacity. This creates a substantial clinical challenge in the setting of large, chronic tears seen clinically. Interest in regenerative healing seen in amphibians and certain strains of mice has arisen in response to the biological behavior of tendon tissue. Bone is also a model of tissue regeneration as healing bone will achieve the mechanical and histologic characteristics of the original tissue. The ultimate goal of the study of genes and mechanisms that contribute to true tissue regeneration is to ultimately attempt to manipulate the expression of those genes and activate these mechanisms in the setting of tendon injury and repair. Clearly, further research is needed to bring this to the forefront, however, study of scarless healing has potential to have meaningful application to tendon healing.

Increased levels of p21((CIP1/WAF1)) correlate with decreased chondrogenic differentiation potential in synovial membrane progenitor cells

Cartilage injuries are a major concern in the field of orthopedics. They occur following trauma, as well as from a variety of pathological conditions including Osteoarthritis (OA). Although cartilage does not exhibit robust endogenous repair, it has been demonstrated that modulating the activity of p21 can increase the regenerative abilities of cartilage in vitro and in vivo. Since the synovial membrane is abundant with mesenchymal progenitor cells (MPCs) capable of differentiating into cartilage both in vitro and in vivo, we examined if p21 expression levels varied between MPCs derived from normal vs. OA knee joints. Analysis of p21 at the mRNA and protein levels within normal and OA MPCs demonstrated differential levels of expression between these two groups, with OA MPCs having higher p21 expression levels. The higher levels of p21 in OA MPCs are also correlated with a decreased chondrogenic differentiation capacity and synovial inflammation, however, there was no evidence of senescence in the OA cells. The results of this study suggest that cell cycle regulation in MPCs may be altered in OA and that modulation of this pathway may have therapeutic potential once the mechanism by which this regulates stem/progenitor cells is better understood.

An Articular Cartilage Repair Model in Common C57Bl/6 Mice

To analyze the genetic and biomolecular mechanisms underlying cartilage repair, an optimized mouse model of osteochondral repair is required. Although several models of articular cartilage injury in mice have recently been established, the articular surface in adult C57Bl/6 mice heals poorly. Since C57Bl/6 mice are the most popular strain of genetically manipulated mice, an articular cartilage repair model using C57Bl/6 mice would be helpful for analysis of the mechanisms of cartilage repair. The purpose of this study was to establish a cartilage repair model in C57Bl/6 mice using immature animals. To achieve this goal, full-thickness injuries were generated in 3-week-old (young), 4-week-old (juvenile), and 8-week-old (adult) C57Bl/6 mice. To investigate the reproducibility and consistency of full-thickness injuries, mice were sacrificed immediately after operation, and cartilage thickness at the patellar groove, depth of the cartilage injury, cross-sectional width, and cross-sectional area were compared among the three age groups. The depth of cartilage injury/cartilage thickness ratio (%depth) and the coefficient of variation (CV) for each parameter were also calculated. At 8 weeks postoperatively, articular cartilage repair was assessed using a histological scoring system. With respect to the reproducibility and consistency of full-thickness injuries, cartilage thickness, depth of cartilage injury, and cross-sectional area were significantly larger in young and juvenile mice than in adult mice, whereas cross-sectional width and %depth were almost equal among the three age groups. CVs of %depths were less than 10% in all groups. With respect to articular cartilage repair, young and juvenile mice showed superior results. In conclusion, we established a novel cartilage repair model in C57Bl/6 mice. This model will be valuable in achieving mechanistic insights into the healing process of the joint surface, as it will facilitate the use of genetically modified mice, which are most commonly developed on a C57Bl/6 background.

Inflammation and Its Correlates in Regenerative Wound Healing: An Alternate Perspective

Objective: The wound healing response may be viewed as partially overlapping sets of two physiological processes, regeneration and wound repair with the former overrepresented in some lower species such as newts and the latter more typical of mammals. A robust and quantitative model of regenerative healing has been described in Murphy Roths Large (MRL) mice in which through-and-through ear hole wounds in the ear pinna leads to scarless healing and replacement of all tissue through blastema formation and including cartilage. Since these mice are naturally autoimmune and display many aspects of an enhanced inflammatory response, we chose to examine the inflammatory status during regenerative ear hole closure and observed that inflammation has a clear positive effect on regenerative healing. Approach: The inflammatory gene expression patterns (Illumina microarrays) of early healing ear tissue from regenerative MRL and nonregenerative C57BL/6 (B6) strains are presented along with a survey of innate inflammatory cells found in this tissue type pre and postinjury. The role of inflammation on healing is tested using a COX-2 inhibitor. Innovation and Conclusion: We conclude that (1) enhanced inflammation is consistent with, and probably necessary, for a full regenerative response and (2) the inflammatory gene expression and cell distribution patterns suggest a novel mast cell population with markers found in both immature and mature mast cells that may be a key component of regeneration.

Regeneration of articular cartilage in healer and non-healer mice

Mammals rarely regenerate their lost or injured tissues into adulthood. MRL/MpJ mouse strain initially identified to heal full-thickness ear wounds now represents a classical example of mammalian wound regeneration since it can heal a spectrum of injuries such as skin and cardiac wounds, nerve injuries and knee articular cartilage lesions. In addition to MRL/MpJ, a few other mouse strains such as LG/J (a parent of MRL/MpJ) and LGXSM-6 (arising from an intercross between LG/J and SM/J mouse strains) have now been recognized to possess regenerative/healing abilities for articular cartilage and ear wound injuries that are similar, if not superior, to MRL/MpJ mice. While some mechanisms underlying regenerative potential have been begun to emerge, a complete set of biological processes and pathways still needs to be elucidated. Using a panel of healer and non-healer mouse strains, our recent work has provided some insights into the genes that could potentially be associated with healing potential. Future mechanistic studies can help seek the Holy Grail of regenerative medicine. This review highlights the regenerative capacity of selected mouse strains for articular cartilage, in particular, and lessons from other body tissues, in general.

Fine-mapping quantitative trait loci affecting murine external ear tissue regeneration in the LG/J by SM/J advanced intercross line

External ear hole closure in LG/J mice represents a model of regenerative response. It is accompanied by the formation of a blastema-like structure and the re-growth of multiple tissues, including cartilage. The ability to regenerate tissue is heritable. An F34 advanced intercross line of mice (Wustl:LG,SM-G34) was generated to identify genomic loci involved in ear hole closure over a 30-day healing period. We mapped 19 quantitative trait loci (QTL) for ear hole closure. Individual gene effects are relatively small (0.08 mm), and most loci have co-dominant effects with phenotypically intermediate heterozygotes. QTL support regions were limited to a median size of 2 Mb containing a median of 19 genes. Positional candidate genes were evaluated using differential transcript expression between LG/J and SM/J healing tissue, function analysis and bioinformatic analysis of single-nucleotide polymorphisms in and around positional candidate genes of interest. Analysis of the set of 34 positional candidate genes and those displaying expression differences revealed over-representation of genes involved in cell cycle regulation/DNA damage, cell migration and adhesion, developmentally related genes and metabolism. This indicates that the healing phenotype in LG/J mice involves multiple physiological mechanisms.

Increased AMP-activated protein kinase in skeletal muscles of Murphy Roth Large mice and its potential role in altered metabolism

Wild‐type Murphy Roth Large (MRL) mice have long been investigated for their superior healing ability when subjected to various wound and disease models. Despite this long history, the mechanisms causing their extraordinary healing ability remain undefined. As we have recently demonstrated that MRL mice with muscular dystrophy are resistant to the associated fibrosis and the Heber‐Katz group has demonstrated MRL mitochondrial mutations, we decided to investigate the skeletal muscle metabolic characteristics of the MRL mouse strain compared to the commonly utilized C57BL/6J control mouse strain. We now have evidence demonstrating an altered metabolism in the MRL quadriceps, triceps brachii, and diaphragm of 8‐week‐old animals compared to tissues from control animals. The MRL skeletal muscles have increased activated phosphorylated AMP‐activated protein kinase (pAMPK). The increased pAMPK signaling coincides with increased skeletal muscle mitochondrial content. These metabolic changes may compensate for insufficient oxidative phosphorylation which is demonstrated by altered quantities of proteins involved in oxidative phosphorylation and ex vivo metabolic investigations. We also demonstrate that the MRL muscle cells have increased metabolic physiologic reserve. These data further the investigations into this important and unique mouse strain. Why the MRL mice have increased pAMPK and how increased pAMPK and the resultant metabolic alterations affect the healing ability in the MRL mouse strain is discussed. Understanding the molecular mechanisms surrounding the super healing characteristics of these mice will lead to relevant clinical intervention points. In conclusion, we present novel data of increased mitochondrial content, pAMPK, and glycolytic indicators in MRL skeletal muscles.

Molecular insight into the association between cartilage regeneration and ear wound healing in genetic mouse models: targeting new genes in regeneration

Tissue regeneration is a complex trait with few genetic models available. Mouse strains LG/J and MRL are exceptional healers. Using recombinant inbred strains from a large (LG/J, healer) and small (SM/J, nonhealer) intercross, we have previously shown a positive genetic correlation between ear wound healing, knee cartilage regeneration, and protection from osteoarthritis. We hypothesize that a common set of genes operates in tissue healing and articular cartilage regeneration. Taking advantage of archived histological sections from recombinant inbred strains, we analyzed expression of candidate genes through branched-chain DNA technology directly from tissue lysates. We determined broad-sense heritability of candidates, Pearson correlation of candidates with healing phenotypes, and Ward minimum variance cluster analysis for strains. A bioinformatic assessment of allelic polymorphisms within and near candidate genes was also performed. The expression of several candidates was significantly heritable among strains. Although several genes correlated with both ear wound healing and cartilage healing at a marginal level, the expression of four genes representing DNA repair (Xrcc2, Pcna) and Wnt signaling (Axin2, Wnt16) pathways was significantly positively correlated with both phenotypes. Cluster analysis accurately classified healers and nonhealers for seven out of eight strains based on gene expression. Specific sequence differences between LG/J and SM/J were identified as potential causal polymorphisms. Our study suggests a common genetic basis between tissue healing and osteoarthritis susceptibility. Mapping genetic variations causing differences in diverse healing responses in multiple tissues may reveal generic healing processes in pursuit of new therapeutic targets designed to induce or enhance regeneration and, potentially, protection from osteoarthritis.

Musculoskeletal regeneration and its implications for the treatment of tendinopathy

Tendinopathies are common muskoloskeletal injuries that lead to pain and disability. Development and pathogenesis of tendinopathy is attributed to progressive pathological changes to the structure, function, and biology of tendon. The nature of this disease state, whether acquired by acute or chronic injury, is being actively investigated. Scarring, disorganized tissue, and loss of function characterize adult tendon healing. Recent work from animal models has begun to reveal the potential for adult mammalian tendon regeneration, the replacement of diseased with innate tissue. This review discusses what is known about musculoskeletal regeneration from a molecular perspective and how these findings can be applied to tendinopathy. Non-mammalian and mammalian models are discussed with emphasis on the potential of Murphy Roths Large mice to serve as a model of adult tendon regeneration. Comparison of regeneration in non-mammals, foetal mammals and adult mammals emphasizes distinctly different contributing factors to effective regeneration.

Genetic and cellular evidence of decreased inflammation associated with reduced incidence of posttraumatic arthritis in MRL/MpJ mice

OBJECTIVE

To examine the relationship between inflammation and posttraumatic arthritis (PTA) in a murine intraarticular fracture model.

METHODS

Male C57BL/6 and MRL/MpJ "superhealer" mice received tibial plateau fractures using a previously established method. Mice were killed on day 0 (within 4 hours of fracture) and days 1, 3, 5, 7, 28, and 56 after fracture. Synovial tissue samples, obtained prior to fracture and on days 0, 1, 3, 5, and 7 after fracture, were examined by reverse transcription-polymerase chain reaction for gene expression of proinflammatory cytokines and chemokines. Synovial fluid and serum samples were collected to measure cytokine concentrations, using enzyme-linked immunosorbent assay. Whole joints were examined histologically for the extent of synovitis and cartilage degradation, and joint tissue samples from all time points were analyzed immunohistochemically to evaluate the distribution of interleukin-1 (IL-1).

RESULTS

Compared to C57BL/6 mice, MRL/MpJ mice had less severe intraarticular and systemic inflammation following joint injury, as evidenced by lower gene expression of tumor necrosis factor α and IL-1β in the synovial tissue and lower protein levels of IL-1α and IL-1β in the synovial fluid, serum, and joint tissues. Furthermore, after joint injury, MRL/MpJ mice had lower gene expression of macrophage inflammatory proteins and macrophage-derived chemokine (CCL22) in the synovial tissue, and also had reduced acute and late-stage infiltration of synovial macrophages.

CONCLUSION

C57BL/6 mice exhibited higher levels of inflammation than MRL/MpJ mice, indicating that MRL/MpJ mice are protected from PTA in this model. These data thus suggest an association between joint tissue inflammation and the development and progression of PTA in mice.

Keratin gene expression profiles after digit amputation in C57BL/6 vs. regenerative MRL mice imply an early regenerative keratinocyte activated-like state

Mouse strains C57BL/6 (B6) and MRL were studied by whole mouse genome chip microarray analyses of RNA isolated from amputation sites at different times pre- and postamputation at the midsecond phalange of the middle digit. Many keratin genes were highly differentially expressed. All keratin genes were placed into three temporal response classes determined by injury/preinjury ratios. One class, containing only Krt6 and Krt16, were uniquely expressed relative to the other two classes and exhibited different temporal responses in MRL vs. B6. Immunohistochemical staining for Krt6 and Krt16 in tissue sections, including normal digit, flank skin, and small intestine, and from normal and injured ear pinna tissue exhibited staining differences in B6 (low) and MRL (high) that were consistent with the microarray results. Krt10 staining showed no injury-induced differences, consistent with microarray expression. We analyzed Krt6 and Krt16 gene association networks and observed in uninjured tissue several genes with higher expression levels in MRL, but not B6, that were associated with the keratinocyte activated state: Krt6, Krt16, S100a8, S100a9, and Il1b; these data suggest that keratinocytes in the MRL strain, but not in B6, are in an activated state prior to wounding. These expression levels decreased in MRL at all times postwounding but rose in the B6, peaking at day 3. Other keratins significantly expressed in the normal basal keratinocyte state showed no significant strain differences. These data suggest that normal MRL skin is in a keratinocyte activated state, which may provide it with superior responses to wounding.

The superhealing MRL background improves muscular dystrophy

Background

Mice from the MRL or “superhealing” strain have enhanced repair after acute injury to the skin, cornea, and heart. We now tested an admixture of the MRL genome and found that it altered the course of muscle pathology and cardiac function in a chronic disease model of skeletal and cardiac muscle. Mice lacking γ-sarcoglycan (Sgcg), a dystrophin-associated protein, develop muscular dystrophy and cardiomyopathy similar to their human counterparts with limb girdle muscular dystrophy. With disruption of the dystrophin complex, the muscle plasma membrane becomes leaky and muscles develop increased fibrosis.

Methods

MRL/MpJ mice were bred with Sgcg mice, and cardiac function was measured. Muscles were assessed for fibrosis and membrane leak using measurements of hydroxyproline and Evans blue dye. Quantitative trait locus mapping was conducted using single nucleotide polymorphisms distinct between the two parental strains.

Results

Introduction of the MRL genome reduced fibrosis but did not alter membrane leak in skeletal muscle of the Sgcg model. The MRL genome was also associated with improved cardiac function with reversal of depressed fractional shortening and the left ventricular ejection fraction. We conducted a genome-wide analysis of genetic modifiers and found that a region on chromosome 2 was associated with cardiac, diaphragm muscle and abdominal muscle fibrosis.

Conclusions

These data are consistent with a model where the MRL genome acts in a dominant manner to suppress fibrosis in this chronic disease setting of heart and muscle disease.

Intra-articular delivery of purified mesenchymal stem cells from C57BL/6 or MRL/MpJ superhealer mice prevents posttraumatic arthritis

Joint injury dramatically enhances the onset of osteoarthritis (OA) and is responsible for an estimated 12% of OA. Posttraumatic arthritis (PTA) is especially common after intra-articular fracture, and no disease-modifying therapies are currently available. We hypothesized that the delivery of mesenchymal stem cells (MSCs) would prevent PTA by altering the balance of inflammation and regeneration after fracture of the mouse knee. Additionally, we examined the hypothesis that MSCs from the MRL/MpJ (MRL) "superhealer" mouse strain would show increased multilineage and therapeutic potentials as compared to those from C57BL/6 (B6) mice, as MRL mice have shown exceptional in vivo regenerative abilities. A highly purified population of MSCs was prospectively isolated from bone marrow using cell surface markers (CD45-/TER119-/PDGFRα+/Sca-1+). B6 MSCs expanded greater than 100,000-fold in 3 weeks when cultured at 2% oxygen and displayed greater adipogenic, osteogenic, and chondrogenic differentiation as compared to MRL MSCs. Mice receiving only a control saline injection after fracture demonstrated PTA after 8 weeks, but the delivery of 10,000 B6 or MRL MSCs to the joint prevented the development of PTA. Cytokine levels in serum and synovial fluid were affected by treatment with stem cells, including elevated systemic interleukin-10 at several time points. The delivery of MSCs did not reduce the degree of synovial inflammation but did show increased bone volume during repair. This study provides evidence that intra-articular stem cell therapy can prevent the development of PTA after fracture and has implications for possible clinical interventions after joint injury before evidence of significant OA.

Heritability of articular cartilage regeneration and its association with ear wound healing in mice

OBJECTIVE

Emerging evidence suggests that genetic components contribute significantly to cartilage degeneration in osteoarthritis pathophysiology, but little information is available on the genetics of cartilage regeneration. Therefore, this study was undertaken to investigate cartilage regeneration in genetic murine models using common inbred strains and a set of recombinant inbred (RI) lines generated from LG/J (healer of ear wounds) and SM/J (nonhealer) inbred mouse strains.

METHODS

An acute full-thickness cartilage injury was introduced in the trochlear groove of 8-week-old mice (n=265) through microsurgery. Mouse knee joints were sagittally sectioned and stained with toluidine blue to evaluate regeneration. For the ear wound phenotype, a bilateral 2-mm through-and-through puncture was created in 6-week-old mice (n=229), and healing outcomes were measured after 30 days. Broad-sense heritability and genetic correlations were calculated for both phenotypes.

RESULTS

Time-course analysis of the RI mouse lines showed no significant regeneration until 16 weeks after surgery; at that time, the strains could be segregated into 3 categories: good, intermediate, and poor healers. Analysis of heritability (H2) showed that both cartilage regeneration (H2=26%; P=0.006) and ear wound closure (H2=53%; P<0.00001) were significantly heritable. The genetic correlations between the two healing phenotypes for common inbred mouse strains (r=0.92) and RI mouse lines (r=0.86) were found to be extremely high.

CONCLUSION

Our findings indicate that articular cartilage regeneration in mice is heritable, the differences between the mouse lines are due to genetic differences, and a strong genetic correlation between the two phenotypes exists, indicating that they plausibly share a common genetic basis. We therefore surmise that LG/J by SM/J intercross mice can be used to dissect the genetic basis of variation in cartilage regeneration.

TRIM28 prevents autoinflammatory T cell development in vivo

TRIM28 is a component of heterochromatin complexes whose function in the immune system is unknown. By studying mice with conditional T cell-specific deletion of TRIM28 (CKO mice), we found that TRIM28 was phosphorylated after stimulation via the T cell antigen receptor (TCR) and was involved in the global regulation of CD4(+) T cells. The CKO mice had a spontaneous autoimmune phenotype that was due in part to early lymphopenia associated with a defect in the production of interleukin 2 (IL-2) as well as incomplete cell-cycle progression of their T cells. In addition, CKO T cells showed derepression of the cytokine TGF-β3, which resulted in an altered cytokine balance; this caused the accumulation of autoreactive cells of the T(H)17 subset of helper T cells and of Foxp3(+) T cells. Notably, CKO Foxp3(+) T cells were unable to prevent the autoimmune phenotype in vivo. Our results show critical roles for TRIM28 in both T cell activation and T cell tolerance.

The super super-healing MRL mouse strain

The Murphy Roths Large (MRL/MpJ) mice provide unique insights into wound repair and regeneration. These mice and the closely related MRL/MpJ-Fas^lpr /J and Large strains heal wounds made in multiple tissues without production of a fibrotic scar. The precise mechanism of this remarkable ability still eludes researchers, but some data has been generated and insights are being revealed. For example, MRL cells reepithelialize over dermal wound sites faster than cells of other mouse strains. This allows a blastema to develop beneath the protective layer. The MRL mice also have an altered basal immune system and an altered immune response to injury. In addition, MRL mice have differences in their tissue resident progenitor cells and certain cell cycle regulatory proteins. The difficulty often lies in separating the causative differences from the corollary differences. Remarkably, not every tissue in these mice heals scarlessly, and the specific type of wound and priming affect regeneration ability as well. The MRL/MpJ, MRL/MpJ-Fas^lpr /J, and Large mouse strains are also being investigated for their autoimmune characteristic. Whether the two phenotypes of regeneration and autoimmunity are related remains an enigma.

Healing quantitative trait loci in a combined cross analysis using related mouse strain crosses

Inbred mouse strains MRL and LG share the ability to fully heal ear hole punches with the full range of appropriate tissues without scarring. They also share a common ancestry, MRL being formed from a multi-strain cross with two final backcrosses to LG before being inbred by brother-sister mating. Many gene-mapping studies for healing ability have been performed using these two strains, resulting in the location of about 20 quantitative trait loci (QTLs). Here, we combine two of these crosses (N = 638), MRL/lpr × C57BL/6NTac and LG/J × SM/J, in a single combined cross analysis to increase the mapping power, decrease QTL support intervals, separate multiple QTLs and establish allelic states at individual QTL. The combined cross analysis located 11 QTLs, 6 affecting only one cross (5 LG × SM and 1 MRL × B6) and 5 affecting both crosses, approximately the number of common QTLs expected given strain SNP similarity. Amongst the five QTLs mapped in both crosses, three had significantly different genetic effects, additive in one cross and over or underdominant in the other. It is possible that allelic states at these three loci are different in SM and B6 because they lead to differences in dominance interactions with the LG and MRL alleles. QTL support intervals are 40% smaller in the combined cross analysis than in either of the single crosses. Combined cross analysis was successful in enhancing the interpretation of earlier QTL results for these strains.

Regenerative phenotype in mice with a point mutation in transforming growth factor beta type I receptor (TGFBR1)

Regeneration of peripheral differentiated tissue in mammals is rare, and regulators of this process are largely unknown. We carried out a forward genetic screen in mice using N-ethyl-N-nitrosourea mutagenesis to identify genetic mutations that affect regenerative healing in vivo. More than 400 pedigrees were screened for closure of a through-and-through punch wound in the mouse ear. This led to the identification of a single pedigree with a heritable, fast, and regenerative wound-healing phenotype. Within 5 wk after ear-punch, a threefold decrease in the diameter of the wound was observed in the mutant mice compared with the wild-type mice. At 22 wk, new cartilage, hair follicles, and sebaceous glands were observed in the newly generated tissue. This trait was mapped to a point mutation in a receptor for TGF-β, TGFBR1. Mouse embryonic fibroblasts from the affected mice had increased expression of a subset of TGF-β target genes, suggesting that the mutation caused partial activation of the receptor. Further, bone marrow stromal cells from the mutant mice more readily differentiated to chondrogenic precursors, providing a plausible explanation for the enhanced development of cartilage islands in the regenerated ears. This mutant mouse strain provides a unique model to further explore regeneration in mammals and, in particular, the role of TGFBR1 in chondrogenesis and regenerative wound healing.

The role of p21 in regulating mammalian regeneration

The MRL (Murphy Roths Large) mouse has provided a unique model of adult mammalian regeneration as multiple tissues show this important phenotype. Furthermore, the healing employs a blastema-like structure similar to that seen in amphibian regenerating tissue. Cells from the MRL mouse display DNA damage, cell cycle G2/M arrest, and a reduced level of p21^CIP1/WAF. A functional role for p21 was confirmed when tissue injury in an adult p21^-/- mouse showed a healing phenotype that matched the MRL mouse, with the replacement of tissues, including cartilage, and with hair follicle formation and a lack of scarring. Since the major canonical function of p21 is part of the p53/p21 axis, we explored the consequences of p53 deletion. A regenerative response was not seen in a p53^-/- mouse and the elimination of p53 from the MRL background had no negative effect on the regeneration of the MRL.p53^-/- mouse. An exploration of other knockout mice to identify p21-dependent, p53-independent regulatory pathways involved in the regenerative response revealed another significant finding showing that elimination of transforming growth factor-β1 displayed a healing response as well. These results are discussed in terms of their effect on senescence and differentiation.

Post-traumatic osteoarthritis: improved understanding and opportunities for early intervention

Even with current treatments of acute joint injuries, more than 40% of people who suffer significant ligament or meniscus tears, or articular surface injuries, will develop osteoarthritis (OA). Correspondingly, 12% or more of all patients with lower extremity OA have a history of joint injury. Recent research suggests that acute joint damage that occurs at the time of an injury initiates a sequence of events that can lead to progressive articular surface damage. New molecular interventions, combined with evolving surgical methods, aim to minimize or prevent progressive tissue damage triggered by joint injury. Seizing the potential for progress in the treatment of joint injuries to forestall OA will depend on advances in (1) quantitative methods of assessing the injury severity, including both structural damage and biologic responses, (2) understanding of the pathogenesis of post-traumatic OA, taking into account potential interactions among the different tissues and the role of post-traumatic incongruity and instability, and (3) application of engineering and molecular research to develop new methods of treating injured joints. This paper highlights recent advances in understanding of the structural damage and the acute biological response following joint injury, and it identifies important directions for future research.

An ear punch model for studying the effect of radiation on wound healing

PURPOSE

Radiation and wound combined injury represents a major clinical challenge because of the synergistic interactions that lead to higher morbidity and mortality than either insult would produce singly. The purpose of this study was to develop a mouse ear punch model to study the physiological mechanisms underlying radiation effects on healing wounds.

MATERIALS AND METHODS

Surgical wounds were induced by a 2 mm surgical punch in the ear pinnae of MRL/MpJ mice. Photographs of the wounds were taken and the sizes of the ear punch wounds were quantified by image analysis. Local radiation to the ear was delivered by orthovoltage X-ray irradiator using a specially constructed jig that shields the other parts of body.

RESULTS

Using this model, we demonstrated that local radiation to the wound area significantly delayed the healing of ear punch wounds in a dose-dependent fashion. The addition of sublethal whole body irradiation (7 Gy) further delayed the healing of ear punch wounds. These results were replicated in C57BL/6 mice; however, wound healing in MRL/MpJ mice was accelerated.

CONCLUSIONS

These data indicate that the mouse ear punch model is a valuable model to study radiation and wound combined injury.

Mechanisms of improved wound healing in Murphy Roths Large (MRL) mice after skin transplantation

Scars arise in the late phase of wound healing and are characterized by fibroplasia. Previous controversial studies have discussed the regenerative wound healing capacity of Murphy Roths Large (MRL) mice. The aim of this study was to investigate the mechanisms of improved wound healing in a skin transplantation model. Skin grafts from MRL and haplotypically identical B10.BR mice were cross-transplanted. At day 10, B10.BR and MRL grafts on B10.BR recipients deposited collagen and showed severe apoptosis. Grafts of MRL recipients were not affected by such alterations and showed an enhanced healing progress. They were characterized by higher partial pressure of tissue oxygen, increased microcirculation, exceptionally intense neovascularization, and a blunted inflammatory response. This phenotype was accompanied by increased vascular endothelial growth factor expression, augmented by enhanced signal transducer and activator of transcription 3 (STAT3) phosphorylation. These effects were combined with a decreased STAT1 expression and phosphorylation. STAT1 pattern variation was associated with decreased Smad7 levels. Furthermore, MRL recipients showed improved stem cell recruitment to the wound area. The basic accelerated wound healing mechanism in MRL mice found in this skin transplantation model is improved engraftment; this is based on enhanced neovascularization and reduced inflammation. These effects are most likely due to higher vascular endothelial growth factor levels and changes in the STAT/Smad signal pathway, which may enhance transforming growth factor-β signaling, reducing proinflammatory responses.

Basic science of intra-articular fractures and posttraumatic osteoarthritis

Intra-articular fractures represent the primary etiologic factor leading to posttraumatic osteoarthritis. The pathomechanisms linking intra-articular fractures to end-stage cartilage destruction are poorly understood. However, fracture-related chondrocyte death has been linked to posttraumatic osteoarthritis. Researchers have made significant progress in understanding the pathomechanical link between injury and chondrocyte death. This article reviews recent basic scientific progress investigating intraarticular fractures and fracture-related chondrocyte death and dysfunction.

Genetic loci that regulate healing and regeneration in LG/J and SM/J mice

MRL mice display unusual healing properties. When MRL ear pinnae are hole punched, the holes close completely without scarring, with regrowth of cartilage and reappearance of both hair follicles and sebaceous glands. Studies using (MRL/lpr x C57BL/6)F(2) and backcross mice first showed that this phenomenon was genetically determined and that multiple loci contributed to this quantitative trait. The lpr mutation itself, however, was not one of them. In the present study we examined the genetic basis of healing in the Large (LG/J) mouse strain, a parent of the MRL mouse and a strain that shows the same healing phenotype. LG/J mice were crossed with Small (SM/J) mice and the F(2) population was scored for healing and their genotypes determined at more than 200 polymorphic markers. As we previously observed for MRL and (MRL x B6)F(2) mice, the wound-healing phenotype was sexually dimorphic, with female mice healing more quickly and more completely than male mice. We found quantitative trait loci (QTLs) on chromosomes (Chrs) 9, 10, 11, and 15. The heal QTLs on Chrs 11 and 15 were linked to differential healing primarily in male animals, whereas QTLs on Chrs 9 and 10 were not sexually dimorphic. A comparison of loci identified in previous crosses with those in the present report using LG/J x SM/J showed that loci on Chrs 9, 11, and 15 colocalized with those seen in previous MRL crosses, whereas the locus on Chr 10 was not seen before and is contributed by SM/J.

Genetic background affects the biomechanical behavior of the postpartum mouse cervix

OBJECTIVE

We hypothesized that the genetic makeup has an impact on the functional behavior of the uterine cervix. Therefore, we compared the biomechanical properties of uterine cervix in postpartum in 2 strains of mice that differ in their underlying regenerative collagen remodeling characteristics: MRL/MpJ+/+ (MRL: high regenerative repair) and C57BL/6 (C57: low regenerative high fibrotic repair).

STUDY DESIGN

Cervical tensile proprieties were assessed on day 3, 15, and 60 postpartum in MRL (n = 14) and C57 (n = 13) mice (4-5 animals at each time point). Stress-strain curves were generated using Shimadzu EZ-test instrumentation. Cervical tissue was stretched by 0.42 mm/min until rupture. Parameters of viscoelasticity including slope (a measure of stiffness), yield point (YP; moment when tissue changes its proprieties from elastic to plastic), and break point (BP; measure of tissue strength) were recorded and analyzed blindly between strains. Data were normalized to the weight of the tissue and analyzed by 2-way analysis of variance. Histological and collagen birefringence evaluation of the uterine cervix (MRL: n = 4; C57: n = 4) was performed 5 days after delivery.

RESULTS

At 3 and 15 days postpartum, cervices of MRL mice were significantly more compliant than those of C57 (P < .001). MRL mice displayed a significant increase in stiffness from day 3 to day 60 (slope, median +/- SEM: day 3: 3.1 +/- 0.5 vs day 15: 20.3 +/- 4.9 vs day 60: 33.1 +/- 3.5 N/mm per gram; P < .001). In contrast, the stiffness of C57 cervices reached maximum on day 15 (slope day 3: 14.1 +/- 4.3 vs day 15: 40.0 +/- 6.5 N/mm per gram; P = .02) and rested at a similar level on day 60 (day 60: 26.1 +/- 7.0 N/mm per gram; day 60 vs day 15: P = .937). More force was required to reach YP in C57 on day 3 (C57: 72.5 +/- 14.7 vs MRL: 19.9 +/- 1.6 N/g; P < .001) but not on either day 15 (C57: 156.1 +/- 27.5 vs MRL: 109.2 +/- 26.0 N/g; P = .120) or on day 60 (C57: 143.4 +/- 26.5 vs MRL: 164.5 +/- 18.7 N/g; P = .412). There was a significant decrease in BP in both strains on both day 15 and day 60 compared with day 3 postpartum (P = .856 for strain, P = .008 for day). MRL mice displayed significantly less cervical collagen birefringence compared with C57 control (P < .001) but increased proteoglycan staining and increased water content.

CONCLUSION

We provide evidence that genetic makeup may have an impact on cervical tissue remodeling and function. There are significant differences in postpartum cervical stiffness and compliance that vary with the regenerative collagen remodeling phenotype.

Evidence for articular cartilage regeneration in MRL/MpJ mice

OBJECTIVE

A major clinical problem in Orthopaedics is the repair of traumatic articular cartilage lesions. The MRL/MpJ strain of mice has the remarkable ability to regenerate ear hole punch wounds seamlessly including the scarless replacement of multiple tissues. The objective of this study was to assess whether articular cartilage defects repair or regenerate in the MRL/MpJ 'healer' strain of mice.

METHOD

Full thickness and partial thickness lesions were introduced into trochlear groove articular cartilage of MRL/MpJ and C57Bl/6 mice, a control strain that does not undergo ear hole regeneration. The wound sites were assessed 6 weeks and 12 weeks post-surgery using a histological scoring scheme and immunohistochemistry for markers of articular cartilage including proteoglycan, collagen II and collagen VI.

RESULTS

The partial thickness lesions did not repair in either strain. However, at both 6 weeks and 12 weeks timepoints the MRL/MpJ mice had a superior healing response of full thickness lesions with abundant chondrocytes and an extracellular matrix rich in proteoglycan, collagen II and collagen VI at the wound site. At the 12 week timepoint the enhanced cartilage healing was restricted to male MRL/MpJ mice. In contrast, the C57Bl/6 control strain produced an extracellular matrix at the wound site that, overall, had significantly less matrix proteoglycan and collagen II.

CONCLUSIONS

Male MRL/MpJ mice appear to possess an intrinsic ability to 'regenerate' articular cartilage. Understanding the biochemical and genetic basis for articular cartilage regeneration may open up new treatment options for traumatic articular cartilage defects.

Naturally occurring mitochondrial DNA heteroplasmy in the MRL mouse

The MRL/MpJ mouse is an inbred laboratory strain of Mus musculus, known to exhibit enhanced autoimmunity, increased wound healing, and increased regeneration properties. We report the full-length mitochondrial DNA (mtDNA) sequence of the MRL mouse (Accession # EU450583), and characterize the discovery of two naturally occurring heteroplasmic sites. The first is a T3900C substitution in the TPsiC loop of the tRNA methionine gene (tRNA-Met; mt-Tm). The second is a heteroplasmic insertion of 1-6 adenine nucleotides in the A-tract of the tRNA arginine gene (tRNA-Arg; mt-Tr) at positions 9821-9826. The level of heteroplasmy varied independently at these two sites in MRL individuals. The length of the tRNA-Arg A-tract increased with age, but heteroplasmy at the tRNA-Met site did not change with age. The finding of naturally occurring mtDNA heteroplasmy in an inbred strain of mouse makes the MRL mouse a powerful new experimental model for studies designed to explore therapeutic measures to alter the cellular burden of heteroplasmy.

Absence of posttraumatic arthritis following intraarticular fracture in the MRL/MpJ mouse

OBJECTIVE

Posttraumatic arthritis is a frequent long-term complication of intraarticular fractures. A model of a closed intraarticular fracture in C57BL/6 mice that progresses to posttraumatic arthritis has been developed. The MRL/MpJ mouse has shown unique regenerative abilities in response to injury. The objective of this study was to determine if the MRL/MpJ mouse is protected from posttraumatic arthritis after intraarticular fractures.

METHODS

Intraarticular fractures were created in MRL/MpJ mice and C57BL/6 control mice (n = 16 each). Limbs were analyzed for posttraumatic arthritis 4 and 8 weeks after fracture using microfocal computed tomography bone morphology, subchondral bone thickness evaluation, and histologic evaluation of cartilage degeneration. Serum cytokines and biomarkers were measured after the mice were killed.

RESULTS

Intraarticular fractures were successfully created in all 32 mice. In the experimental fractured limbs, C57BL/6 mice had a decrease in bone density, increased subchondral bone thickness, and increased cartilage degeneration compared with normal contralateral control limbs. In the MRL/MpJ mice, no differences in bone density, subchondral bone thickness, or histologic grading of cartilage degeneration were seen between fractured and contralateral control limbs. Cytokine analysis showed lower systemic levels of the proinflammatory cytokine interleukin-1alpha (IL-1alpha) and higher levels of the antiinflammatory cytokines IL-4 and IL-10 in the MRL/MpJ mice.

CONCLUSION

This study shows that the MRL/MpJ mouse is relatively protected from posttraumatic arthritis after intraarticular fracture. Further investigation into the mechanism involved in this response will hopefully provide new insight into the pathogenesis, prevention, and treatment of posttraumatic arthritis after intraarticular fracture.

Neuroblast protuberances in the subventricular zone of the regenerative MRL/MpJ mouse

The MRL mouse is unique in its capacity for regenerative healing of wounds. This regenerative ability includes complete closure, with little scarring, of wounds to the ear pinna and repair of cardiac muscle, without fibrosis, following cryoinjury. Here, we examine whether neurogenic zones within the MRL brain show enhanced regenerative capacity. The largest neurogenic zone in the adult brain, the subventricular zone (SVZ), lies adjacent to the lateral wall of the lateral ventricle and is responsible for replacement of interneuron populations within the olfactory bulb. Initial gross observation of the anterior forebrain in MRL mice revealed enlarged lateral ventricles; however, little neurodegeneration was detected within the SVZ or surrounding tissues. Instead, increased proliferation within the SVZ was observed, based on incorporation of the thymidine analogue bromodeoxyuridine. Closer examination using electron microscopy revealed that a significant number of SVZ astrocytes interpolated within the ependyma and established contact with the ventricle. In addition, subependymal, protuberant nests of cells, consisting primarily of neuroblasts, were found along the anterior SVZ of MRL mice. Whole mounts of the lateral wall of the lateral ventricle stained for the neuroblast marker doublecortin revealed normal formation of chains of migratory neuroblasts along the entire wall and introduction of enhanced green fluorescent protein-tagged retrovirus into the lateral ventricles confirmed that newly generated neuroblasts were able to track into the olfactory bulb.