The super super-healing MRL mouse strain

Influencing factors and repair advancements in rodent models of peripheral nerve regeneration

Peripheral nerve injuries lead to severe functional impairments, with rodent models essential for studying regeneration. This review examines key factors affecting outcomes. Age-related declines, like reduced nerve fiber density and impaired axonal transport of vesicles, hinder recovery. Hormonal differences influence regeneration, with BDNF/trkB critical for testosterone and nerve growth factor for estrogen signaling pathways. Species and strain selection impact outcomes, with C57BL/6 mice and Sprague-Dawley rats exhibiting varying regenerative capacities. Injury models - crush for early regeneration, chronic constriction for neuropathic pain, stretch for traumatic elongation and transection for severe lacerations - provide insights into clinically relevant scenarios. Repair techniques, such as nerve grafts and conduits, show that autografts are the gold standard for gaps over 3 cm, with success influenced by graft type and diameter. Time course analysis highlights crucial early degeneration and regeneration phases within the first month, with functional recovery stabilizing by three to six months. Early intervention optimizes regeneration by reducing scar tissue formation, while later interventions focus on remyelination. Understanding these factors is vital for designing robust preclinical studies and translating research into effective clinical treatments for peripheral nerve injuries.

Spiny mice are primed but fail to regenerate volumetric skeletal muscle loss injuries

BACKGROUND

In recent years, the African spiny mouse Acomys cahirinus has been shown to regenerate a remarkable array of severe internal and external injuries in the absence of a fibrotic response, including the ability to regenerate full-thickness skin excisions, ear punches, severe kidney injuries, and complete transection of the spinal cord. While skeletal muscle is highly regenerative in adult mammals, Acomys displays superior muscle regeneration properties compared with standard laboratory mice following several injuries, including serial cardiotoxin injections of skeletal muscle and volumetric muscle loss (VML) of the panniculus carnosus muscle following full-thickness excision injuries. VML is an extreme muscle injury defined as the irrecoverable ablation of muscle mass, most commonly resulting from combat injuries or surgical debridement. Barriers to the treatment of VML injury include early and prolonged inflammatory responses that promote fibrotic repair and the loss of structural and mechanical cues that promote muscle regeneration. While the regeneration of the panniculus carnosus in Acomys is impressive, its direct relevance to the study of VML in patients is less clear as this muscle has largely been lost in humans, and, while striated, is not a true skeletal muscle. We therefore sought to test the ability of Acomys to regenerate a skeletal muscle more commonly used in VML injury models.

METHODS

We performed two different VML injuries of the Acomys tibialis anterior muscle and compared the regenerative response to a standard laboratory mouse strain, Mus C57BL6/J.

RESULTS

Neither Acomys nor Mus recovered lost muscle mass or myofiber number within three months following VML injury, and Acomys also failed to recover force production better than Mus. In contrast, Acomys continued to express eMHC within the injured area even three months following injury, whereas Mus ceased expressing eMHC less than one-month post-injury, suggesting that Acomys muscle was primed, but failed, to regenerate.

CONCLUSIONS

While the panniculus carnosus muscle in Acomys regenerates following VML injury in the context of full-thickness skin excision, this regenerative ability does not translate to regenerative repair of a skeletal muscle.

PLOD2, a key factor for MRL MSC metabolism and chondroprotective properties

BACKGROUND

Initially discovered for its ability to regenerate ear holes, the Murphy Roth Large (MRL) mouse has been the subject of multiple research studies aimed at evaluating its ability to regenerate other body tissues and at deciphering the mechanisms underlying it. These enhanced abilities to regenerate, retained during adulthood, protect the MRL mouse from degenerative diseases such as osteoarthritis (OA). Here, we hypothesized that mesenchymal stromal/stem cells (MSC) derived from the regenerative MRL mouse could be involved in their regenerative potential through the release of pro-regenerative mediators.

METHOD

To address this hypothesis, we compared the secretome of MRL and BL6 MSC and identified several candidate molecules expressed at significantly higher levels by MRL MSC than by BL6 MSC. We selected one candidate, Plod2, and performed functional in vitro assays to evaluate its role on MRL MSC properties including metabolic profile, migration, and chondroprotective effects. To assess its contribution to MRL protection against OA, we used an experimental model for osteoarthritis induced by collagenase (CiOA).

RESULTS

Among the candidate molecules highly expressed by MRL MSC, we focused our attention on procollagen-lysine,2-oxoglutarate 5-dioxygenase 2 (PLOD2). Plod2 silencing induced a decrease in the glycolytic function of MRL MSC, resulting in the alteration of their migratory and chondroprotective abilities in vitro. In vivo, we showed that Plod2 silencing in MRL MSC significantly impaired their capacity to protect mouse from developing OA.

CONCLUSION

Our results demonstrate that the chondroprotective and therapeutic properties of MRL MSC in the CiOA experimental model are in part mediated by PLOD2.

Intersection of Immunity, Metabolism, and Muscle Regeneration in an Autoimmune-Prone MRL Mouse Model

Due to the limited capacity of mammals to regenerate complex tissues, researchers have worked to understand the mechanisms of tissue regeneration in organisms that maintain that capacity. One example is the MRL/MpJ mouse strain with unique regenerative capacity in ear pinnae that is absent from other strains, such as the common C57BL/6 strain. The MRL/MpJ mouse has also been associated with an autoimmune phenotype even in the absence of the mutant Fas gene described in its parent strain MRL/lpr. Due to these findings, the differences between the responses of MRL/MpJ versus C57BL/6 strain are evaluated in volumetric muscle injury and subsequent material implantation. One salient feature of the MRL/MpJ response to injury is robust adipogenesis within the muscle. This is associated with a decrease in M2-like polarization in response to biologically derived extracellular matrix scaffolds. In pro-fibrotic materials, such as polyethylene, there are fewer foreign body giant cells in the MRL/MpJ mice. As there are reports of both positive and negative influences of adipose tissue and adipogenesis on wound healing, this model can provide an important lens to investigate the interplay between stem cells, adipose tissue, and immune responses in trauma and material implantation.

ECM-Focused Proteomic Analysis of Ear Punch Regeneration in Acomys Cahirinus

In mammals, significant injury is generally followed by the formation of a fibrotic scar which provides structural integrity but fails to functionally restore damaged tissue. Spiny mice of the genus Acomys represent the first example of full skin autotomy in mammals. Acomys cahirinus has evolved extremely weak skin as a strategy to avoid predation and is able to repeatedly regenerate healthy tissue without scar after severe skin injury or full-thickness ear punches. Extracellular matrix (ECM) composition is a critical regulator of wound repair and scar formation and previous studies have suggested that alterations in its expression may be responsible for the differences in regenerative capacity observed between Mus musculus and A. cahirinus , yet analysis of this critical tissue component has been limited in previous studies by its insolubility and resistance to extraction. Here, we utilize a 2-step ECM-optimized extraction to perform proteomic analysis of tissue composition during wound repair after full-thickness ear punches in A. cahirinus and M. musculus from weeks 1 to 4 post-injury. We observe changes in a wide range of ECM proteins which have been previously implicated in wound regeneration and scar formation, including collagens, coagulation and provisional matrix proteins, and matricryptic signaling peptides. We additionally report differences in crosslinking enzyme activity and ECM protein solubility between Mus and Acomys. Furthermore, we observed rapid and sustained increases in CD206, a marker of pro-regenerative M2 macrophages, in Acomys, whereas little or no increase in CD206 was detected in Mus. Together, these findings contribute to a comprehensive understanding of tissue cues which drive the regenerative capacity of Acomys and identify a number of potential targets for future pro-regenerative therapies.

Ectopic adipogenesis in response to injury and material implantation in an autoimmune mouse model

Due to the limited capacity of mammals to regenerate complex tissues, researchers have worked to understand the mechanisms of tissue regeneration in organisms that maintain that capacity. One example is the MRL/MpJ mouse strain with unique regenerative capacity in ear pinnae that is absent from other strains, such as the common C57BL/6 strain. The MRL/MpJ mouse has also been associated with an autoimmune phenotype even in the absence of the mutant Fas gene described in its parent strain MRL/lpr. Due to these findings, we evaluated the differences between the responses of MRL/MpJ versus C57BL/6 strain in traumatic muscle injury and subsequent material implantation. One salient feature of the MRL/MpJ response to injury was a robust adipogenesis within the muscle. This was associated with a decrease in M2-like polarization in response to biologically derived extracellular matrix scaffolds. In pro-fibrotic materials, such as polyethylene, there were fewer foreign body giant cells in the MRL/MpJ mice. As there are reports of both positive and negative influences of adipose tissue and adipogenesis on wound healing, this model could provide an important lens to investigate the interplay between stem cells, adipose tissue, and immune responses in trauma and materials implantation.

Cis-regulatory arbitrators of regeneration

Mammals favor healing with scaring over functional tissue regeneration.1 In this issue of Cell Stem Cell, Mack et al. use "super-healer" mice to identify cis-regulatory variations that direct regenerative versus fibrotic gene expression in wound fibroblasts and they uncover complement factor H as a molecular driver of skin regeneration.2.

Allele-specific expression reveals genetic drivers of tissue regeneration in mice

In adult mammals, skin wounds typically heal by scarring rather than through regeneration. In contrast, "super-healer" Murphy Roths Large (MRL) mice have the unusual ability to regenerate ear punch wounds; however, the molecular basis for this regeneration remains elusive. Here, in hybrid crosses between MRL and non-regenerating mice, we used allele-specific gene expression to identify cis-regulatory variation associated with ear regeneration. Analyzing three major cell populations (immune, fibroblast, and endothelial), we found that genes with cis-regulatory differences specifically in fibroblasts were associated with wound-healing pathways and also co-localized with quantitative trait loci for ear wound-healing. Ectopic treatment with one of these proteins, complement factor H (CFH), accelerated wound repair and induced regeneration in typically fibrotic wounds. Through single-cell RNA sequencing (RNA-seq), we observed that CFH treatment dramatically reduced immune cell recruitment to wounds, suggesting a potential mechanism for CFH's effect. Overall, our results provide insights into the molecular drivers of regeneration with potential clinical implications.

Earlier proteoglycan turnover promotes higher efficiency matrix remodeling in MRL/MpJ tendons

While most mammalian tissue regeneration is limited, the Murphy Roths Large (MRL/MpJ) mouse has been identified to regenerate several tissues, including tendon. Recent studies have indicated that this regenerative response is innate to the tendon tissue and not reliant on a systemic inflammatory response. Therefore, we hypothesized that MRL/MpJ mice may also exhibit a more robust homeostatic regulation of tendon structure in response to mechanical loading. To assess this, MRL/MpJ and C57BL/6J flexor digitorum longus tendon explants were subjected to stress-deprived conditions in vitro for up to 14 days. Explant tendon health (metabolism, biosynthesis, and composition), matrix metalloproteinase (MMP) activity, gene expression, and tendon biomechanics were assessed periodically. We found a more robust response to the loss of mechanical stimulus in the MRL/MpJ tendon explants, exhibiting an increase in collagen production and MMP activity consistent with previous in vivo studies. This greater collagen turnover was preceded by an early expression of small leucine-rich proteoglycans and proteoglycan-degrading MMP-3, promoting efficient regulation and organization of newly synthesized collagen and allowing for more efficient overall turnover in MRL/MpJ tendons. Therefore, mechanisms of MRL/MpJ matrix homeostasis may be fundamentally different from that of B6 tendons and may indicate better recovery from mechanical microdamage in MRL/MpJ tendons. We demonstrate here the utility of the MRL/MpJ model in elucidating mechanisms of efficient matrix turnover and its potential to shed light on new targets for more effective treatments for degenerative matrix changes brought about by injury, disease, or aging.

Matrisomal components involved in regenerative wound healing in axolotl and Acomys: implications for biomaterial development

Achieving regeneration in humans has been a long-standing goal of many researchers. Whereas amphibians like the axolotl (Ambystoma mexicanum) are capable of regenerating whole organs and even limbs, most mammals heal their wounds via fibrotic scarring. Recently, the African spiny mouse (Acomys sp.) has been shown to be injury resistant and capable of regenerating several tissue types. A major focal point of research with Acomys has been the identification of drivers of regeneration. In this search, the matrisome components related to the extracellular matrix (ECM) are often overlooked. In this review, we compare Acomys and axolotl skin wound healing and blastema-mediated regeneration by examining their wound healing responses and comparing the expression pattern of matrisome genes, including glycosaminoglycan (GAG) related genes. The goal of this review is to identify matrisome genes that are upregulated during regeneration and could be potential candidates for inclusion in pro-regenerative biomaterials. Research papers describing transcriptomic or proteomic coverage of either skin regeneration or blastema formation in Acomys and axolotl were selected. Matrisome and GAG related genes were extracted from each dataset and the resulting lists of genes were compared. In our analysis, we found several genes that were consistently upregulated, suggesting possible involvement in regenerative processes. Most of the components have been implicated in regulation of cell behavior, extracellular matrix remodeling and wound healing. Incorporation of such pro-regenerative factors into biomaterials may help to shift pro-fibrotic processes to regenerative responses in treated wounds.

African Spiny Mice (Acomys) Exhibit Mild Osteoarthritis Following Meniscal Injury

OBJECTIVE

Hyaline cartilage has limited innate healing abilities and hyaline cartilage loss is a hallmark of osteoarthritis (OA). Animal models can provide important insights into cartilage regeneration potential. One such animal model, the African spiny mouse (Acomys), is capable of regenerating skin, skeletal muscle, and elastic cartilage. This study aims to evaluate whether these regenerative abilities protect Acomys with meniscal injury from OA-related joint damage and behaviors indicative of joint pain and dysfunction.

DESIGN

Acomys received destabilization of the medial meniscus (DMM) surgery (n = 11) or a skin incision (n = 10). Gait testing occurred at 4, 6, 8, 10, and 12 weeks after surgery. At endpoint, joints were processed for histology to assess cartilage damage.

RESULTS

Following joint injury, Acomys with DMM surgery altered their walking patterns by increasing the percent stance time on the contralateral limb relative to the operated limb, thereby reducing the amount of time the injured limb must bear weight on its own throughout the gait cycle. Histological grading indicated evidence of OA-related joint damage in Acomys with DMM surgery; these changes were primarily driven by loss of structural integrity in the hyaline cartilage.

CONCLUSIONS

Acomys developed gait compensations, and the hyaline cartilage in Acomys is not fully protected from OA-related joint damage following meniscal injury, although this damage was less severe than that historically found in C57BL/6 mice with an identical injury. Thus, Acomys do not appear to be completely protected from OA-related changes, despite the ability to regenerate other wounded tissues.

MRL/MpJ Mice Resist to Age-Related and Long-Term Ovariectomy-Induced Bone Loss: Implications for Bone Regeneration and Repair

Osteoporosis and age-related bone loss increase bone fracture risk and impair bone healing. The need for identifying new factors to prevent or treat bone loss is critical. Previously, we reported that young MRL/MpJ mice have superior bone microarchitecture and biomechanical properties as compared to wild-type (WT) mice. In this study, MRL/MpJ mice were tested for resistance to age-related and long-term ovariectomy-induced bone loss to uncover potential beneficial factors for bone regeneration and repair. Bone tissues collected from 14-month-old MRL/MpJ and C57BL/6J (WT) mice were analyzed using micro-CT, histology, and immunohistochemistry, and serum protein markers were characterized using ELISAs or multiplex assays. Furthermore, 4-month-old MRL/MpJ and WT mice were subjected to ovariectomy (OV) or sham surgery and bone loss was monitored continuously using micro-CT at 1, 2, 4, and 6 months (M) after surgery with histology and immunohistochemistry performed at 6 M post-surgery. Sera were collected for biomarker detection using ELISA and multiplex assays at 6 M after surgery. Our results indicated that MRL/MpJ mice maintained better bone microarchitecture and higher bone mass than WT mice during aging and long-term ovariectomy. This resistance of bone loss observed in MRL/MpJ mice correlated with the maintenance of higher OSX⁺ osteoprogenitor cell pools, higher activation of the pSMAD5 signaling pathway, more PCNA⁺ cells, and a lower number of osteoclasts. Systemically, lower serum RANKL and DKK1 with higher serum IGF1 and OPG in MRL/MpJ mice relative to WT mice may also contribute to the maintenance of higher bone microarchitecture during aging and less severe bone loss after long-term ovariectomy. These findings may be used to develop therapeutic approaches to maintain bone mass and improve bone regeneration and repair due to injury, disease, and aging.

Evaluation of peripheral nerve regeneration in Murphy Roths Large mouse strain following transection injury

Aim: Murphy Roths Large (MRL/MpJ) mice have demonstrated the ability to heal with minimal or no scar formation in several tissue types. In order to identify a novel animal model, this study sought to evaluate whether this attribute applies to peripheral nerve regeneration. Materials & methods: This was a two-phase study. 6-week-old male mice were divided into two interventional groups: nerve repair and nerve graft. The MRL/MpJ was compared with the C57BL/6J strain for evaluation of both functional and histological outcomes. Results: MRL/MpJ strain demonstrated superior axon myelination and less scar formation, however functional outcomes did not show significant difference between strains. Conclusion: Superior histological outcomes did not translate into superior peripheral nerve regeneration in MRL/MpJ strain.

Prolyl-hydroxylase inhibitor-induced regeneration of alveolar bone and soft tissue in a mouse model of periodontitis through metabolic reprogramming

Bone injuries and fractures reliably heal through a process of regeneration with restoration to original structure and function when the gap between adjacent sides of a fracture site is small. However, when there is significant volumetric loss of bone, bone regeneration usually does not occur. In the present studies, we explore a particular case of volumetric bone loss in a mouse model of human periodontal disease (PD) in which alveolar bone surrounding teeth is permanently lost and not replaced. This model employs the placement a ligature around the upper second molar for 10 days leading to inflammation and bone breakdown and faithfully replicates the bacterially-induced inflammatory etiology of human PD to induce bone degeneration. After ligature removal, mice are treated with a timed-release formulation of a small molecule inhibitor of prolylhydroxylases (PHDi; 1,4-DPCA) previously shown to induce epimorphic regeneration of soft tissue in non-regenerating mice. This PHDi induces high expression of HIF-1α and is able to shift the metabolic state from OXPHOS to aerobic glycolysis, an energetic state used by stem cells and embryonic tissue. This regenerative response was completely blocked by siHIF1a. In these studies, we show that timed-release 1,4-DPCA rapidly and completely restores PD-affected bone and soft tissue with normal anatomic fidelity and with increased stem cell markers due to site-specific stem cell migration and/or de-differentiation of local tissue, periodontal ligament (PDL) cell proliferation, and increased vascularization. In-vitro studies using gingival tissue show that 1,4-DPCA indeed induces de-differentiation and the expression of stem cell markers but does not exclude the role of migrating stem cells. Evidence of metabolic reprogramming is seen by the expression of not only HIF-1a, its gene targets, and resultant de-differentiation markers, but also the metabolic genes Glut-1, Gapdh, Pdk1, Pgk1 and Ldh-a in jaw periodontal tissue.

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.

Wound healing, fibroblast heterogeneity, and fibrosis

Fibroblasts are highly dynamic cells that play a central role in tissue repair and fibrosis. However, the mechanisms by which they contribute to both physiologic and pathologic states of extracellular matrix deposition and remodeling are just starting to be understood. In this review article, we discuss the current state of knowledge in fibroblast biology and heterogeneity, with a primary focus on the role of fibroblasts in skin wound repair. We also consider emerging techniques in the field, which enable an increasingly nuanced and contextualized understanding of these complex systems, and evaluate limitations of existing methodologies and knowledge. Collectively, this review spotlights a diverse body of research examining an often-overlooked cell type-the fibroblast-and its critical functions in wound repair and beyond.

Modified foreign body reaction to silicone imbedded in subcutaneous tissues by different mouse systemic immune conditions

Foreign body reaction (FBR) causes unexpected adverse effects due to implanted materials in humans and animals. Inflammation and subsequent fibrosis during FBR seems to be affected by recipient immunity, such as the balance of T helper (Th) response that has the potential to regulate FBR-related macrophage function. Here, the immunological effects of FBR on subcutaneously imbedded silicone tubes (ST) at 8 weeks were investigated histologically by comparing Th1-biased C57BL/6N, Th2-biased MRL/MpJ, and autoimmune disease-prone MRL/MpJ-Fas^lpr/lpr . Tissue surrounding ST (TSS) was analyzed at day (D) 7 and 14 (reaction phase) or D35 (stability phase) after surgery. In all strains, the TSS was composed of a thin layer (TL) containing fibrous tissues and loose connective tissues formed outside the TL. Few lymphocytes and mast cells, several neutrophils, and numerous macrophages infiltrated the TSS. Active vascularization was observed at D14 in all strains. For the examined indices, M1-type macrophage density in the TSS of C57BL/6N mice was significantly higher at D14 compared to other strains. No significant strain difference relating to M2-type macrophages was detected, suggesting the effects of Th1-biased immunity on FBR-related inflammation. Collagen fibers in the TSS increased in density and became stable with age in all strains. In particular, MRL/MpJ-Fas^lpr/lpr showed progressive fibrotic features. Serum autoantibody levels in MRL/MpJ-Fas^lpr/lpr mice were inversely correlated with M1-type macrophage density. These data from MRL/MpJ-Fas^lpr/lpr mice suggested modifications of FBR-related inflammation and fibrosis by autoimmune abnormalities. The results provide crucial insights into the pathological modification of FBR by recipient immunity and emphasize its clinicopathological importance in humans and animals.

Inducing Vertebrate Limb Regeneration: A Review of Past Advances and Future Outlook

Limb loss due to traumatic injury or amputation is a major biomedical burden. Many vertebrates exhibit the ability to form and pattern normal limbs during embryogenesis from amorphous clusters of precursor cells, hinting that this process could perhaps be activated later in life to rebuild missing or damaged limbs. Indeed, some animals, such as salamanders, are proficient regenerators of limbs throughout their life span. Thus, research over the last century has sought to stimulate regeneration in species that do not normally regenerate their appendages. Importantly, these efforts are not only a vital aspect of regenerative medicine, but also have fundamental implications for understanding evolution and the cellular control of growth and form throughout the body. Here we review major recent advances in augmenting limb regeneration, summarizing the degree of success that has been achieved to date in frog and mammalian models using genetic, biochemical, and bioelectrical interventions. While the degree of whole limb repair in rodent models has been modest to date, a number of new technologies and approaches comprise an exciting near-term road map for basic and clinical progress in regeneration.

Molekularne mechanizmy działania czynnika transkrypcyjnego FOXN1 w skórze

Artykuł jest przeglądem wyników badań dotyczących funkcji i mechanizmu działania czynnika transkrypcyjnego FOXN1. Lokalizacja FOXN1 u przedstawicieli wszystkich ssaków ogranicza się do nabłonka tylko dwóch organów: skóry i grasicy. W skórze FOXN1 stymuluje różnicowanie się keratynocytów, reguluje proces pigmentacji i bierze udział w rozwoju włosów. W skórze objętej urazem FOXN1 jest zaangażowany w bliznowy proces gojenia poprzez udział w reepitelializacji oraz w procesie przejścia epitelialno-mezenchymalnego (epithelial-mesenchymal transition; EMT). Pozbawione aktywnego czynnika transkrypcyjnego FOXN1 dorosłe myszy (Foxn1-/-) goją urazy skórne w unikalnym, charakterystycznym jedynie dla płodów ssaków, procesie bezbliznowej (scar-free) regeneracji. Analiza porównawcza transkryptomów skóry: dorosłych myszy Foxn1-/- oraz skóry płodów myszy (14. dzień rozwoju płodowego) wykazała istotne podobieństwa w ekspresji genów związanych przede wszystkim z przebudową tkanek, budową cytoszkieletu, gojeniem urazów, odpowiedzią immunologiczną oraz różnicowaniem. Wyniki te wskazują, iż FOXN1 może być głównym elementem szlaku sygnałowego na drodze tzw. punktu tranzycyjnego czyli przejścia z etapu gojenia bezbliznowego (płodowego) do bliznowego (dorosłego) w trakcie rozwoju płodowego.

Morphological Features of the Testis among Autoimmune Mouse Model and Healthy Strains

Autoimmune diseases play a critical role in the progression of infertility in both sexes and their severity has been reported to increase with age. However, few reports have discussed their effect on the morphological features of the testis. Therefore, we compared the morphological alterations in the testes of autoimmune model mice (MRL/MpJ-Faslpr) and the control strain (MRL/MpJ) with those of their background strain (C57BL/6N) at 3 and 6 months. Furthermore, we analyzed the changes in spermatocytes, Sertoli cells, immune cells, and Zonula occludens-1 junctional protein by immunohistochemical staining. The MRL/MpJ-Faslpr mice showed a significant increase in the serum Anti-double stranded DNA antibody level, relative spleen weight, and seminiferous luminal area when compared with other studied two strains. In contrast, a significant decrease in the relative testis weight, and numbers of both Sertoli, meiotic spermatocyte was observed in MRL/MpJ-Faslpr and MRL/MpJ mice compared with C57BL/6N mice especially at 6 months. Similarly, Zonula occludens-1 junctional protein positive cells showed a significant decrease in the same strains at 6 months. However, no immune cell infiltration could be observed among the studied three strains. Our findings suggest that the increase in autoimmune severity especially with age could lead to infertility through loss of spermatogenic and Sertoli cells, rather than the disturbance of the blood-testis barrier.

Evaluation of Pseudomonas aeruginosa pathogenesis and therapeutics in military-relevant animal infection models

Modern combat-related injuries are often associated with acute polytrauma. As a consequence of severe combat-related injuries, a dysregulated immune response results in serious infectious complications. The gram-negative bacterium Pseudomonas aeruginosa is an opportunistic pathogen that often causes life-threatening bloodstream, lung, bone, urinary tract, and wound infections following combat-related injuries. The rise in the number of multidrug-resistant P. aeruginosa strains has elevated its importance to civilian clinicians and military medicine. Development of novel therapeutics and treatment options for P. aeruginosa infections is urgently needed. During the process of drug discovery and therapeutic testing, in vivo testing in animal models is a critical step in the bench-to-bedside approach, and required for Food and Drug Administration approval. Here, we review current and past literature with a focus on combat injury-relevant animal models often used to understand infection development, the interplay between P. aeruginosa and the host, and evaluation of novel treatments. Specifically, this review focuses on the following animal infection models: wound, burn, bone, lung, urinary tract, foreign body, and sepsis.

Aberrant Gut Microbiome Contributes to Intestinal Oxidative Stress, Barrier Dysfunction, Inflammation and Systemic Autoimmune Responses in MRL/lpr Mice

Microbiome composition and function have been implicated as contributing factors in the pathogenesis of autoimmune diseases (ADs), including systemic lupus erythematosus (SLE), rheumatoid arthritis and autoimmune hepatitis (AIH). Furthermore, dysbiosis of gut microbiome is associated with impaired barrier function and mucosal immune dysregulation. However, mechanisms by which gut microbiome contributes to the ADs and whether antioxidant treatment can restore gut homeostasis and ameliorate the disease outcome are not known. This study was, therefore, focused on examining the involvement of gut microbiome and host responses in the pathogenesis of SLE using unique female mouse models (C57BL/6, MRL+/+ and MRL/lpr) of 6 and 18 weeks with varying degrees of disease progression. Fecal microbiome diversity and composition, gut oxidative stress (OS), barrier function and inflammation, as well as systemic autoimmunity were determined. Interestingly, each mouse strain had distinct bacterial community as revealed by β-diversity. A lower Firmicutes/Bacteroidetes ratio in 6-week-old MRL/lpr mice was observed, evidenced by decrease in Peptostreptococcaceae under Firmicutes phylum along with enrichment of Rikenellaceae under Bacteroidetes phylum. Additionally, we observed increases in colonic OS [4-hydroxynonenal (HNE)-adducts and HNE-specific immune complexes], permeability changes (lower tight junction protein ZO-2; increased fecal albumin and IgA levels) and inflammatory responses (increased phos-NF-κB, IL-6 and IgG levels) in 18-week-old MRL/lpr mice. These changes were associated with markedly elevated AD markers (antinuclear and anti-smooth muscle antibodies) along with hepatic portal inflammation and severe glomerulonephritis. Notably, antioxidant N-acetylcysteine treatment influenced the microbial composition (decreased Rikenellaceae; increased Akkeransiaceae, Erysipelotrichaceae and Muribaculaceae) and attenuated the systemic autoimmunity in MRL/lpr mice. Our data thus show that gut microbiome dysbiosis is associated with increased colonic OS, barrier dysfunction, inflammatory responses and systemic autoimmunity markers. These findings apart from delineating a role for gut microbiome dysbiosis, also support the contribution of gut OS, permeability changes and inflammatory responses in the pathogenesis of ADs.

Anatomy and histology of the foramen of ovarian bursa opening to the peritoneal cavity and its changes in autoimmune disease-prone mice

The ovarian bursa is a small peritoneal cavity enclosed by the mesovarium and mesosalpinx, which surrounds the ovaries and oviductal infundibulum in mammals. The ovarian bursa is considered as the structure facilitating the transport of ovulated oocytes into the oviduct. Our previous study revealed reduced oocyte pick-up function in the oviduct of lupus-prone MRL/MpJ-Faslpr/lpr mouse, suggesting the possibility of an escape of ovulated oocytes into the peritoneal cavity, despite the presence of an almost complete ovarian bursa in the mouse. In this study, we revealed anatomical and histological characteristics of the ovarian bursa in C57BL/6 N, MRL/MpJ, and MRL/MpJ-Faslpr/lpr mice. All strains had the foramen of ovarian bursa (FOB), with a size of approximately 0.04 to 0.12 cm2 , surrounded by the ligament of ovarian bursa (LOB), which is part of the mesosalpinx. The LOB was partially lined with the cuboidal mesothelial cells and consisted of a thick smooth muscle layer in all strains. In 6-month-old MRL/MpJ-Faslpr/lpr mice, in which the systemic autoimmune abnormality deteriorated and oocyte pick-up function was impaired, the size of the FOB tended to be larger than that of other strains. Additionally, in MRL/MpJ-Faslpr/lpr mice at 6 months of age, there was infiltration by numerous immune cells in the mesosalpinx suspending the isthmus; however, the LOB prevented severe inflammation and showed deposition of collagen fibers. These results not only indicate that the FOB is a common structure within mice, but also imply the physiological function of the LOB and its role in maintaining the microenvironment around the ovary, as well as regulating healthy reproduction.

The role of the macrophage in tendinopathy and tendon healing

The role of the macrophage is an area of emerging interest in tendinopathy and tendon healing. The macrophage has been found to play a key role in regulating the healing process of the healing tendon. The specific function of the macrophage depends on its functional phenotype. While the M1 macrophage phenotype exhibits a phagocytic and proinflammatory function, the M2 macrophage phenotype is associated with the resolution of inflammation and tissue deposition. Several studies have been conducted on animal models looking at enhancing or suppressing macrophage function, targeting specific phenotypes. These studies include the use of exogenous biological and pharmacological substances and more recently the use of transgenic and genetically modified animals. The outcomes of these studies have been promising. In particular, enhancement of M2 macrophage activity in the healing tendon of animal models have shown decreased scar formation, accelerated healing, decreased inflammation and even enhanced biomechanical strength. Currently our understanding of the role of the macrophage in tendinopathy and tendon healing is limited. Furthermore, the roles of therapies targeting macrophages to enhance tendon healing is unclear. Clinical Significance: An increased understanding of the significance of the macrophage and its functional phenotypes in the healing tendon may be the key to enhancing tendon healing. This review will present the current literature on the function of macrophages in tendinopathy and tendon healing and the potential of therapies targeting macrophages to enhance tendon healing.

Genetic correlations between cartilage regeneration and degeneration reveal an inverse relationship

OBJECTIVE

The etiology of osteoarthritis (OA) is unknown, however, there appears to be a significant contribution from genetics. We have identified recombinant inbred strains of mice derived from LG/J (large) and SM/J (small) strains that vary significantly in their ability to repair articular cartilage and susceptibility to post-traumatic OA due to their genetic composition. Here, we report cartilage repair phenotypes in the same strains of mice in which OA susceptibility was analyzed previously, and determine the genetic correlations between phenotypes.

DESIGN

We used 12 recombinant inbred strains, including the parental strains, to test three phenotypes: ear-wound healing (n = 263), knee articular cartilage repair (n = 131), and post-traumatic OA (n = 53) induced by the surgical destabilization of the medial meniscus (DMM). Genetic correlations between various traits were calculated as Pearson's correlation coefficients of strain means.

RESULTS

We found a significant positive correlation between ear-wound healing and articular cartilage regeneration (r = 0.71; P = 0.005). We observed a strong inverse correlation between articular cartilage regeneration and susceptibility to OA based on maximum (r = -0.54; P = 0.036) and summed Osteoarthritis Research Society International (OARSI) scores (r = -0.56; P = 0.028). Synovitis was not significantly correlated with articular cartilage regeneration but was significantly positively correlated with maximum (r = 0.63; P = 0.014) and summed (r = 0.70; P = 0.005) OARSI scores. Ectopic calcification was significantly positively correlated with articular cartilage regeneration (r = 0.59; P = 0.021).

CONCLUSIONS

Using recombinant inbred strains, our study allows, for the first time, the measurement of genetic correlations of regeneration phenotypes with degeneration phenotypes, characteristic of OA (cartilage degeneration, synovitis). We demonstrate that OA is positively correlated with synovitis and inversely correlated with the ability to repair cartilage. These results suggest an addition to the risk paradigm for OA from a focus on degeneration to regeneration.

RSK-3 promotes cartilage regeneration via interacting with rpS6 in cartilage stem/progenitor cells

Rationale: Cartilage stem/progenitor cells (CSPC) are a promising cellular source to promote endogenous cartilage regeneration in osteoarthritis (OA). Our previous work indicates that ribosomal s6 kinase 3 (RSK-3) is a target of 4-aminobiphenyl, a chemical enhancing CSPC-mediated cartilage repair in OA. However, the primary function and mechanism of RSK-3 in CSPC-mediated cartilage pathobiology remain undefined.

Methods: We systematically assessed the association of RSK-3 with OA in three mouse strains with varying susceptibility to OA (MRL/MpJ>CBA>STR/Ort), and also RSK-3^-/- mice. Bioinformatic analysis was used to identify the possible mechanism of RSK-3 affecting CSPC, which was further verified in OA mice and CSPC with varying RSK-3 expression induced by chemicals or gene modification.

Results: We demonstrated that the level of RSK-3 in cartilage was positively correlated with cartilage repair capacities in three mouse strains (MRL/MpJ>CBA>STR/Ort). Enhanced RSK-3 expression by 4-aminobiphenyl markedly attenuated cartilage injury in OA mice and inhibition or deficiency of RSK-3 expression, on the other hand, significantly aggravated cartilage damage. Transcriptional profiling of CSPC from mice suggested the potential role of RSK-3 in modulating cell proliferation. It was further shown that the in vivo and in vitro manipulation of the RSK-3 expression indeed affected the CSPC proliferation. Mechanistically, ribosomal protein S6 (rpS6) was activated by RSK-3 to accelerate CSPC growth.

Conclusion: RSK-3 is identified as a key regulator to enhance cartilage repair, at least partly by regulating the functionality of the cartilage-resident stem/progenitor cells.

The MRL/MpJ Mouse Strain Is Not Protected From Muscle Atrophy and Weakness After Rotator Cuff Tear

Chronic rotator cuff tears are a common source of shoulder pain and disability. Patients with rotator cuff tears often have substantial weakness, fibrosis, and fat accumulation, which limit successful surgical repair and postoperative rehabilitation. The Murphy Roths Large (MRL) strain of mice have demonstrated superior healing and protection against pathological changes in several disease and injury conditions. We tested the hypothesis that, compared with the commonly used C57Bl/6 (B6) strain, MRL mice would have less muscle fiber atrophy and fat accumulation, and be protected against the loss in force production that occurs after cuff tear. Adult male B6 and MRL mice were subjected to a rotator cuff tear, and changes in muscle fiber contractility and histology were measured. RNA sequencing and shotgun metabolomics and lipidomics were also performed. The muscles were harvested one month after tear. B6 and MRL mice had a 40% reduction in relative muscle force production after rotator cuff tear. RNA sequencing identified an increase in fibrosis-associated genes and a reduction in mitochondrial metabolism genes. The markers of glycolytic metabolism increased in B6 mice, while MRL mice appeared to increase amino acid metabolism after tear. There was an accumulation of lipid after injury, although there was a divergent response between B6 and MRL mice in the types of lipid species that accrued. There were strain-specific differences between the transcriptome, metabolome, and lipidome of B6 and MRL mice, but these differences did not protect MRL mice from weakness and pathological changes after rotator cuff tear. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:811-822, 2020.

Hypoxia-inducible factor 1α (HIF-1α) is a major determinant in the enhanced function of muscle-derived progenitors from MRL/MpJ mice

Although the mouse strain Murphy Roths Large (MRL/MpJ) possesses high regenerative potential, the mechanism of tissue regeneration, including skeletal muscle, in MRL/MpJ mice after injury is still unclear. Our previous studies have shown that muscle-derived stem/progenitor cell (MDSPC) function is significantly enhanced in MRL/MpJ mice when compared with MDSPCs isolated from age-matched wild-type (WT) mice. Using mass spectrometry-based proteomic analysis, we identified increased expression of hypoxia-inducible factor (HIF) 1α target genes (expression of glycolytic factors and antioxidants) in sera from MRL/MpJ mice compared with WT mice. Therefore, we hypothesized that HIF-1α promotes the high muscle healing capacity of MRL/MpJ mice by increasing the potency of MDSPCs. We demonstrated that treating MRL/MpJ MDSPCs with dimethyloxalylglycine and CoCl₂ increased the expression of HIF-1α and target genes, including angiogenic and cell survival genes. We also observed that HIF-1α activated the expression of paired box (Pax)7 through direct interaction with the Pax7 promoter. Furthermore, we also observed a higher myogenic potential of MDSPCs derived from prolyl hydroxylase (Phd) 3-knockout (Phd3^-/-) mice, which displayed higher stability of HIF-1α. Taken together, our findings suggest that HIF-1α is a major determinant in the increased MDSPC function of MRL/MpJ mice through enhancement of cell survival, proliferation, and myogenic differentiation.-Sinha, K. M., Tseng, C., Guo, P., Lu, A., Pan, H., Gao, X., Andrews, R., Eltzschig, H., Huard, J. Hypoxia-inducible factor 1α (HIF-1α) is a major determinant in the enhanced function of muscle-derived progenitors from MRL/MpJ mice.

Variable outcomes of human heart attack recapitulated in genetically diverse mice

Clinical variation in patient responses to myocardial infarction (MI) has been difficult to model in laboratory animals. To assess the genetic basis of variation in outcomes after heart attack, we characterized responses to acute MI in the Collaborative Cross (CC), a multi-parental panel of genetically diverse mouse strains. Striking differences in post-MI functional, morphological, and myocardial scar features were detected across 32 CC founder and recombinant inbred strains. Transcriptomic analyses revealed a plausible link between increased intrinsic cardiac oxidative phosphorylation levels and MI-induced heart failure. The emergence of significant quantitative trait loci for several post-MI traits indicates that utilizing CC strains is a valid approach for gene network discovery in cardiovascular disease, enabling more accurate clinical risk assessment and prediction.

Evidence for Genetic Contribution to Variation in Posttraumatic Osteoarthritis in Mice

OBJECTIVE

Recombinant inbred mouse strains generated from an LG/J and SM/J intercross offer a unique resource to study complex genetic traits such as osteoarthritis (OA). We undertook this study to determine the susceptibility of 14 strains to various phenotypes characteristic of posttraumatic OA. We hypothesized that phenotypic variability is associated with genetic variability.

METHODS

Ten-week-old male mice underwent surgical destabilization of the medial meniscus (DMM) to induce posttraumatic OA. Mice were killed 8 weeks after surgery, and knee joints were processed for histology to score cartilage degeneration and synovitis. Micro-computed tomography was used to analyze trabecular bone parameters including subchondral bone plate thickness and synovial ectopic calcifications. Gene expression in the knees was assessed using a QuantiGene Plex assay.

RESULTS

Broad-sense heritability ranged from 0.18 to 0.58, which suggested that the responses to surgery were moderately heritable. The LGXSM-33, LGXSM-5, LGXSM-46, and SM/J strains were highly susceptible to OA, while the LGXSM-131b, LGXSM-163, LGXSM-35, LGXSM-128a, LGXSM-6, and LG/J strains were relatively OA resistant. This study was the first to accomplish measurement of genetic correlations of phenotypes that are characteristic of posttraumatic OA. Cartilage degeneration was significantly positively associated with synovitis (r = 0.83-0.92), and subchondral bone plate thickness was negatively correlated with ectopic calcifications (r = -0.59). Moreover, we showed that 40 of the 78 genes tested were significantly correlated with various OA phenotypes. However, unlike the OA phenotypes, there was no evidence for genetic variation in differences in gene expression levels between DMM-operated and sham-operated knees.

CONCLUSION

For these mouse strains, various characteristics of posttraumatic OA varied with genetic composition, which demonstrated a genetic basis for susceptibility to posttraumatic OA. The heritability of posttraumatic OA was established. Phenotypes exhibited various degrees of correlations; cartilage degeneration was positively correlated with synovitis, but not with the formation of ectopic calcifications. Further investigation of the genome regions that contain genes implicated in OA, as well as further investigation of gene expression data, will be useful for studying mechanisms of OA and identifying therapeutic targets.

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.

MRL/MpJ mice produce more oocytes and exhibit impaired fertilisation and accelerated luteinisation after superovulation treatment

MRL/MpJ mice exhibit distinct phenotypes in several biological processes, including wound healing. Herein we report two unique phenotypes in the female reproductive system of MRL/MpJ mice that affect ovulation and luteinisation. We found that superovulation treatment resulted in the production of significantly more oocytes in MRL/MpJ than C57BL/6 mice (71.0±13.4 vs 26.8±2.8 respectively). However, no exon mutations were detected in genes coding for female reproductive hormones or their receptors in MRL/MpJ mice. In addition, the fertilisation rate was lower for ovulated oocytes from MRL/MpJ than C57BL/6 mice, with most of the fertilised oocytes showing abnormal morphology, characterised by deformation and cytolysis. Histological tracing of luteinisation showed that MRL/MpJ mice formed corpora lutea within 36h after ovulation, whereas C57BL/6 mice were still at the corpora haemorrhagica formation stage after 36h. The balance between the expression of matrix metalloproteinases and their tissue inhibitors shifted towards the former earlier after ovulation in MRL/MpJ than C57BL/6 mice. This result indicates a possible link between accelerated extracellular matrix remodelling in the ovulated or ruptured follicles and luteinisation in MRL/MpJ mice. Together, these findings reveal novel phenotypes in MRL/MpJ mice that provide novel insights into reproductive biology.

Comparative Transcriptomics Identifies Novel Genes and Pathways Involved in Post-Traumatic Osteoarthritis Development and Progression

Anterior cruciate ligament (ACL) injuries often result in post-traumatic osteoarthritis (PTOA). To better understand the molecular mechanisms behind PTOA development following ACL injury, we profiled ACL injury-induced transcriptional changes in knee joints of three mouse strains with varying susceptibility to OA: STR/ort (highly susceptible), C57BL/6J (moderately susceptible) and super-healer MRL/MpJ (not susceptible). Right knee joints of the mice were injured using a non-invasive tibial compression injury model and global gene expression was quantified before and at 1-day, 1-week, and 2-weeks post-injury using RNA-seq. Following injury, injured and uninjured joints of STR/ort and injured C57BL/6J joints displayed significant cartilage degeneration while MRL/MpJ had little cartilage damage. Gene expression analysis suggested that prolonged inflammation and elevated catabolic activity in STR/ort injured joints, compared to the other two strains may be responsible for the severe PTOA phenotype observed in this strain. MRL/MpJ had the lowest expression values for several inflammatory cytokines and catabolic enzymes activated in response to ACL injury. Furthermore, we identified several genes highly expressed in MRL/MpJ compared to the other two strains including B4galnt2 and Tpsab1 which may contribute to enhanced healing in the MRL/MpJ. Overall, this study has increased our knowledge of early molecular changes associated with PTOA development.

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.

Foxn1 in Skin Development, Homeostasis and Wound Healing

Intensive research effort has focused on cellular and molecular mechanisms that regulate skin biology, including the phenomenon of scar-free skin healing during foetal life. Transcription factors are the key molecules that tune gene expression and either promote or suppress gene transcription. The epidermis is the source of transcription factors that regulate many functions of epidermal cells such as proliferation, differentiation, apoptosis, and migration. Furthermore, the activation of epidermal transcription factors also causes changes in the dermal compartment of the skin. This review focuses on the transcription factor Foxn1 and its role in skin biology. The regulatory function of Foxn1 in the skin relates to physiological (development and homeostasis) and pathological (skin wound healing) conditions. In particular, the pivotal role of Foxn1 in skin development and the acquisition of the adult skin phenotype, which coincides with losing the ability of scar-free healing, is discussed. Thus, genetic manipulations with Foxn1 expression, specifically those introducing conditional Foxn1 silencing in a Foxn1+/+ organism or its knock-in in a Foxn1−/− model, may provide future perspectives for regenerative medicine.

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.

Skeletal Muscle Metabolism in Duchenne and Becker Muscular Dystrophy-Implications for Therapies

The interactions between nutrition and metabolism and skeletal muscle have long been known. Muscle is the major metabolic organ—it consumes more calories than other organs—and therefore, there is a clear need to discuss these interactions and provide some direction for future research areas regarding muscle pathologies. In addition, new experiments and manuscripts continually reveal additional highly intricate, reciprocal interactions between metabolism and muscle. These reciprocal interactions include exercise, age, sex, diet, and pathologies including atrophy, hypoxia, obesity, diabetes, and muscle myopathies. Central to this review are the metabolic changes that occur in the skeletal muscle cells of muscular dystrophy patients and mouse models. Many of these metabolic changes are pathogenic (inappropriate body mass changes, mitochondrial dysfunction, reduced adenosine triphosphate (ATP) levels, and increased Ca²⁺) and others are compensatory (increased phosphorylated AMP activated protein kinase (pAMPK), increased slow fiber numbers, and increased utrophin). Therefore, reversing or enhancing these changes with therapies will aid the patients. The multiple therapeutic targets to reverse or enhance the metabolic pathways will be discussed. Among the therapeutic targets are increasing pAMPK, utrophin, mitochondrial number and slow fiber characteristics, and inhibiting reactive oxygen species. Because new data reveals many additional intricate levels of interactions, new questions are rapidly arising. How does muscular dystrophy alter metabolism, and are the changes compensatory or pathogenic? How does metabolism affect muscular dystrophy? Of course, the most profound question is whether clinicians can therapeutically target nutrition and metabolism for muscular dystrophy patient benefit? Obtaining the answers to these questions will greatly aid patients with muscular dystrophy.

Wound healing related agents: Ongoing research and perspectives

Wound healing response plays a central part in chronic inflammation, affecting millions of people worldwide. It is a dynamic process that can lead to fibrosis, if tissue damage is irreversible and wound resolution is not attained. It is clear that there is a tight interconnection among wound healing, fibrosis and a variety of chronic disease conditions, demonstrating the heterogeneity of this pathology. Based on our further understanding of the cellular and molecular mechanisms underpinning tissue repair, new therapeutic approaches have recently been developed that target different aspects of the wound healing process and fibrosis. Nevertheless, several issues still need to be taken into consideration when designing modern wound healing drug delivery formulations. In this review, we highlight novel pharmacological agents that hold promise for targeting wound repair and fibrosis. We also focus on drug-delivery systems that may enhance current and future therapies.

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).

Cartilage regeneration and ageing: Targeting cellular plasticity in osteoarthritis

Ageing processes play a major contributing role for the development of Osteoarthritis (OA). This prototypic degenerative condition of ageing is the most common form of arthritis and is accompanied by a general decline, chronic pain and mobility deficits. The disease is primarily characterized by articular cartilage degradation, followed by subchondral bone thickening, osteophyte formation, synovial inflammation and joint degeneration. In the early stages, osteoarthritic chondrocytes undergo phenotypic changes that increase cell proliferation and cluster formation and enhance the production of matrix-remodelling enzymes. In fact, chondrocytes exhibit differentiation plasticity and undergo phenotypic changes during the healing process. Current studies are focusing on unravelling whether OA is a consequence of an abnormal wound healing response. Recent investigations suggest that alterations in different proteins, such as TGF-ß/BMPs, NF-Kß, Wnt, and Cx43, or SASP factors involved in signalling pathways in wound healing response, could be directly implicated in the initiation of OA. Several findings suggest that osteoarthritic chondrocytes remain in an immature state expressing stemness-associated cell surface markers. In fact, the efficacy of new disease-modifying OA drugs that promote chondrogenic differentiation in animal models indicates that this may be a drug-sensible state. In this review, we highlight the current knowledge regarding cellular plasticity in chondrocytes and OA. A better comprehension of the mechanisms involved in these processes may enable us to understand the molecular pathways that promote abnormal repair and cartilage degradation in OA. This understanding would be advantageous in identifying novel targets and designing therapies to promote effective cartilage repair and successful joint ageing by preventing functional limitations and disability.

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.

Early induction of NRF2 antioxidant pathway by RHBDF2 mediates rapid cutaneous wound healing

Rhomboid family protein RHBDF2, an upstream regulator of the epidermal growth factor (EGF) receptor signaling, has been implicated in cutaneous wound healing. However, the underlying molecular mechanisms are still emerging. In humans, a gain-of-function mutation in the RHBDF2 gene accelerates cutaneous wound healing in an EGFR-dependent manner. Likewise, a gain-of-function mutation in the mouse Rhbdf2 gene (Rhbdf2^cub/cub) shows a regenerative phenotype (rapid ear-hole closure) resulting from constitutive activation of the EGFR pathway. Because the RHBDF2-regulated EGFR pathway is relevant to cutaneous wound healing in humans, we used Rhbdf2^cub/cub mice to investigate the biological networks and pathways leading to accelerated ear-hole closure, with the goal of identifying therapeutic targets potentially effective in promoting wound healing in humans. Comparative transcriptome analysis of ear pinna tissue from Rhbdf2^cub/cub and Rhbdf2^+/+ mice at 0h, 15min, 2h, and 24h post-wounding revealed an early induction of the nuclear factor E2-related factor 2 (NRF2)-mediated anti-oxidative pathway (0h and 15min), followed by the integrin-receptor aggregation pathway (2h) as early-stage events immediately and shortly after wounding in Rhbdf2^cub/cub mice. Additionally, we observed genes enriched for the Fc fragment of the IgG receptor IIIa (FCGR3A)-mediated phagocytosis pathway 24h post-wounding. Although cutaneous wound repair in healthy individuals is generally non-problematic, it can be severely impaired due to aging, diabetes, and chronic inflammation. This study suggests that activation of the NRF2-antioxidant pathway by rhomboid protein RHBDF2 might be beneficial in treating chronic non-healing wounds.

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.

Neotenic phenomenon in gene expression in the skin of Foxn1- deficient (nude) mice - a projection for regenerative skin wound healing

Background

Mouse fetuses up to 16 day of embryonic development and nude (Foxn1- deficient) mice are examples of animals that undergo regenerative (scar-free) skin healing. The expression of transcription factor Foxn1 in the epidermis of mouse fetuses begins at embryonic day 16.5 which coincides with the transition point from scar-free to scar-forming skin wound healing. In the present study, we tested the hypothesis that Foxn1 expression in the skin is an essential condition to establish the adult skin phenotype and that Foxn1 inactivity in nude mice keeps skin in the immature stage resembling the phenomena of neoteny.

Results

Uninjured skin of adult C57BL/6J (B6) mice, mouse fetuses at days 14 (E14) and 18 (E18) of embryonic development and B6.Cg-Foxn1 nu (nude) mice were characterized for their gene expression profiles by RNA sequencing that was validated through qRT-PCR, Western Blot and immunohistochemistry. Differentially regulated genes indicated that nude mice were more similar to E14 (model of regenerative healing) and B6 were more similar to E18 (model of reparative healing). The up-regulated genes in nude and E14 mice were associated with tissue remodeling, cytoskeletal rearrangement, wound healing and immune response, whereas the down-regulated genes were associated with differentiation. E14 and nude mice exhibit prominent up-regulation of keratin (Krt23, -73, -82, -16, -17), involucrin (Ivl) and filaggrin (Flg2) genes. The transcription factors associated with the Hox genes known to specify cell fate during embryonic development and promote embryonic stem cells differentiation were down-regulated in both nude and E14. Among the genes enriched in the nude skin but not shared with E14 fetuses were members of the Wnt and matrix metalloproteinases (Mmps) families whereas Bmp and Notch related genes were down-regulated.

Conclusions

In summary, Foxn1 appears to be a pivotal control element of the developmental program and skin maturation. Nude mice may be considered as a model of neoteny among mammals. The resemblance of gene expression profiles in the skin of both nude and E14 mice are direct or indirect consequences of the Foxn1 deficiency. Foxn1 appears to regulate the balance between cell proliferation and differentiation and its inactivity creates a pro-regenerative environment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3401-z) contains supplementary material, which is available to authorized users.

Hepatic Adaptations to a High Fat Diet in the MRL Mouse Strain are Associated with an Inefficient Oxidative Phosphorylation System

The MRL mice are resistant to a 12-week high fat diet (HFD) feeding protocol, with the proximal cause being an increased basal pAMPK^T172 expression in the skeletal muscle. Here, we test if this lack of pathology extends to the liver at both the tissue and cellular levels and its correlation to pAMPK^T172 levels. MRL and B6 mice were subjected to 12 weeks of diet intervention and tissues were either fixed for histology or snap-frozen for further processing (n= 3-6, per group). The HFD MRL mice remain insulin and glucose sensitive after 12 weeks of HFD. This phenomenon is correlated to increased liver pAMPK^T172. The HFD-fed B6 control strain demonstrates the opposite trend with decreased pAMPK^T172 expression after the HFD period. We have found further evidence of differential MRL metabolic adaptations. These differences include reduced glycogen content, reduced ectopic fat storage, and increased expression of Complex II (CII) and Complex V of the Electron Transport Chain (ETC). Whereas, B6 HFD control show unchanged glycogen content, increased ectopic fat and increased expression of Complex I and Complex V of the ETC. Taken together, the MRL adaptations point to an inefficient energy-producing phenotype that leads to glycogen depletion and attenuation of ectopic fat as secondary consequences with AMPK as the signaling mediator of these HFD- hepatic adaptations.

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.

A liposomal steroid nano-drug for treating systemic lupus erythematosus

Background

Glucocorticoids have been known for years to be the most effective therapy in systemic lupus erythematosus. Their use, however, is limited by the need for high doses due to their unfavorable pharmacokinetics and biodistribution. We have previously developed a novel liposome-based steroidal (methylprednisolone hemisuccinate (MPS)) nano-drug and demonstrated its specific accumulation in inflamed tissues, as well as its superior therapeutic efficacy over that of free glucocorticoids (non-liposomal) in the autoimmune diseases, including the adjuvant arthritis rat model and the experimental autoimmune encephalomyelitis mouse model.

Objectives

In the present work we have evaluated the therapeutic effect of the above liposome-based steroidal (MPS) nano-drug in the MRL-lpr/lpr murine model of SLE and compared it with similar doses of the free MPS.

Methods

MRL-lpr/lpr mice were treated with daily injections of free MPS or weekly injections of 10% dextrose, empty nano-liposomes or the steroidal nano-drug and the course of their disease was followed up to the age of 24 weeks.

Results

Treatment with the steroidal nano-drug was found to be significantly superior to the free MPS in suppressing anti-dsDNA antibody levels, proliferation of lymphoid tissue and renal damage, and in prolonging survival of animals.

Conclusion

This significant superiority of our liposome based steroidal nano-drug administered weekly compared with daily injections of free methylprednisolone hemisuccinate in suppressing murine lupus indicates this glucocorticoid nano-drug formulation may be a good candidate for the treatment of human SLE.

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.