Digit tip regrowth and differential gene expression in MRL/Mpj, DBA/2, and C57BL/6 mice

OA susceptibility in mice is partially mediated by the gut microbiome, is transferrable via microbiome transplantation and is associated with immunophenotype changes

OBJECTIVES

The Murphy Roths Large (MRL)/MpJ 'superhealer' mouse strain is protected from post-traumatic osteoarthritis (OA), although no studies have evaluated the microbiome in the context of this protection. This study characterised microbiome differences between MRL and wild-type mice, evaluated microbiome transplantation and OA and investigated microbiome-associated immunophenotypes.

METHODS

Cecal material from mixed sex C57BL6/J (B6) or female MRL/MpJ (MRL) was transplanted into B6 and MRL mice, then OA was induced by disruption of the medial meniscus surgery (DMM). In other experiments, transplantation was performed after DMM and transplantation was performed into germ-free mice. Transplanted mice were bred through F2. OARSI, synovitis and osteophyte scores were determined blindly 8 weeks after DMM. 16S microbiome sequencing was performed and metagenomic function was imputed. Immunophenotypes were determined using mass cytometry.

RESULTS

MRL-into-B6 transplant prior to DMM showed reduced OA histopathology (OARSI score 70% lower transplant vs B6 control), synovitis (60% reduction) and osteophyte scores (30% reduction) 8 weeks after DMM. When performed 48 hours after DMM, MRL-into-B6 transplant improved OA outcomes but not when performed 1-2 weeks after DMM. Protection was seen in F1 (60% reduction) and F2 progeny (30% reduction). Several cecal microbiome clades were correlated with either better (eg, Lactobacillus, R=-0.32, p=0.02) or worse (eg, Rikenellaceae, R=0.43, p=0.001) OA outcomes. Baseline immunophenotypes associated with MRL-into-B6 transplants and MRL included reduced double-negative T cells and increased CD25+CD4+ T cells.

CONCLUSION

The gut microbiome is responsible in part for OA protection in MRL mice and is transferrable by microbiome transplantation. Transplantation induces resting systemic immunophenotyping changes that correlate with OA protection.

Biomaterial Design Inspired by Regenerative Research Organisms

The efficacy of implanted biomaterials is largely dependent on the response of the host's immune and stromal cells. Severe foreign body response (FBR) can impede the integration of the implant into the host tissue and compromise the intended mechanical and biochemical function. Many features of FBR, including late-stage fibrotic encapsulation of implants, parallel the formation of fibrotic scar tissue after tissue injury. Regenerative organisms like zebrafish and salamanders can avoid fibrosis after injury entirely, but FBR in these research organisms is rarely investigated because their immune competence is much lower than humans. The recent characterization of a regenerative mammal, the spiny mouse (Acomys), has inspired us to take a closer look at cellular regulation in regenerative organisms across the animal kingdom for insights into avoiding FBR in humans. Here, we highlight how major features of regeneration, such as blastema formation, macrophage polarization, and matrix composition, can be modulated across a range of regenerative research organisms to elucidate common features that may be harnessed to minimize FBR. Leveraging a deeper understanding of regenerative biology for biomaterial design may help to reduce FBR and improve device integration and performance.

Modulating Cellular Responses to Mechanical Forces to Promote Wound Regeneration

Significance: Skin scarring poses a major biomedical burden for hundreds of millions of patients annually. However, this burden could be mitigated by therapies that promote wound regeneration, with full recovery of skin's normal adnexa, matrix ultrastructure, and mechanical strength. Recent Advances: The observation of wound regeneration in several mouse models suggests a retained capacity for postnatal mammalian skin to regenerate under the right conditions. Mechanical forces are a major contributor to skin fibrosis and a prime target for devices and therapeutics that could promote skin regeneration. Critical Issues: Wound-induced hair neogenesis, Acomys "spiny" mice, Murphy Roths Large mice, and mice treated with mechanotransduction inhibitors all show various degrees of wound regeneration. Comparison of regenerating wounds in these models against scarring wounds reveals differences in extracellular matrix interactions and in mechanosensitive activation of key signaling pathways, including Wnt, Sonic hedgehog, focal adhesion kinase, and Yes-associated protein. The advent of single-cell "omics" technologies has deepened this understanding and revealed that regeneration may recapitulate development in certain contexts, although it is unknown whether these mechanisms are relevant to healing in tight-skinned animals such as humans. Future Directions: While early findings in mice are promising, comparison across model systems is needed to resolve conflicting mechanisms and to identify conserved master regulators of skin regeneration. There also remains a dire need for studies on mechanomodulation of wounds in large, tight-skinned animals, such as red Duroc pigs, which better approximate human wound healing.

Regenerative Drug Discovery Using Ear Pinna Punch Wound Model in Mice

The ear pinna is a complex tissue consisting of the dermis, cartilage, muscles, vessels, and nerves. Ear pinna healing is a model of regeneration in mammals. In some mammals, including rabbits, punch wounds in the ear pinna close spontaneously; in common-use laboratory mice, they remain for life. Agents inducing ear pinna healing are potential regenerative drugs. We tested the effects of selected bioactive agents on 2 mm ear pinna wound closure in BALB/c mice. Our previous research demonstrated that a DNA methyltransferase inhibitor, zebularine, remarkably induced ear pinna regeneration. Although experiments with two other demethylating agents, RG108 and hydralazine, were unsuccessful, a histone deacetylase inhibitor, valproic acid, was another epigenetic agent found to increase ear hole closure. In addition, we identified a pro-regenerative activity of 4-ketoretinoic acid, a retinoic acid metabolite. Attempts to counteract the regenerative effects of the demethylating agent zebularine, with folates as methyl donors, failed. Surprisingly, a high dose of methionine, another methyl donor, promoted ear hole closure. Moreover, we showed that the regenerated areas of ear pinna were supplied with nerve fibre networks and blood vessels. The ear punch model proved helpful in testing the pro-regenerative activities of small-molecule compounds and observations of peripheral nerve regeneration.

p21-/- Mice Exhibit Spontaneous Articular Cartilage Regeneration Post-Injury

OBJECTIVE

Recent studies have implicated the cyclin dependent kinase inhibitor, p21, in enhanced tissue regeneration observed in MRL/MpJ "super-healer" mice. Specifically, p21 is downregulated in MRL cells and similar ear hole closure to MRL mice has been observed in p21^-/- mice. However, the direct implications of p21 deletion in endogenous articular cartilage regeneration remain unknown. In this study, we investigated the role of p21 deletion in the ability of mice to heal full-thickness cartilage defects (FTCDs).

DESIGN

C57BL/6 and p21^-/- (Cdkn1a^tm1Tyj) mice were subjected to FTCD and assessment of cartilage healing was performed at 1 hour, 3 days, 1 week, 2 weeks, and 4 weeks post-FTCD using a 14-point histological scoring system. X-ray microscopy was used to quantify cartilage healing parameters (e.g., cartilage thickness, surface area/volume) between C57BL/6 and p21^-/- mice.

RESULTS

Absence of p21 resulted in increased spontaneous articular cartilage regeneration by 3 days post-FTCD. Furthermore, p21^-/- mice presented with increased cartilage thickness at 1 and 2 weeks post-FTCD compared with uninjured controls, returning to baseline by 4 weeks post-FTCD.

CONCLUSIONS

We report that p21^-/- mice display enhanced articular cartilage regeneration post-FTCD compared with C57BL/6 mice. Furthermore, cartilage thickness was increased in p21^-/- mice at 1 week post-FTCD compared with uninjured p21^-/- mice and C57BL/6 mice.

Ear wound healing in MRL/MpJ mice is associated with gut microbiome composition and is transferable to non-healer mice via microbiome transplantation

Objective

Adult elastic cartilage has limited repair capacity. MRL/MpJ (MRL) mice, by contrast, are capable of spontaneously healing ear punctures. This study was undertaken to characterize microbiome differences between healer and non-healer mice and to evaluate whether this healing phenotype can be transferred via gut microbiome transplantation.

Methods

We orally transplanted C57BL/6J (B6) mice with MRL/MpJ cecal contents at weaning and as adults (n = 57) and measured ear hole closure 4 weeks after a 2.0mm punch and compared to vehicle-transplanted MRL and B6 (n = 25) and B6-transplanted MRL (n = 20) mice. Sex effects, timing of transplant relative to earpunch, and transgenerational heritability were evaluated. In a subset (n = 58), cecal microbiomes were profiled by 16S sequencing and compared to ear hole closure. Microbial metagenomes were imputed using PICRUSt.

Results

Transplantation of B6 mice with MRL microbiota, either in weanlings or adults, improved ear hole closure. B6-vehicle mice healed ear hole punches poorly (0.25±0.03mm, mm ear hole healing 4 weeks after a 2mm ear hole punch [2.0mm—final ear hole size], mean±SEM), whereas MRL-vehicle mice healed well (1.4±0.1mm). MRL-transplanted B6 mice healed roughly three times as well as B6-vehicle mice, and half as well as MRL-vehicle mice (0.74±0.05mm, P = 6.9E-10 vs. B6-vehicle, P = 5.2E-12 vs. MRL-vehicle). Transplantation of MRL mice with B6 cecal material did not reduce MRL healing (B6-transplanted MRL 1.3±0.1 vs. MRL-vehicle 1.4±0.1, p = 0.36). Transplantation prior to ear punch was associated with the greatest ear hole closure. Offspring of transplanted mice healed significantly better than non-transplanted control mice (offspring:0.63±0.03mm, mean±SEM vs. B6-vehicle control:0.25±0.03mm, n = 39 offspring, P = 4.6E-11). Several microbiome clades were correlated with healing, including Firmicutes (R = 0.84, P = 8.0E-7), Lactobacillales (R = 0.65, P = 1.1E-3), and Verrucomicrobia (R = -0.80, P = 9.2E-6). Females of all groups tended to heal better than males (B6-vehicle P = 0.059, MRL-transplanted B6 P = 0.096, offspring of MRL-transplanted B6 P = 0.0038, B6-transplanted MRL P = 1.6E-6, MRL-vehicle P = 0.0031). Many clades characteristic of female mouse cecal microbiota vs. males were the same as clades characteristic of MRL and MRL-transplanted B6 mice vs. B6 controls, including including increases in Clostridia and reductions in Verrucomicrobia in female mice.

Conclusion

In this study, we found an association between the microbiome and tissue regeneration in MRL mice and demonstrate that this trait can be transferred to non-healer mice via microbiome transplantation. We identified several microbiome clades associated with healing.

Unlocking mammalian regeneration through hypoxia inducible factor one alpha signaling

Historically, the field of regenerative medicine has aimed to heal damaged tissue through the use of biomaterials scaffolds or delivery of foreign progenitor cells. Despite 30 years of research, however, translation and commercialization of these techniques has been limited. To enable mammalian regeneration, a more practical approach may instead be to develop therapies that evoke endogenous processes reminiscent of those seen in innate regenerators. Recently, investigations into tadpole tail regrowth, zebrafish limb restoration, and the super-healing Murphy Roths Large (MRL) mouse strain, have identified ancient oxygen-sensing pathways as a possible target to achieve this goal. Specifically, upregulation of the transcription factor, hypoxia-inducible factor one alpha (HIF-1α) has been shown to modulate cell metabolism and plasticity, as well as inflammation and tissue remodeling, possibly priming injuries for regeneration. Since HIF-1α signaling is conserved across species, environmental or pharmacological manipulation of oxygen-dependent pathways may elicit a regenerative response in non-healing mammals. In this review, we will explore the emerging role of HIF-1α in mammalian healing and regeneration, as well as attempts to modulate protein stability through hyperbaric oxygen treatment, intermittent hypoxia therapy, and pharmacological targeting. We believe that these therapies could breathe new life into the field of regenerative medicine.

Understanding cartilage protection in OA and injury: a spectrum of possibilities

BACKGROUND

Osteoarthritis (OA) is a prevalent musculoskeletal disease resulting in progressive degeneration of the hyaline articular cartilage within synovial joints. Current repair treatments for OA often result in poor quality tissue that is functionally ineffective compared to the hyaline cartilage and demonstrates increased failure rates post-treatment. Complicating efforts to improve clinical outcomes, animal models used in pre-clinical research show significant heterogeneity in their regenerative and degenerative responses associated with their species, age, genetic/epigenetic traits, and context of cartilage injury or disease. These can lead to variable outcomes when testing and validating novel therapeutic approaches for OA. Furthermore, it remains unclear whether protection against OA among different model systems is driven by inhibition of cartilage degeneration, enhancement of cartilage regeneration, or any combination thereof.

MAIN TEXT

Understanding the mechanistic basis underlying this context-dependent duality is essential for the rational design of targeted cartilage repair and OA therapies. Here, we discuss some of the critical variables related to the cross-species paradigm of degenerative and regenerative abilities found in pre-clinical animal models, to highlight that a gradient of regenerative competence within cartilage may exist across species and even in the greater human population, and likely influences clinical outcomes.

CONCLUSIONS

A more complete understanding of the endogenous regenerative potential of cartilage in a species specific context may facilitate the development of effective therapeutic approaches for cartilage injury and/or OA.

Impact of Pb on Chlamydomonas reinhardtii at Physiological and Transcriptional Levels

Trace elements stress is one of the most damaging abiotic stresses in environment. Nevertheless, the defense mechanism in microalgae remains poorly understood. In this study, physiological and molecular methods were performed to analyze the defense responses in green alga Chlamydomonas reinhardtii. It was speculated that the defense responses might mainly be due to the regulation of hormone signaling, indicating its potential role in alleviating the Pb toxicity besides other physiological and molecular defense responses like decrease in growth rate, chlorophyll content and photosynthesis efficiency, intensification of antioxidative mechanisms, regulation of transcription factors, trace elements chelation, and sequestration into vacuole via trace elements transporters. The sole differentially expressed ATP-binding cassette (ABC) transporters indicated that ABC transporters might play a very important role in the transport and relocation of Pb in C. reinhardtii. Additionally, our data provide the required knowledge for future investigations regarding Pb toxicity and defense mechanisms in algae, and detection of trace elements pollution in environment.

Transcriptomic analysis of bone and fibrous tissue morphogenesis during digit tip regeneration in the adult mouse

Humans have limited regenerative potential of musculoskeletal tissues following limb or digit loss. The murine digit has been used to study mammalian regeneration, where stem/progenitor cells (the "blastema") completely regenerate the digit tip after distal, but not proximal, amputation. However, the molecular mechanisms responsible for this response remain to be determined. Here, we evaluated the spatiotemporal formation of bone and fibrous tissues after level-dependent amputation of the murine terminal phalanx and quantified the transcriptome of the repair tissue. Distal (regenerative) and proximal (non-regenerative) amputations showed significant differences in temporal gene expression and tissue regrowth over time. Genes that direct skeletal system development and limb morphogenesis are transiently upregulated during blastema formation and differentiation, including distal Hox genes. Overall, our results suggest that digit tip regeneration is controlled by a gene regulatory network that recapitulates aspects of limb development, and that failure to activate this developmental program results in fibrotic wound healing.

Duck Tembusu Virus Utilizes miR-221-3p Expression to Facilitate Viral Replication via Targeting of Suppressor of Cytokine Signaling 5

Duck Tembusu virus (DTMUV), a member of Flaviviridae family, causes acute egg-drop syndrome in ducks. MicroRNAs (miRNAs) have been found to be involved in various biological processes, including tumor genesis, viral infection, and immune response. However, the functional effect of miRNAs on DTMUV replication remains largely unclear. This study aimed to elucidate the role of host microRNA-221-3p (miR-221-3p) in regulating DTMUV replication. Here, we indicated that the expression of miR-221-3p was significantly upregulated in duck embryo fibroblasts (DEFs) during DTMUV infection. Transfection of miR-221-3p mimic significantly reduced interferon (IFN) β production, whereas transfection of miR-221-3p inhibitor conversely significantly increased the expression of IFN-β in DTMUV-infected DEF. Moreover, we found that viral RNA copies, viral E protein expression level, and virus titer, which represent the replication and proliferation of virus, were all enhanced when transfecting the miR-221-3p mimic into DEF; reverse results were also observed by transfecting the miR-221-3p inhibitor. We also found that the expression of suppressor of cytokine signaling 5 (SOCS5) was downregulated in DEF infected with DTMUV. Besides, we further proved that SOCS5 is a target of miR-221-3p and that miR-221-3p could negatively modulate SOCS5 expression at both mRNA and protein levels. Finally, our results showed that overexpression of SOCS5 inhibited DTMUV replication and knockdown of SOCS5 enhanced DTMUV replication. Thus, our findings reveal a novel host evasion mechanism adopted by DTMUV via miR-221-3p, which may hew out novel strategies for designing miRNA-based vaccines and therapies.

High bone microarchitecture, strength, and resistance to bone loss in MRL/MpJ mice correlates with activation of different signaling pathways and systemic factors

The MRL/MpJ mice have demonstrated an enhanced tissue regeneration capacity for various tissues. In the present study, we systematically characterized bone microarchitecture and found that MRL/MpJ mice exhibit higher bone microarchitecture and strength compared to both C57BL/10J and C57BL/6J WT mice at 2, 4, and 10 months of age. The higher bone mass in MRL/MpJ mice was correlated to increased osteoblasts, decreased osteoclasts, higher cell proliferation, and bone formation, and enhanced pSMAD5 signaling earlier during postnatal development (2-month old) in the spine trabecular bone, and lower bone resorption rate at later age. Furthermore, these mice exhibit accelerated fracture healing via enhanced pSMAD5, pAKT and p-P38MAPK pathways compared to control groups. Moreover, MRL/MpJ mice demonstrated resistance to ovariectomy-induced bone loss as evidenced by maintaining higher bone volume/tissue volume (BV/TV) and lower percentage of bone loss later after ovariectomy. The consistently higher serum IGF1 level and lower RANKL level in MRL/MpJ mice may contribute to the maintenance of high bone mass in uninjured and injured bone. In conclusion, our results indicate that enhanced pSMAD5, pAKT, and p-P38MAPK signaling, higher serum IGF-1, and lower RANKL level contribute to the higher bone microarchitecture and strength, accelerated healing, and resistance to osteoporosis in MRL/MpJ mice.

Epigenetic inhibitor zebularine activates ear pinna wound closure in the mouse

BACKGROUND

Most studies on regenerative medicine focus on cell-based therapies and transplantations. Small-molecule therapeutics, though proved effective in different medical conditions, have not been extensively investigated in regenerative research. It is known that healing potential decreases with development and developmental changes are driven by epigenetic mechanisms, which suggests epigenetic repression of regenerative capacity.

METHODS

We applied zebularine, a nucleoside inhibitor of DNA methyltransferases, to stimulate the regenerative response in a model of ear pinna injury in mice.

FINDINGS

We observed the regeneration of complex tissue that was manifested as improved ear hole repair in mice that received intraperitoneal injections of zebularine. Six weeks after injury, the mean hole area decreased by 83.2 ± 9.4% in zebularine-treated and by 43.6 ± 15.4% in control mice (p < 10^-30). Combined delivery of zebularine and retinoic acid potentiated and accelerated this effect, resulting in complete ear hole closure within three weeks after injury. We found a decrease in DNA methylation and transcriptional activation of neurodevelopmental and pluripotency genes in the regenerating tissues.

INTERPRETATION

This study is the first to demonstrate an effective induction of complex tissue regeneration in adult mammals using zebularine. We showed that the synergistic action of an epigenetic drug (zebularine) and a transcriptional activator (retinoic acid) could be effectively utilized to induce the regenerative response, thus delineating a novel pharmacological strategy for regeneration. The strategy was effective in the model of ear pinna regeneration in mice, but zebularine acts on different cell types, therefore, a similar approach can be tested in other tissues and organs.

Characterization of articular cartilage homeostasis and the mechanism of superior cartilage regeneration of MRL/MpJ mice

This study investigated articular cartilage (AC) homeostasis and different signaling pathways involved in the superior cartilage regeneration of Murphy Roths large (MRL/MpJ) mice previously reported. We collected uninjured and destabilized medial meniscus (DMM)-injured knees from 8-wk-old C57BL/6J and MRL/MpJ mice. We used micro-computed tomography (microCT), histology, and immunohistochemistry to evaluate AC homeostasis and repair. We used the ear punch model to investigate the role of angiogenesis and inflammation in the superior healing of MRL/MpJ mice. We found fewer β-catenin and more pSMAD5 positive cells in the uninjured AC of MRL/MpJ mice than that from C57BL/6J mice. MRL/MpJ mice exhibited better AC repair in DMM-induced OA, as indicated by microCT results, Alcian blue, and Safranin O staining. Mechanistically, fewer β-catenin, pSMAD2-, pSMAD3-, a disintegrin and metalloproteinase with thrombospondin motifs 4-, matrix metalloproteinase (MMP) 9-, and MMP13-positive cells and more proliferating cell nuclear antigen- and pSMAD5-positive cells were found in the DMM-injured AC in MRL/MpJ mice than in normal mice. The accelerated ear wound healing of MRL/MpJ mice correlated with enhanced angiogenesis and macrophage polarization toward the M2a phenotype through elevated IL-10 and IL-4 expressing cells. Collectively, our study revealed that down-regulation of pSMAD2/3, β-catenin, and MMPs and up-regulation of pSMAD5 and M2a macrophage polarization contribute to the enhanced cartilage repair observed in MRL/MpJ mice.-Deng, Z., Gao, X., Sun, X., Amra, S., Lu, A., Cui, Y., Eltzschig, H. K., Lei, G., Huard, J. Characterization of articular cartilage homeostasis and the mechanism of superior cartilage regeneration of MRL/MpJ mice.

Markers of Accelerated Skeletal Muscle Regenerative Response in Murphy Roths Large Mice: Characteristics of Muscle Progenitor Cells and Circulating Factors

The "super-healing" Murphy Roths Large (MRL/MpJ) mouse possesses a superior regenerative capacity for repair of many tissues, which makes it an excellent animal model for studying molecular and cellular mechanisms during tissue regeneration. As the role of muscle progenitor cells (MPCs) in muscle-healing capacity of MRL/MpJ mice has not been previously studied, we investigated the muscle regenerative capacity of MRL/MpJ mice following muscle injury, and the results were compared to results from C57BL/6J (B6) age-matched control mice. Our results show that muscle healing upon cardiotoxin injury was accelerated in MRL/MpJ mice and characterized by reduced necrotic muscle area, reduced macrophage infiltration, and more regenerated myofibers (embryonic myosin heavy chain+/centronucleated fibers) at 3, 5, and 12 days postinjury, when compared to B6 age-matched control mice. These observations were associated with enhanced function of MPCs, including improved cell proliferation, differentiation, and resistance to stress, as well as increased muscle regenerative potential when compared to B6 MPCs. Mass spectrometry of serum proteins revealed higher levels of circulating antioxidants in MRL/MpJ mice when compared to B6 mice. Indeed, we found relatively higher gene expression of superoxide dismutase 1 (Sod1) and catalase (Cat) in MRL/MpJ MPCs. Depletion of Sod1 or Cat by small interfering RNA impaired myogenic potential of MRL/MpJ MPCs, indicating a role for these antioxidants in muscle repair. Taken together, these findings provide evidence that improved function of MPCs and higher levels of circulating antioxidants play important roles in accelerating muscle-healing capacity of MRL/MpJ mice. Stem Cells 2019;37:357-367.

Acupuncture Resolves Persistent Pain and Neuroinflammation in a Mouse Model of Chronic Overlapping Pain Conditions

Patients with chronic overlapping pain conditions have decreased levels of catechol-O-methyltransferase (COMT), an enzyme that metabolizes catecholamines. Consistent with clinical syndromes, we previously demonstrated that COMT inhibition in rodents produces persistent pain and heightened immune responses. Here, we sought to determine the efficacy of manual acupuncture in resolving persistent pain and neuroinflammation in the classic inbred C57BL/6 strain and the rapid-wound healing MRL/MpJ strain. Mice received subcutaneous osmotic minipumps to deliver the COMT inhibitor OR486 or vehicle for 13 days. On day 7 after pump implantation, acupuncture was performed at the Zusanli (ST36) point or a non-acupoint for 6 consecutive days. Behavioral responses to mechanical stimuli were measured throughout the experiment. Immunohistochemical analysis of spinal phosphorylated p38 mitogen-activated protein kinase, a marker of inflammation, and glial fibrillary acidic protein, a marker of astrogliosis, was performed on day 13. Results demonstrated that ST36, but not sham, acupuncture resolved mechanical hypersensitivity and reduced OR486-dependent increases in phosphorylated p38 and glial fibrillary acidic protein in both strains. The magnitude of the analgesic response was greater in MRL/MpJ mice. These findings indicate acupuncture as an effective treatment for persistent pain linked to abnormalities in catecholamine signaling and, furthermore, that analgesic efficacy may be influenced by genetic differences. PERSPECTIVE: Chronic overlapping pain conditions remain ineffectively managed by conventional pharmacotherapies. Here, we demonstrate that acupuncture alleviates persistent pain and neuroinflammation linked to heightened catecholaminergic tone. Mice with superior healing capacity exhibit greater analgesic efficacy. Findings indicate acupuncture as an effective treatment for chronic overlapping pain conditions and provide insight into treatment response variability.

New insight into functional limb regeneration: A to Z approaches

Limb/digit amputation is a common event in humans caused by trauma, medical illness, or surgery. Although the loss of a digit is not lethal, it affects quality of life and imposes high costs on amputees. In recent years, the increasing interest in limb regeneration has led to enhanced scientific knowledge. However, the limited ability to develop functional limb regeneration in the clinical setting suggests that a challenging issue remains in limb regeneration. Recently, the emergence of regenerative engineering is a promising field to address this challenge and close the gap between science and clinical applications. Cell signalling and molecular mechanisms involved in the limb regeneration process have been extensively studied; however, there is still insufficient data on cell therapy and tissue engineering for limb regeneration. In this review, we intend to focus on therapeutic approaches for limb regeneration that are closely related to gene, immune, and stem cell therapies, as well as tissue engineering approaches that take into consideration the peculiar developmental properties of the limbs. In addition, we attempt to identify the challenges of these strategies for limb regeneration studies in terms of clinical settings and as a road map to accomplish the goal of functional human limb regeneration.

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).

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.

Regeneration and Regrowth Potentials of Digit Tips in Amphibians and Mammals

Tissue regeneration and repair have received much attention in the medical field over the years. The study of amphibians, such as newts and salamanders, has uncovered many of the processes that occur in these animals during full-limb/digit regeneration, a process that is highly limited in mammals. Understanding these processes in amphibians could shed light on how to develop and improve this process in mammals. Amputation injuries in mammals usually result in the formation of scar tissue with limited regrowth of the limb/digit; however, it has been observed that the very tips of digits (fingers and toes) can partially regrow in humans and mice under certain conditions. This review will summarize and compare the processes involved in salamander limb regeneration, mammalian wound healing, and digit regeneration in mice and humans.

A concise review of common animal models for the study of limb regeneration

Correct selection of an appropriate animal mode to closely mimic human extremity diseases or to exhibit desirable phenotypes of limb regeneration is the first critical step for all scientists in biomedical and regenerative researches. The commonly-used animals in limb regeneration and repairing studies, such as axolotl, mice, and rats, are discussed in the review and other models including cockroaches, dogs, and horses are also mentioned. The review weighs the general advantages, disadvantages, and precedent uses of each model in the context of limb and peripheral injury and subsequent regeneration. We hope that this review can provide the reader an overview of each model, from which to select one for their specific purpose.

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.

Analogous cellular contribution and healing mechanisms following digit amputation and phalangeal fracture in mice

Regeneration of amputated structures is severely limited in humans and mice, with complete regeneration restricted to the distal portion of the terminal phalanx (P3). Here, we investigate the dynamic tissue repair response of the second phalangeal element (P2) post amputation in the adult mouse, and show that the repair response of the amputated bone is similar to the proximal P2 bone fragment in fracture healing. The regeneration‐incompetent P2 amputation response is characterized by periosteal endochondral ossification resulting in the deposition of new trabecular bone, corresponding to a significant increase in bone volume; however, this response is not associated with bone lengthening. We show that cells of the periosteum respond to amputation and fracture by contributing both chondrocytes and osteoblasts to the endochondral ossification response. Based on our studies, we suggest that the amputation response represents an attempt at regeneration that ultimately fails due to the lack of a distal organizing influence that is present in fracture healing.

A cellular, molecular, and pharmacological basis for appendage regeneration in mice

Regenerative medicine aims to restore normal tissue architecture and function. However, the basis of tissue regeneration in mammalian solid organs remains undefined. Remarkably, mice lacking p21 fully regenerate injured ears without discernable scarring. Here we show that, in wild-type mice following tissue injury, stromal-derived factor-1 (Sdf1) is up-regulated in the wound epidermis and recruits Cxcr4-expressing leukocytes to the injury site. In p21-deficient mice, Sdf1 up-regulation and the subsequent recruitment of Cxcr4-expressing leukocytes are significantly diminished, thereby permitting scarless appendage regeneration. Lineage tracing demonstrates that this regeneration derives from fate-restricted progenitor cells. Pharmacological or genetic disruption of Sdf1-Cxcr4 signaling enhances tissue repair, including full reconstitution of tissue architecture and all cell types. Our findings identify signaling and cellular mechanisms underlying appendage regeneration in mice and suggest new therapeutic approaches for regenerative medicine.

Transcriptome profiling reveals distinctive traits of retinol metabolism and neonatal parallels in the MRL/MpJ mouse

Background

The MRL/MpJ mouse is a laboratory inbred strain known for regenerative abilities which are manifested by scarless closure of ear pinna punch holes. Enhanced healing responses have been reported in other organs. A remarkable feature of the strain is that the adult MRL/MpJ mouse retains several embryonic biochemical characteristics, including increased expression of stem cell markers.

Results

We explored the transcriptome of the MRL/MpJ mouse in the heart, liver, spleen, bone marrow and ears. We used two reference strains, thus increasing the chances to discover the genes responsible for the exceptional properties of the regenerative strain. We revealed several distinctive characteristics of gene expression patterns in the MRL/MpJ mouse, including the repression of immune response genes, the up-regulation of those associated with retinol metabolism and PPAR signalling, as well as differences in expression of the genes engaged in wounding response. Another crucial finding is that the gene expression patterns in the adult MRL/MpJ mouse and murine neonates share a number of parallels, which are also related to immune and wounding response, PPAR pathway, and retinol metabolism.

Conclusions

Our results indicate the significance of retinol signalling and neonatal transcriptomic relics as the distinguishing features of the MRL/MpJ mouse. The possibility that retinoids could act as key regulatory molecules in this regeneration model brings important implications for regenerative medicine.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2075-2) contains supplementary material, which is available to authorized users.

Hyperbaric Oxygen Promotes Proximal Bone Regeneration and Organized Collagen Composition during Digit Regeneration

Oxygen is critical for optimal bone regeneration. While axolotls and salamanders have retained the ability to regenerate whole limbs, mammalian regeneration is restricted to the distal tip of the digit (P3) in mice, primates, and humans. Our previous study revealed the oxygen microenvironment during regeneration is dynamic and temporally influential in building and degrading bone. Given that regeneration is dependent on a dynamic and changing oxygen environment, a better understanding of the effects of oxygen during wounding, scarring, and regeneration, and better ways to artificially generate both hypoxic and oxygen replete microenvironments are essential to promote regeneration beyond wounding or scarring. To explore the influence of increased oxygen on digit regeneration in vivo daily treatments of hyperbaric oxygen were administered to mice during all phases of the entire regenerative process. Micro-Computed Tomography (μCT) and histological analysis showed that the daily application of hyperbaric oxygen elicited the same enhanced bone degradation response as two individual pulses of oxygen applied during the blastema phase. We expand past these findings to show histologically that the continuous application of hyperbaric oxygen during digit regeneration results in delayed blastema formation at a much more proximal location after amputation, and the deposition of better organized collagen fibers during bone formation. The application of sustained hyperbaric oxygen also delays wound closure and enhances bone degradation after digit amputation. Thus, hyperbaric oxygen shows the potential for positive influential control on the various phases of an epimorphic regenerative response.

Collection of Wound Exudate From Human Digit Tip Amputations Does Not Impair Regenerative Healing: A Randomized Trial

The regrowth of amputated digit tips represents a unique regenerative healing in mammals with subcutaneous volume regrowth, restoration of dactylogram, and suppression of scar formation. Although factor analysis in amphibians and even in mice is easy to obtain, safety of harvesting biomaterial from human digit tip amputations for analysis has not yet been described.The aim of this study was to evaluate if recovering wound exudate does hamper clinical outcome or influence microbiologic or inflammation status.A predefined cohort of 18 patients with fresh digit tip amputations was randomly assigned to receive standard therapy (debridement, occlusive dressing) with (n = 9) or without (n = 9) collection of the whole wound exudate in every dressing change. Primary endpoint (lengthening) and secondary endpoints (regeneration of dactylogram, nail bed and bone healing, time to complete wound closure, scar formation, 2-point discrimination, microbiologic analysis, inflammatory factors interleukin (IL)-1α, tumor necrosis factor-α, IL-4, and IL-6) were determined by an independent, blinded observer.Patients' characteristics showed no significant differences between the groups. All patients completed the study to the end of 3 months follow-up. Exudate collection did not influence primary and secondary endpoints. Furthermore, positive microbiologic findings as well as pus- and necrosis-like appearance neither impaired tissue restoration nor influenced inflammatory factor release.Here, the authors developed an easy and safe protocol for harvesting wound exudate from human digit tip amputations. For the first time, it was shown that harvesting does not impair regenerative healing. Using this method, further studies can be conducted to analyze regeneration associated factors in the human digit tip.DRKS.de Identifier: DRKS00006882 (UTN: U1111-1166-5723).

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.

The strategy and method in modulating finger regeneration

The tip of the human finger can regenerate if the amputation is distal to the nail bed, usually in young children. Studies in regeneration of rodent digits have shown that regeneration occurs if the amputation is distal to the mid-third phalanx for certain ages. The digit contains many different components, such as muscle, tendon, bone, skin, nerves and blood vessels, which must all be regrown in the proper location in order to restore functionality. The mechanism behind the complex healing/regeneration processes is still under investigation; however, improvements in injured finger regeneration have been gradually developing in animal models over the past few years. This review discusses a few strategies and methods to possibly enhance digit regeneration beyond current natural limits, focusing on aspects including scarless wound healing, cell-based treatments, tissue engineering and electrical stimulation.

Mapping Novel Subcutaneous Angiogenesis Quantitative Trait Loci in [B6×MRL]F2 Mice

Objective: MRL/MpJ mice are known for enhanced healing, but mechanistic details or how specific aspects of wounding (e.g., angiogenesis) contribute to healing are unknown. While previous studies investigated the systemic effects of immunity in MRL/MpJ healing, few have focused on tissue-intrinsic effects. Approach:Ex vivo skin biopsies from MRL/MpJ and C57BL/6J mice were cultured in ex vivo conditions that favor endothelial cell growth to compare their angiogenic potential. We localized enhanced angiogenesis quantitative trait loci (QTL) in an F2 intercross. We then performed an expression analysis in cultured skin biopsies from MRL/MpJ and C57BL/6J mice to determine the pathways that are associated with the capacity for differential growth. Results: MRL/MpJ biopsies have a two- to threefold greater growth potential than C57BL/6J mice, supporting the hypothesis that angiogenesis may contribute to enhanced healing in MRL/MpJ skin. We mapped two QTLs that are unique from previously mapped MRL/MpJ wound healing QTLs and detected interactions between wound healing QTLs and loci in this cross. Additionally, we found that pathways previously implicated in MRL/MpJ healing are also enriched in skin biopsies. Innovation: We have developed a novel approach to determine how specific aspects of tissue development contribute to wound healing that will ultimately lead to the discovery of unidentified genes that contribute to enhanced healing. Conclusion: We have shown that, consistent with previous studies following wound closure in MRL/MpJ mice, vessel growth during healing is also influenced by genetic background. Our ongoing work will identify the genetic factors that should be useful biomarkers or as therapeutic targets for enhanced wound healing.

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.

Lin28 enhances tissue repair by reprogramming cellular metabolism

Regeneration capacity declines with age, but why juvenile organisms show enhanced tissue repair remains unexplained. Lin28a, a highly conserved RNA-binding protein expressed during embryogenesis, plays roles in development, pluripotency, and metabolism. To determine whether Lin28a might influence tissue repair in adults, we engineered the reactivation of Lin28a expression in several models of tissue injury. Lin28a reactivation improved hair regrowth by promoting anagen in hair follicles and accelerated regrowth of cartilage, bone, and mesenchyme after ear and digit injuries. Lin28a inhibits let-7 microRNA biogenesis; however, let-7 repression was necessary but insufficient to enhance repair. Lin28a bound to and enhanced the translation of mRNAs for several metabolic enzymes, thereby increasing glycolysis and oxidative phosphorylation (OxPhos). Lin28a-mediated enhancement of tissue repair was negated by OxPhos inhibition, whereas a pharmacologically induced increase in OxPhos enhanced repair. Thus, Lin28a enhances tissue repair in some adult tissues by reprogramming cellular bioenergetics. PAPERCLIP:

The role of mouse strain differences in the susceptibility to fibrosis: a systematic review

In humans, a number of genetic factors have been linked to the development of fibrosis in a variety of different organs. Seeking a wider understanding of this observation in man is ethically important. There is mounting evidence suggesting that inbred mouse strains with different genetic backgrounds demonstrate variable susceptibility to a fibrotic injury. We performed a systematic review of the literature describing strain and organ specific response to injury in order to determine whether genetic susceptibility plays a role in fibrogenesis. Data were collected from studies that were deemed eligible for analysis based on set inclusion criteria, and findings were assessed in relation to strain of mouse, type of injury and organ of investigation. A total of 44 studies were included covering 21 mouse strains and focusing on fibrosis in the lung, liver, kidney, intestine and heart. There is evidence that mouse strain differences influence susceptibility to fibrosis and this appears to be organ specific. For instance, C57BL/6J mice are resistant to hepatic, renal and cardiac fibrosis but susceptible to pulmonary and intestinal fibrosis. However, BALB/c mice are resistant to pulmonary fibrosis but susceptible to hepatic fibrosis. Few studies have assessed the effect of the same injury stimulus in different organ systems using the same strains of mouse. Such mouse strain studies may prove useful in elucidating the genetic as well as epigenetic factors in humans that could help determine why some people are more susceptible to the development of certain organ specific fibrosis than others.

Epigenetic basis of regeneration: analysis of genomic DNA methylation profiles in the MRL/MpJ mouse

Epigenetic regulation plays essential role in cell differentiation and dedifferentiation, which are the intrinsic processes involved in regeneration. To investigate the epigenetic basis of regeneration capacity, we choose DNA methylation as one of the most important epigenetic mechanisms and the MRL/MpJ mouse as a model of mammalian regeneration known to exhibit enhanced regeneration response in different organs. We report the comparative analysis of genomic DNA methylation profiles of the MRL/MpJ and the control C57BL/6J mouse. Methylated DNA immunoprecipitation followed by microarray analysis using the Nimblegen '3 × 720 K CpG Island Plus RefSeq Promoter' platform was applied in order to carry out genome-wide DNA methylation profiling covering 20 404 promoter regions. We identified hundreds of hypo- and hypermethylated genes and CpG islands in the heart, liver, and spleen, and 37 of them in the three tissues. Decreased inter-tissue diversification and the shift of DNA methylation balance upstream the genes distinguish the genomic methylation patterns of the MRL/MpJ mouse from the C57BL/6J. Homeobox genes and a number of other genes involved in embryonic morphogenesis are significantly overrepresented among the genes hypomethylated in the MRL/MpJ mouse. These findings indicate that epigenetic patterning might be a likely molecular basis of regeneration capability in the MRL/MpJ mouse.

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.

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.

Cytoskeletal regulation of dermal regeneration

Wound healing results in the repair of injured tissues however fibrosis and scar formation are, more often than not the unfortunate consequence of this process. The ability of lower order vertebrates and invertebrates to regenerate limbs and tissues has been all but lost in mammals; however, there are some instances where glimpses of mammalian regenerative capacity do exist. Here we describe the unlocked potential that exists in mammals that may help us understand the process of regeneration post-injury and highlight the potential role of the actin cytoskeleton in this process. The precise function and regulation of the cytoskeleton is critical to the success of the healing process and its manipulation may therefore facilitate regenerative healing. The gelsolin family of actin remodelling proteins in particular has been shown to have important functions in wound healing and family member Flightless I (Flii) is involved in both regeneration and repair. Understanding the interactions between different cytoskeletal proteins and their dynamic control of processes including cellular adhesion, contraction and motility may assist the development of therapeutics that will stimulate regeneration rather than repair.

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.

A strategy for promoting bone regeneration by inducible expression of 15-LOX-1

Repairing large bone defects is a major orthopaedic problem, with current treatments being constrained by the regenerative capacity of human tissue. Current methods for repairing bone defects include osteogenesis, osteoconduction, osteoinduction, and tissue engineering; however, the cumulative effect of these methods is, as of yet, rather unsatisfactory. Recent research has demonstrated that knockout of the cell cycle inhibitor p21, which works as a switch to control regenerative capacity, can promote cell proliferation and appendage regeneration. The enzyme 15-lipoxygenase type 1 (15-LOX-1), which is involved in arachidonic and linoleic acid metabolism, has the potential to down-regulate the expression of p21. Therefore, we hypothesise that the construction of a new bone substitute that expresses 15-LOX-1 locally will promote osteoblast proliferation through inhibition of p21, resulting in an effective and inducible method for promoting bone regeneration.

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.

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.

Enhanced functional recovery in MRL/MpJ mice after spinal cord dorsal hemisection

Adult MRL/MpJ mice have been shown to possess unique regeneration capabilities. They are able to heal an ear-punched hole or an injured heart with normal tissue architecture and without scar formation. Here we present functional and histological evidence for enhanced recovery following spinal cord injury (SCI) in MRL/MpJ mice. A control group (C57BL/6 mice) and MRL/MpJ mice underwent a dorsal hemisection at T9 (thoracic vertebra 9). Our data show that MRL/MpJ mice recovered motor function significantly faster and more completely. We observed enhanced regeneration of the corticospinal tract (CST). Furthermore, we observed a reduced astrocytic response and fewer micro-cavities at the injury site, which appear to create a more growth-permissive environment for the injured axons. Our data suggest that the reduced astrocytic response is in part due to a lower lesion-induced increase of cell proliferation post-SCI, and a reduced astrocytic differentiation of the proliferating cells. Interestingly, we also found an increased number of proliferating microglia, which could be involved in the MRL/MpJ spinal cord repair mechanisms. Finally, to evaluate the molecular basis of faster spinal cord repair, we examined the difference in gene expression changes in MRL/MpJ and C57BL/6 mice after SCI. Our microarray data support our histological findings and reveal a transcriptional profile associated with a more efficient spinal cord repair in MRL/MpJ mice.

Enhanced retinal pigment epithelium regeneration after injury in MRL/MpJ mice

Regenerative medicine holds the promise of restoring cells and tissues that are destroyed in human disease, including degenerative eye disorders. However, development of this approach in the eye has been limited by a lack of animal models that show robust regeneration of ocular tissue. Here, we test whether MRL/MpJ mice, which exhibit enhanced wound healing, can efficiently regenerate the retinal pigment epithelium (RPE) after an injury that mimics the loss of this tissue in age-related macular degeneration. The RPE of MRL/MpJ and control AKR/J mice was injured by retro-orbital injection of sodium iodate at 20 mg/kg body weight, which titration studies indicated was optimal for highlighting strain differences in the response to injury. Five days after sodium iodate injection at this dose, electroretinography of both strains revealed equivalent retinal responses that were significantly reduced compared to untreated mice. At one and two months post-injection, retinal responses were restored in MRL/MpJ but not AKR/J mice. Bright field and fluorescence microscopy of eyecup cryosections indicated an initial central loss of RPE cells and RPE65 immunostaining in MRL/MpJ and AKR/J mice, with preservation of peripheral RPE. Phalloidin staining of posterior eye whole mounts confirmed this pattern of RPE loss, and revealed a transition region characterized by RPE cell shedding and restructuring in both strains, suggesting a similar initial response to injury. At one month post-injection, central RPE cells, RPE65 immunostaining and phalloidin staining were restored in MRL/MpJ but not AKR/J mice. BrdU incorporation was observed throughout the RPE of MRL/MpJ but not AKR/J mice after one month of administration following sodium iodate treatment, consistent with RPE proliferation. These findings provide evidence for a dramatic regeneration of the RPE after injury in MRL/MpJ mice that supports full recovery of retinal function, which has not been observed previously in mammalian eyes. This model should prove useful for understanding molecular mechanisms that underlie regeneration, and for identifying factors that promote RPE regeneration in age-related macular degeneration and related diseases.

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.

Limitations of the MRL mouse as a model for cardiac regeneration

OBJECTIVE

Myocardial repair following injury in mammals is restricted such that damaged areas are replaced by scar tissue, impairing cardiac function. MRL mice exhibit exceptional regenerative healing in an ear punch wound model. Some myocardial repair with restoration of heart function has also been reported following cryoinjury. Increased cardiomyocyte proliferation and a foetal liver stem cell population were implicated. We investigated molecular mechanisms facilitating myocardial repair in MRL mice to identify potential therapeutic targets in non-regenerative species.

METHODS

Expressions of specific cell-cycle regulators that might account for regeneration (CDKs 1, 2, 4 and 6; cyclins A, E, D1 and B1; p21, p27 and E2F5) were compared by immunoblotting in MRL and control C57BL/6 ventricles during development. Flow cytometry was used to investigate stem cell populations in livers from foetal mice, and infarct sizes were compared in coronary artery-ligated and sham-treated MRL and C57BL/6 adult mice.

KEY FINDINGS

No differences in the expressions of cell cycle regulators were observed between the two strains. Expressions of CD34+Sca1+ckit-, CD34+Sca1+ckit+ and CD34+Sca1-ckit+ increased in livers from C57BL/6 vs MRL mice. No differences were observed in infarct sizes, levels of fibrosis, Ki67 staining or cardiac function between MRL and C57BL/6 mice.

CONCLUSIONS

No intrinsic differences were observed in cell cycle control molecules or stem cell populations between MRL and control C57BL mouse hearts. Pathophysiologically relevant ischaemic injury is not repaired more efficiently in MRL myocardium, questioning the use of the MRL mouse as a reliable model for cardiac regeneration in response to pathophysiologically relevant forms of injury.