Complement Receptor C5aR1 Plays an Evolutionarily Conserved Role in Successful Cardiac Regeneration

Establishing a semi-homology-directed recombination method for precision gene integration in axolotls

The axolotl is broadly used in regenerative, developmental, and evolutionary biology research. Targeted gene knock-in is crucial for precision transgenesis, enabling disease modeling, visualization, tracking, and functional manipulation of specific cells or genes of interest (GOIs). Existing CRISPR/Cas9-mediated homology-independent method for gene knock-in often causes "scars/indels" at integration junctions. Here, we develop a CRISPR/Cas9-mediated semi-homology-directed recombination (HDR) knock-in method using a donor construct containing a single homology arm for precise GOI integration. This semi-HDR approach achieves seamless single-end integration of the Cherry reporter gene and a large inducible Cre cassette into intronless genes like Sox2 and Neurod6 in axolotls, which are challenging to modify with the homology-independent method. Additionally, we integrate the inducible Cre cassette into intron-containing loci (e.g., Nkx2.2 and FoxA2) without introducing indels via semi-HDR. GOIs are properly expressed in F0 founders, with approximately 5%-10% showing precise integration confirmed by genotyping. Furthermore, using the Nkx2.2:CreERT2 line, we fate-map spinal cord p3 neural progenitor cells, revealing that Nkx2.2+ cells adopt different lineages in development and regeneration, preferentially generating motoneurons over oligodendrocytes during regeneration. Overall, this semi-HDR method balances efficiency and precision in GOI integration, providing a valuable tool for generating knock-in axolotls and potentially extending to other species.

The role and treatment potential of the complement pathway in chronic pain

The role of the complement system in pain syndromes has garnered attention on the back of preclinical and clinical evidence supporting its potential as a target for new analgesic pharmacotherapies. Of the components that make up the complement system, component 5a (C5a) and component 3a (C3a) are most strongly and consistently associated with pain. Receptors for C5a are widely found in immune resident cells (microglia, astrocytes, sensory neuron-associated macrophages (sNAMs)) in the central nervous system (CNS) as well as hematogenous immune cells (mast cells, macrophages, T-lymphocytes, etc.). When active, as is often observed in chronic pain conditions, these cells produce various inflammatory mediators including pro-inflammatory cytokines. These events can trigger nervous tissue inflammation (neuroinflammation) which coexists with and potentially maintains peripheral and central sensitization. C5a has a likely critical role in initiating this process highlighting its potential as a promising non-opioid target for treating pain. This review summarizes the most up-to-date research on the role of the complement system in pain with emphasis on the C5 pathway in peripheral tissue, dorsal root ganglia (DRG) and the CNS, and explores advances in complement-targeted drug development and sex differences. A perspective on the optimal application of different C5a inhibitors for different types (e.g., neuropathic, post-surgical and chemotherapy-induced pain, osteoarthritis pain) and stages (e.g., acute, subacute, chronic) of pain is also provided to help guide future clinical trials. PERSPECTIVE: This review highlights the role and mechanisms of complement components and their receptors in physiological and pathological pain. The potential of complement-targeted therapeutics for the treatment of chronic pain is also explored with a focus on C5a inhibitors to help guide future clinical trials.

A cellular triad for linking cardiac niche to regeneration

Cardiovascular disease is the leading cause of mortality with very limited therapeutic interventions, thus holding great hope for cardiac regenerative medicine. A recent work from Martin's laboratory reports their identification of a fetal-like cardiomyocyte progenitor, adult cardiomyocyte type 2 (aCM2), and its potential interactions with C3+ cardiac fibroblasts and C3ar1+ macrophages to form a regenerative cellular triad, which is only present in the regenerative heart models, YAP5SA-expressing adult hearts and neonatal hearts. The complement signaling is essential for cellular interactions in this regenerative triad. This Highlight summarizes these major findings and provides brief perspectives on the impact of this regenerative niche during cardiac regeneration in the future.

Transition from fetal to postnatal state in the heart: Crosstalk between metabolism and regeneration

Cardiovascular disease is the leading cause of mortality worldwide. Myocardial injury resulting from ischemia can be fatal because of the limited regenerative capacity of adult myocardium. Mammalian cardiomyocytes rapidly lose their proliferative capacities, with only a small fraction of adult myocardium remaining proliferative, which is insufficient to support post-injury recovery. Recent investigations have revealed that this decline in myocardial proliferative capacity is closely linked to perinatal metabolic shifts. Predominantly glycolytic fetal myocardial metabolism transitions towards mitochondrial fatty acid oxidation postnatally, which not only enables efficient production of ATP but also causes a dramatic reduction in cardiomyocyte proliferative capacity. Extensive research has elucidated the mechanisms behind this metabolic shift, as well as methods to modulate these metabolic pathways. Some of these methods have been successfully applied to enhance metabolic reprogramming and myocardial regeneration. This review discusses recently acquired insights into the interplay between metabolism and myocardial proliferation, emphasizing postnatal metabolic transitions.

Cardiomyocyte-derived exosomes promote cardiomyocyte proliferation and neonatal heart regeneration

Heart regeneration was mainly achieved by intrinsic capacity. Exosomes are crucial in cardiovascular disease, yet their involvement in myocardial regeneration remains underexplored. To understand the role of cardiomyocyte-derived exosomes (CM-Exos) in heart regeneration. We established mouse models of myocardial infarction and apical resection in neonates to investigate the potential benefits of exosomes in response to injury. Rab27a knockout (KO) mice were constructed as an exosome decrease model. Distinct fibrosis appears in the infarcted and resection area in the KO mice 21 days after heart injury. The proliferation marker pH 3, Ki67, and Aurora B were detected 3 days after surgery, which decreased in KO mice compared to WT mice. Intravenous injection of CM-Exos increased cardiomyocyte proliferation and partially restored heart function in KO mice. Rab27a knockdown in vitro reduced the expression of pH 3, Ki67, and Aurora B positive cardiomyocytes. However, the supplementation of CM-Exos increased the proliferation of cardiomyocytes. Exosomal miRNA sequencing was subsequently applied, and miR-21-5p was a promising candidate that promoted cardiomyocyte proliferation through its target genes Spry-1 and PDCD4. Intravenous injection of miR-21-5p exhibited similar proliferative effects as CM-Exos. Our results indicate that CM-Exos promotes cardiomyocyte cycle reentry by delivering miR-21-5p, highlighting the endogenous factors of myocardial regeneration.

Engineered model of heart tissue repair for exploring fibrotic processes and therapeutic interventions

Advancements in human-engineered heart tissue have enhanced the understanding of cardiac cellular alteration. Nevertheless, a human model simulating pathological remodeling following myocardial infarction for therapeutic development remains essential. Here we develop an engineered model of myocardial repair that replicates the phased remodeling process, including hypoxic stress, fibrosis, and electrophysiological dysfunction. Transcriptomic analysis identifies nine critical signaling pathways related to cellular fate transitions, leading to the evaluation of seventeen modulators for their therapeutic potential in a mini-repair model. A scoring system quantitatively evaluates the restoration of abnormal electrophysiology, demonstrating that the phased combination of TGFβ inhibitor SB431542, Rho kinase inhibitor Y27632, and WNT activator CHIR99021 yields enhanced functional restoration compared to single factor treatments in both engineered and mouse myocardial infarction model. This engineered heart tissue repair model effectively captures the phased remodeling following myocardial infarction, providing a crucial platform for discovering therapeutic targets for ischemic heart disease.

Homeostasis and evolution in relation to regeneration and repair

Homeostasis constitutes a key concept in physiology and refers to self-regulating processes that maintain internal stability when adjusting to changing external conditions. It diminishes internal entropy constituting a driving force behind evolution. Natural selection might act on homeostatic regulatory mechanisms and control mechanisms including homeodynamics, allostasis, hormesis and homeorhesis, where different stable stationary states are reached. Regeneration is under homeostatic control through hormesis. Damage to tissues initiates a response to restore the impaired equilibrium caused by mild stress using cell proliferation, cell differentiation and cell death to recover structure and function. Repair is a homeorhetic change leading to a new stable stationary state with decreased functionality and fibrotic scarring without reconstruction of the 3-D pattern. Mechanisms determining entrance of the tissue or organ to regeneration or repair include the balance between innate and adaptive immune cells in relation to cell plasticity and stromal stem cell responses, and redox balance. The regenerative and reparative capacities vary in different species, distinct tissues and organs, and at different stages of development including ageing. Many cell signals and pathways play crucial roles determining regeneration or repair by regulating protein synthesis, cellular growth, inflammation, proliferation, autophagy, lysosomal function, metabolism and metalloproteinase cell signalling. Attempts to favour the entrance of damaged tissues to regeneration in those with low proliferative rates have been made; however, there are evolutionary constraint mechanisms leading to poor proliferation of stem cells in unfavourable environments or tumour development. More research is required to better understand the regulatory processes of these mechanisms.

Regeneration in Mice of Injured Skin, Heart, and Spinal Cord by α-Gal Nanoparticles Recapitulates Regeneration in Amphibians

The healing of skin wounds, myocardial, and spinal cord injuries in salamander, newt, and axolotl amphibians, and in mouse neonates, results in scar-free regeneration, whereas injuries in adult mice heal by fibrosis and scar formation. Although both types of healing are mediated by macrophages, regeneration in these amphibians and in mouse neonates also involves innate activation of the complement system. These differences suggest that localized complement activation in adult mouse injuries might induce regeneration instead of the default fibrosis and scar formation. Localized complement activation is feasible by antigen/antibody interaction between biodegradable nanoparticles presenting α-gal epitopes (α-gal nanoparticles) and the natural anti-Gal antibody which is abundant in humans. Administration of α-gal nanoparticles into injuries of anti-Gal-producing adult mice results in localized complement activation which induces rapid and extensive macrophage recruitment. These macrophages bind anti-Gal-coated α-gal nanoparticles and polarize into M2 pro-regenerative macrophages that orchestrate accelerated scar-free regeneration of skin wounds and regeneration of myocardium injured by myocardial infarction (MI). Furthermore, injection of α-gal nanoparticles into spinal cord injuries of anti-Gal-producing adult mice induces recruitment of M2 macrophages, that mediate extensive angiogenesis and axonal sprouting, which reconnects between proximal and distal severed axons. Thus, α-gal nanoparticle treatment in adult mice mimics physiologic regeneration in amphibians. These studies further suggest that α-gal nanoparticles may be of significance in the treatment of human injuries.

YAP induces a neonatal-like pro-renewal niche in the adult heart

After myocardial infarction (MI), mammalian hearts do not regenerate, and the microenvironment is disrupted. Hippo signaling loss of function with activation of transcriptional co-factor YAP induces heart renewal and rebuilds the post-MI microenvironment. In this study, we investigated adult renewal-competent mouse hearts expressing an active version of YAP, called YAP5SA, in cardiomyocytes (CMs). Spatial transcriptomics and single-cell RNA sequencing revealed a conserved, renewal-competent CM cell state called adult (a)CM2 with high YAP activity. aCM2 co-localized with cardiac fibroblasts (CFs) expressing complement pathway component C3 and macrophages (MPs) expressing C3ar1 receptor to form a cellular triad in YAP5SA hearts and renewal-competent neonatal hearts. Although aCM2 was detected in adult mouse and human hearts, the cellular triad failed to co-localize in these non-renewing hearts. C3 and C3ar1 loss-of-function experiments indicated that C3a signaling between MPs and CFs was required to assemble the pro-renewal aCM2, C3+ CF and C3ar1+ MP cellular triad.

The C5aR1 complement receptor: A novel immunomodulator of insulin action in skeletal muscle

The complement system constitutes an integral component of the innate immune system and plays a critical role in adaptive immunity. Activation of this system engenders the production of complement peptide fragments, including C5a, which engage G-protein coupled receptors predominantly expressed in immune-associated cells, such as neutrophils, initiating pro-inflammatory responses. Intriguingly, our investigation has unveiled the presence of C5a receptor 1 (C5aR1) expression within skeletal muscle, a key metabolic tissue and primary target of insulin. Herein, we demonstrate that C5aR1 activation by C5a in differentiated human skeletal muscle cells elicits acute suppression of insulin signalling. This suppression manifests as impaired insulin-dependent association between IRS1 and the p85 subunit of PI3-kinase, a 50% reduction in Akt phosphorylation, and a 60% decline in insulin-stimulated glucose uptake. This impairment in insulin signalling is associated with a three-fold elevation in intramyocellular diacylglycerol (DAG) levels and a two-fold increase in cytosolic calcium content, which promote PKC-mediated IRS1 inhibition via enhanced phosphorylation at IRS1 Ser1101. Significantly, our findings demonstrate that structurally diverse C5aR1 antagonists, along with genetic deletion or stable silencing of C5aR1 by 80% using short-hairpin RNA, effectively attenuate repression of insulin signalling by C5a in LHCN-M2 human skeletal myotubes. These results underscore the potential of heightened C5aR1 activation, characteristic of obesity and chronic inflammatory conditions, to detrimentally impact insulin function within skeletal muscle cells. Additionally, the study suggests that agents targeting the C5a-C5aR axis, originally devised for mitigating complement-dependent inflammatory conditions, may offer therapeutic avenues to ameliorate immune-driven insulin resistance in key peripheral metabolic tissues, including skeletal muscle.

A Novel Superparamagnetic Multifunctional Nerve Scaffold: A Remote Actuation Strategy to Boost In Situ Extracellular Vesicles Production for Enhanced Peripheral Nerve Repair

Extracellular vesicles (EVs) have inherent advantages over cell-based therapies in regenerative medicine because of their cargos of abundant bioactive cues. Several strategies are proposed to tune EVs production in vitro. However, it remains a challenge for manipulation of EVs production in vivo, which poses significant difficulties for EVs-based therapies that aim to promote tissue regeneration, particularly for long-term treatment of diseases like peripheral neuropathy. Herein, a superparamagnetic nanocomposite scaffold capable of controlling EVs production on-demand is constructed by incorporating polyethyleneglycol/polyethyleneimine modified superparamagnetic nanoparticles into a polyacrylamide/hyaluronic acid double-network hydrogel (Mag-gel). The Mag-gel is highly sensitive to a rotating magnetic field (RMF), and can act as mechano-stimulative platform to exert micro/nanoscale forces on encapsulated Schwann cells (SCs), an essential glial cell in supporting nerve regeneration. By switching the ON/OFF state of the RMF, the Mag-gel can scale up local production of SCs-derived EVs (SCs-EVs) both in vitro and in vivo. Further transcriptome sequencing indicates an enrichment of transcripts favorable in axon growth, angiogenesis, and inflammatory regulation of SCs-EVs in the Mag-gel with RMF, which ultimately results in optimized nerve repair in vivo. Overall, this research provides a noninvasive and remotely time-scheduled method for fine-tuning EVs-based therapies to accelerate tissue regeneration, including that of peripheral nerves.

PLK3 promotes the proneural-mesenchymal transition in glioblastoma via transcriptional regulation of C5AR1

BACKGROUND

Accumulating evidence suggests that polo-like kinase 3 (PLK3) plays an essential role in tumor cells and induces cell proliferation and may have implications for the prognosis of various cancers. We sought to define the role of PLK3-dependent proneural-mesenchymal transition (PMT) in the glioblastoma (GBM) therapy.

METHODS AND RESULTS

We analyzed the expression data for PLK3 by using the TCGA database. PLK3 expression in GBM cell lines was determined by qRT-PCR and Western blotting. PLK3 levels were modulated using Lentivirus infection, and the effects on symptoms, tumor volume, and survival in mice intracranial xenograft models were determined. Irradiation (IR) was performed to induce PMT. PLK3 expression was significantly elevated in mesenchymal subtype GBM and promoted tumor proliferation in GBM. Additionally enriched PLK3 expression could be associated with poor prognosis in GBM patients compared with those who have lower PLK3 expression. Mechanically, PLK3-dependent PMT induced radioresistance in GBM cells via transcriptional regulation of complement C5a receptor 1 (C5AR1). In therapeutic experiments conducted in vitro, targeting PLK3 by using small molecule inhibitor decreased tumor growth and radioresistance of GBM cells both in vitro and in vivo.

CONCLUSIONS

PLK3-C5AR1 axis induced PMT thus enhanced radioresistance in GBM and could become a novel potential therapeutic target for GBM.

Complement System Inhibitory Drugs in a Zebrafish (Danio rerio) Model: Computational Modeling

The dysregulation of complement system activation usually results in acute or chronic inflammation and can contribute to the development of various diseases. Although the activation of complement pathways is essential for innate defense, exacerbated activity of this system may be harmful to the host. Thus, drugs with the potential to inhibit the activation of the complement system may be important tools in therapy for diseases associated with complement system activation. The synthetic peptides Cp40 and PMX205 can be highlighted in this regard, given that they selectively inhibit the C3 and block the C5a receptor (C5aR1), respectively. The zebrafish (Danio rerio) is a robust model for studying the complement system. The aim of the present study was to use in silico computational modeling to investigate the hypothesis that these complement system inhibitor peptides interact with their target molecules in zebrafish, for subsequent in vivo validation. For this, we analyzed molecular docking interactions between peptides and target molecules. Our study demonstrated that Cp40 and the cyclic peptide PMX205 have positive interactions with their respective zebrafish targets, thus suggesting that zebrafish can be used as an animal model for therapeutic studies on these inhibitors.

Comparative Analysis of Heart Regeneration: Searching for the Key to Heal the Heart-Part II: Molecular Mechanisms of Cardiac Regeneration

Cardiovascular diseases are the leading cause of death worldwide, among which ischemic heart disease is the most representative. Myocardial infarction results from occlusion of a coronary artery, which leads to an insufficient blood supply to the myocardium. As it is well known, the massive loss of cardiomyocytes cannot be solved due the limited regenerative ability of the adult mammalian hearts. In contrast, some lower vertebrate species can regenerate the heart after an injury; their study has disclosed some of the involved cell types, molecular mechanisms and signaling pathways during the regenerative process. In this 'two parts' review, we discuss the current state-of-the-art of the main response to achieve heart regeneration, where several processes are involved and essential for cardiac regeneration.

Antibody production and tolerance to the α-gal epitope as models for understanding and preventing the immune response to incompatible ABO carbohydrate antigens and for α-gal therapies

This review describes the significance of the α-gal epitope (Galα-3Galβ1-4GlcNAc-R) as the core of human blood-group A and B antigens (A and B antigens), determines in mouse models the principles underlying the immune response to these antigens, and suggests future strategies for the induction of immune tolerance to incompatible A and B antigens in human allografts. Carbohydrate antigens, such as ABO antigens and the α-gal epitope, differ from protein antigens in that they do not interact with T cells, but B cells interacting with them require T-cell help for their activation. The α-gal epitope is the core of both A and B antigens and is the ligand of the natural anti-Gal antibody, which is abundant in all humans. In A and O individuals, anti-Gal clones (called anti-Gal/B) comprise >85% of the so-called anti-B activity and bind to the B antigen in facets that do not include fucose-linked α1-2 to the core α-gal. As many as 1% of B cells are anti-Gal B cells. Activation of quiescent anti-Gal B cells upon exposure to α-gal epitopes on xenografts and some protozoa can increase the titer of anti-Gal by 100-fold. α1,3-Galactosyltransferase knockout (GT-KO) mice lack α-gal epitopes and can produce anti-Gal. These mice simulate human recipients of ABO-incompatible human allografts. Exposure for 2-4 weeks of naïve and memory mouse anti-Gal B cells to α-gal epitopes in the heterotopically grafted wild-type (WT) mouse heart results in the elimination of these cells and immune tolerance to this epitope. Shorter exposures of 7 days of anti-Gal B cells to α-gal epitopes in the WT heart result in the production of accommodating anti-Gal antibodies that bind to α-gal epitopes but do not lyse cells or reject the graft. Tolerance to α-gal epitopes due to the elimination of naïve and memory anti-Gal B cells can be further induced by 2 weeks in vivo exposure to WT lymphocytes or autologous lymphocytes engineered to present α-gal epitopes by transduction of the α1,3-galactosyltransferase gene. These mouse studies suggest that autologous human lymphocytes similarly engineered to present the A or B antigen may induce corresponding tolerance in recipients of ABO-incompatible allografts. The review further summarizes experimental works demonstrating the efficacy of α-gal therapies in amplifying anti-viral and anti-tumor immune-protection and regeneration of injured tissues.

Physical exercise modulates the microglial complement pathway in mice to relieve cortical circuitry deficits induced by mutant human TDP-43

The aggregation of TAR DNA binding protein 43 kDa (TDP-43) is related to different neurodegenerative diseases, which leads to microglial activation and neuronal loss. The molecular mechanism driving neuronal death by reactive microglia, however, has not been completely resolved. In this study, we generated a mouse model by overexpressing mutant human TDP-43 (M337V) in the primary motor cortex, leading to prominent motor-learning deficits. In vivo 2-photon imaging shows an active approach of microglia toward parvalbumin interneurons, resulting in disrupted cortical excitatory-inhibitory balance. Proteomics studies suggest that activation of the complement pathway induces microglial activity. To develop an early interventional strategy, treadmill exercise successfully prevents the deterioration of motor dysfunction under enhanced adipocytic release of clusterin to block the complement pathway. These results demonstrate a previously unrecognized pathway by which TDP-43 induces cortical deficits and provide additional insights for the mechanistic explanation of exercise training in disease intervention.

Surviving without oxygen involves major tissue specific changes in the proteome of crucian carp (Carassius carassius)

The crucian carp (Carassius carassius) can survive complete oxygen depletion (anoxia) for several months at low temperatures, making it an excellent model for studying molecular adaptations to anoxia. Still, little is known about how its global proteome responds to anoxia and reoxygenation. By applying mass spectrometry-based proteome analyses on brain, heart and liver tissue from crucian carp exposed to normoxia, five days anoxia, and reoxygenation, we found major changes in particularly cardiac and hepatic protein levels in response to anoxia and reoxygenation. These included tissue-specific differences in mitochondrial proteins involved in aerobic respiration and mitochondrial membrane integrity. Enzymes in the electron transport system (ETS) decreased in heart and increased massively in liver during anoxia and reoxygenation but did not change in the brain. Importantly, the data support a special role for the liver in succinate handling upon reoxygenation, as suggested by a drastic increase of components of the ETS and uncoupling protein 2, which could allow for succinate metabolism without excessive formation of reactive oxygen species (ROS). Also during reoxygenation, the levels of proteins involved in the cristae junction organization of the mitochondria changed in the heart, possibly functioning to suppress ROS formation. Furthermore, proteins involved in immune (complement) system activation changed in the anoxic heart compared to normoxic controls. The results emphasize that responses to anoxia are highly tissue-specific and related to organ function.

A modified apical resection model with high accuracy and reproducibility in neonatal mouse and rat hearts

Neonatal mouse heart can regenerate after left ventricle (LV) apical resection (AR). Since current AR rodent method is accomplished by resecting LV apex until exposure of LV chamber, it is relatively difficult to operate reproducibly. We aimed to develop a modified AR method with high accuracy and reproducibility and to investigate whether cardiac regenerative capacity could be replicated in neonatal rats. For 15% AR of whole heart weight in 1-day-old (P1) neonatal mice, a modified 10 μL pipette tip cut to 0.48 mm in internal diameter was connected to a vacuum pump working at 0.06 ± 0.005 MPa and gently kept in touch with LV apex for nearly but no more than 12 s. LV apex was resected by a single incision adjacent to the pipette tip. The modified AR method in P1 mice achieved cardiac structural and functional recovery at 21 days post resection (dpr). Data from different operators showed smaller variation of resected LV apex and higher reproducibility using the modified AR method. Furthermore, we showed that 5% AR of whole heart weight in P1 neonatal rats using a modified 200 μL pipette tip cut to 0.63 mm in internal diameter led to complete regeneration of LV apex and full preservation of cardiac function at 42 dpr. In conclusion, the modified AR rodent model leads to accurate resection of LV apex with high homogeneity and reproducibility and it is practically convenient for the study of structural, functional, and molecular mechanisms of cardiac regeneration in both neonatal mice and rats.

Targeting immunoregulation for cardiac regeneration

Inducing endogenous cardiomyocyte proliferation and heart regeneration is a promising strategy to treat ischemic heart failure. The immune response has recently been considered critical in cardiac regeneration. Thus, targeting the immune response is a potent strategy to improve cardiac regeneration and repair after myocardial infarction. Here we reviewed the characteristics of the relationship between the postinjury immune response and heart regenerative capacity and summarized the latest studies focusing on inflammation and heart regeneration to identify potent targets of the immune response and strategies in the immune response to promote cardiac regeneration.

Molecular Actors of Inflammation and Their Signaling Pathways: Mechanistic Insights from Zebrafish

Inflammation is a hallmark of the physiological response to aggressions. It is orchestrated by a plethora of molecules that detect the danger, signal intracellularly, and activate immune mechanisms to fight the threat. Understanding these processes at a level that allows to modulate their fate in a pathological context strongly relies on in vivo studies, as these can capture the complexity of the whole process and integrate the intricate interplay between the cellular and molecular actors of inflammation. Over the years, zebrafish has proven to be a well-recognized model to study immune responses linked to human physiopathology. We here provide a systematic review of the molecular effectors of inflammation known in this vertebrate and recapitulate their modes of action, as inferred from sterile or infection-based inflammatory models. We present a comprehensive analysis of their sequence, expression, and tissue distribution and summarize the tools that have been developed to study their function. We further highlight how these tools helped gain insights into the mechanisms of immune cell activation, induction, or resolution of inflammation, by uncovering downstream receptors and signaling pathways. These progresses pave the way for more refined models of inflammation, mimicking human diseases and enabling drug development using zebrafish models.

C1q and the classical complement cascade in geographic atrophy secondary to age-related macular degeneration

Geographic atrophy (GA) secondary to age-related macular degeneration (AMD) is a retinal neurodegenerative disorder. Human genetic data support the complement system as a key component of pathogenesis in AMD, which has been further supported by pre-clinical and recent clinical studies. However, the involvement of the different complement pathways (classical, lectin, alternative), and thus the optimal complement inhibition target, has yet to be fully defined. There is evidence that C1q, the initiating molecule of the classical pathway, is a key driver of complement activity in AMD. C1q is expressed locally by infiltrating phagocytic cells and C1q-activating ligands are present at disease onset and continue to accumulate with disease progression. The accumulation of C1q on photoreceptor synapses with age and disease is consistent with its role in synapse elimination and neurodegeneration that has been observed in other neurodegenerative disorders. Furthermore, genetic deletion of C1q, local pharmacologic inhibition within the eye, or genetic deletion of downstream C4 prevents photoreceptor cell damage in mouse models. Hence, targeting the classical pathway in GA could provide a more specific therapeutic approach with potential for favorable efficacy and safety.

α-Gal Nanoparticles Mediated Homing of Endogenous Stem Cells for Repair and Regeneration of External and Internal Injuries by Localized Complement Activation and Macrophage Recruitment

This review discusses a novel experimental approach for the regeneration of original tissue structure by recruitment of endogenous stem-cells to injured sites following administration of α-gal nanoparticles, which harness the natural anti-Gal antibody. Anti-Gal is produced in large amounts in all humans, and it binds the multiple α-gal epitopes (Galα1-3Galβ1-4GlcNAc-R) presented on α-gal nanoparticles. In situ binding of anti-Gal to α-gal nanoparticles activates the complement system and generates complement cleavage chemotactic-peptides that rapidly recruit macrophages. Macrophages reaching anti-Gal coated α-gal nanoparticles bind them via Fc/Fc receptor interaction and polarize into M2 pro-reparative macrophages. These macrophages secrete various cytokines that orchestrate regeneration of the injured tissue, including VEGF inducing neo-vascularization and cytokines directing homing of stem-cells to injury sites. Homing of stem-cells is also directed by interaction of complement cleavage peptides with their corresponding receptors on the stem-cells. Application of α-gal nanoparticles to skin wounds of anti-Gal producing mice results in decrease in healing time by half. Furthermore, α-gal nanoparticles treated wounds restore the normal structure of the injured skin without fibrosis or scar formation. Similarly, in a mouse model of occlusion/reperfusion myocardial-infarction, near complete regeneration after intramyocardial injection of α-gal nanoparticles was demonstrated, whereas hearts injected with saline display ~20% fibrosis and scar formation of the left ventricular wall. It is suggested that recruitment of stem-cells following anti-Gal/α-gal nanoparticles interaction in injured tissues may result in induction of localized regeneration facilitated by conducive microenvironments generated by pro-reparative macrophage secretions and “cues” provided by the extracellular matrix in the injury site.

Zebrafish fin regeneration involves generic and regeneration-specific osteoblast injury responses

Successful regeneration requires the coordinated execution of multiple cellular responses to injury. In amputated zebrafish fins, mature osteoblasts dedifferentiate, migrate towards the injury and form proliferative osteogenic blastema cells. We show that osteoblast migration is preceded by cell elongation and alignment along the proximodistal axis, which require actomyosin, but not microtubule turnover. Surprisingly, osteoblast dedifferentiation and migration can be uncoupled. Using pharmacological and genetic interventions, we found that NF-ĸB and retinoic acid signalling regulate dedifferentiation without affecting migration, while the complement system and actomyosin dynamics affect migration but not dedifferentiation. Furthermore, by removing bone at two locations within a fin ray, we established an injury model containing two injury sites. We found that osteoblasts dedifferentiate at and migrate towards both sites, while accumulation of osteogenic progenitor cells and regenerative bone formation only occur at the distal-facing injury. Together, these data indicate that osteoblast dedifferentiation and migration represent generic injury responses that are differentially regulated and can occur independently of each other and of regenerative growth. We conclude that successful fin bone regeneration appears to involve the coordinated execution of generic and regeneration-specific responses of osteoblasts to injury.

Aging Activates the Immune System and Alters the Regenerative Capacity in the Zebrafish Heart

Age-associated organ failure and degenerative diseases have a major impact on human health. Cardiovascular dysfunction has an increasing prevalence with age and is one of the leading causes of death. In contrast to humans, zebrafish have extraordinary regeneration capacities of complex organs including the heart. In addition, zebrafish has recently become a model organism in research on aging. Here, we have compared the ventricular transcriptome as well as the regenerative capacity after cryoinjury of old and young zebrafish hearts. We identified the immune system as activated in old ventricles and found muscle organization to deteriorate upon aging. Our data show an accumulation of immune cells, mostly macrophages, in the old zebrafish ventricle. Those immune cells not only increased in numbers but also showed morphological and behavioral changes with age. Our data further suggest that the regenerative response to cardiac injury is generally impaired and much more variable in old fish. Collagen in the wound area was already significantly enriched in old fish at 7 days post injury. Taken together, these data indicate an ‘inflammaging’-like process in the zebrafish heart and suggest a change in regenerative response in the old.

Cryo-injury procedure-induced cardiac regeneration shows unique gene expression profiles in the newt Pleurodeles waltl

BACKGROUND

Cardiac regeneration in the adult mouse is not substantial. Some vertebrates, such as newts and zebrafish, regenerate the heart throughout their lives. To understand how regenerative abilities differ among animal species, comparative research has been conducted in animals like mouse, zebrafish, and newt. For those purposes, cryo-injury is suitable as an experimental model for the pathological condition of human myocardial infarction. In fact, cryo-injury procedures are common in mouse and zebrafish.

RESULTS

In the present study, we induced cryo-damage on the ventricle in Iberian ribbed newts using a liquid nitrogen-chilled probe. We observed that the injured area recovered within 8 weeks, with remodeling of scar tissue and proliferation of cardiomyocytes. We investigated the subsequent recovery of cryo-injured and amputated tissues by comparative analysis of the gene expression profiles following these two procedures.

CONCLUSION

Notably, we established a cryo-injury procedure for the newt and confirmed that regeneration of the cryo-damaged myocardial tissue is achieved by changes in gene expression that are milder than those observed in the amputation model. Our results suggest that the cryo-injury method is suitable for comparing the process of cardiac regeneration in the newt with that in other animal models. This article is protected by copyright. All rights reserved.

Biosynthesis of α-Gal Epitopes (Galα1-3Galβ1-4GlcNAc-R) and Their Unique Potential in Future α-Gal Therapies

The α-gal epitope is a carbohydrate antigen which appeared early in mammalian evolution and is synthesized in large amounts by the glycosylation enzyme α1,3galactosyltransferase (α1,3GT) in non-primate mammals, lemurs, and New-World monkeys. Ancestral Old-World monkeys and apes synthesizing α-gal epitopes underwent complete extinction 20–30 million years ago, and their mutated progeny lacking α-gal epitopes survived. Humans, apes, and Old-World monkeys which evolved from the surviving progeny lack α-gal epitopes and produce the natural anti-Gal antibody which binds specifically to α-gal epitopes. Because of this reciprocal distribution of the α-gal epitope and anti-Gal in mammals, transplantation of organs from non-primate mammals (e.g., pig xenografts) into Old-World monkeys or humans results in hyperacute rejection following anti-Gal binding to α-gal epitopes on xenograft cells. The in vivo immunocomplexing between anti-Gal and α-gal epitopes on molecules, pathogens, cells, or nanoparticles may be harnessed for development of novel immunotherapies (referred to as “α-gal therapies”) in various clinical settings because such immune complexes induce several beneficial immune processes. These immune processes include localized activation of the complement system which can destroy pathogens and generate chemotactic peptides that recruit antigen-presenting cells (APCs) such as macrophages and dendritic cells, targeting of antigens presenting α-gal epitopes for extensive uptake by APCs, and activation of recruited macrophages into pro-reparative macrophages. Some of the suggested α-gal therapies associated with these immune processes are as follows: 1. Increasing efficacy of enveloped-virus vaccines by synthesizing α-gal epitopes on vaccinating inactivated viruses, thereby targeting them for extensive uptake by APCs. 2. Conversion of autologous tumors into antitumor vaccines by expression of α-gal epitopes on tumor cell membranes. 3. Accelerating healing of external and internal injuries by α-gal nanoparticles which decrease the healing time and diminish scar formation. 4. Increasing anti-Gal–mediated protection against zoonotic viruses presenting α-gal epitopes and against protozoa, such as Trypanosoma, Leishmania, and Plasmodium, by vaccination for elevating production of the anti-Gal antibody. The efficacy and safety of these therapies were demonstrated in transgenic mice and pigs lacking α-gal epitopes and producing anti-Gal, raising the possibility that these α-gal therapies may be considered for further evaluation in clinical trials.

Near Complete Repair After Myocardial Infarction in Adult Mice by Altering the Inflammatory Response With Intramyocardial Injection of α-Gal Nanoparticles

Background: Neonatal mice, but not older mice, can regenerate their hearts after myocardial-infarction (MI), a process mediated by pro-reparative macrophages. α-Gal nanoparticles applied to skin wounds in adult-mice bind the anti-Gal antibody, activate the complement cascade and generate complement chemotactic peptides that recruit pro-reparative macrophages which are further activated by these nanoparticles. The recruited macrophages decrease wound healing time by ~50%, restore the normal skin structure and prevent fibrosis and scar formation in mice. Objectives: The objective of this study is to determine if α-gal nanoparticles injected into the reperfused myocardium after MI in adult-mice can induce myocardial repair that restores normal structure, similar to that observed in skin injuries. Methods and Results: MI was induced by occluding the mid-portion of the left anterior descending (LAD) coronary artery for 30 min. Immediately following reperfusion, each mouse received two 10 μl injections of 100 μg α-gal nanoparticles in saline into the LAD territory (n = 20), or saline for controls (n = 10). Myocardial infarct size was measured by planimetry following Trichrome staining and macrophage recruitment by hematoxylin-eosin staining. Left ventricular (LV) function was measured by echocardiography. Control mice displayed peak macrophage infiltration at 4-days, whereas treated mice had a delayed peak macrophage infiltration at 7-days. At 28-days, control mice demonstrated large transmural infarcts with extensive scar formation and poor contractile function. In contrast, mice treated with α-gal nanoparticles demonstrated after 28-days a marked reduction in infarct size (~10-fold smaller), restoration of normal myocardium structure and contractile function. Conclusions: Intramyocardial injection of α-gal nanoparticles post-MI in anti-Gal producing adult-mice results in near complete repair of the infarcted territory, with restoration of normal LV structure and contractile function. The mechanism responsible for this benefit likely involves alteration of the usual inflammatory response post-MI, as previously observed with regeneration of injured hearts in adult zebrafish, salamanders and neonatal mice.

Interleukin-11 signaling promotes cellular reprogramming and limits fibrotic scarring during tissue regeneration

Damage-induced fibrotic scarring limits tissue regeneration in mammals and is a leading cause of morbidity. In contrast, species like zebrafish can regenerate damaged tissues without excessive fibrosis. However, whether specific signaling pathways can both limit fibrosis and promote regeneration is unclear. Here, we show that interleukin-11 (Il-11)/Stat3 signaling has such a dual function. Zebrafish lacking Il-11 receptor function display severely compromised heart, fin, and scale regeneration. Deep phenotyping and transcriptional analysis of adult hearts and fins show that Il-11 signaling drives cellular reprogramming to orchestrate global and tissue-specific regenerative programs and broadly antagonizes hallmarks of adult mammalian scarring. Mechanistically, our data indicate that IL-11 signaling in endothelial cells antagonizes profibrotic transforming growth factor–β signaling and endothelial-to-mesenchymal transition, limiting scarring and promoting cardiomyocyte repopulation, after injury. Overall, our findings position damage-induced Il-11/Stat3 signaling in a key role limiting fibrosis and promoting regeneration, revealing novel targets for regenerative therapies.

A neonatal leporine model of age-dependent natural heart regeneration after myocardial infarction

OBJECTIVES

Neonatal rodents and piglets naturally regenerate the injured heart after myocardial infarction. We hypothesized that neonatal rabbits also exhibit natural heart regeneration after myocardial infarction.

METHODS

New Zealand white rabbit kits underwent sham surgery or left coronary ligation on postnatal day 1 (n = 94), postnatal day 4 (n = 11), or postnatal day 7 (n = 52). Hearts were explanted 1 day postsurgery to confirm ischemic injury, at 1 week postsurgery to assess cardiomyocyte proliferation, and at 3 weeks postsurgery to assess left ventricular ejection fraction and scar size. Data are presented as mean ± standard deviation.

RESULTS

Size of ischemic injury as a percentage of left ventricular area was similar after myocardial infarction on postnatal day 1 versus on postnatal day 7 (42.3% ± 5.4% vs 42.3% ± 4.7%, P = .9984). Echocardiography confirmed severely reduced ejection fraction at 1 day after postnatal day 1 myocardial infarction (33.7% ± 5.3% vs 65.2% ± 5.5% for postnatal day 1 sham, P = .0001), but no difference at 3 weeks after postnatal day 1 myocardial infarction (56.0% ± 4.0% vs 58.0% ± 3.3% for postnatal day 1 sham, P = .2198). Ejection fraction failed to recover after postnatal day 4 myocardial infarction (49.2% ± 1.8% vs 58.5% ± 5.8% for postnatal day 4 sham, P = .0109) and postnatal day 7 myocardial infarction (39.0% ± 7.8% vs 60.2% ± 5.0% for postnatal day 7 sham, P < .0001). At 3 weeks after infarction, fibrotic scar represented 5.3% ± 1.9%, 14.3% ± 4.9%, and 25.4% ± 13.3% of the left ventricle area in the postnatal day 1, postnatal day 4, and postnatal day 7 groups, respectively. An increased proportion of peri-infarct cardiomyocytes expressed Ki67 (15.9% ± 1.8% vs 10.2% ± 0.8%, P = .0039) and aurora B kinase (4.0% ± 0.9% vs 1.5% ± 0.6%, P = .0088) after postnatal day 1 myocardial infarction compared with sham, but no increase was observed after postnatal day 7 myocardial infarction.

CONCLUSIONS

A neonatal leporine myocardial infarction model reveals that newborn rabbits are capable of age-dependent natural heart regeneration.

Identification of Independent and Communal Differentially Expressed Genes as Well as Potential Therapeutic Targets in Ischemic Heart Failure and Non-Ischemic Heart Failure

Background

Heart failure (HF) is a rapidly growing public health problem, and its two main etiological types are non-ischemic heart failure (NIHF) and ischemic heart failure (IHF). However, the independent and common mechanisms of NIHF and IHF have not been fully elucidated. Here, bioinformatic analysis was used to characterize the difference and independent pathways for IHF and NIHF, and more importantly, to unearth the common potential markers and therapeutic targets in IHF and NIHF.

Methods

Two data sets with accession numbers GSE26887 and GSE84796 were downloaded from the Gene Expression Omnibus (GEO) database. After identifying the independent and communal DEGs of NIHF and IHF, a functional annotation, protein-protein interaction (PPI) network analysis, co-expression and drug-gene interaction prediction analysis, and mRNA-miRNA regulatory network analysis were performed for DEGs.

Results

We found 1146 independent DEGs (DEGs2) of NIHF mainly enriched in transcription-related and 2595 independent DEGs (DEGs3) of IHF mainly enriched in immune-related. Moreover, 185 communal DEGs (DEGs1) were found between NIHF and IHF, including 93 upregulated genes and 92 downregulated genes. Pathway enrichment analysis results showed that GPCR pathways and biological processes are closely related to the occurrence of HF. In addition, three hub genes were identified from PPI network, including CCL5, C5 and TLR3.

Conclusion

The identification of DEGs and hub genes in this study contributes to a novel perception for potential functional mechanisms and biomarkers or therapeutic targets in NIHF and IHF.

Cardiac regenerative capacity: an evolutionary afterthought?

Cardiac regeneration is the outcome of the highly regulated interplay of multiple processes, including the inflammatory response, cardiomyocyte dedifferentiation and proliferation, neovascularization and extracellular matrix turnover. Species-specific traits affect these injury-induced processes, resulting in a wide variety of cardiac regenerative potential between species. Indeed, while mammals are generally considered poor regenerators, certain amphibian and fish species like the zebrafish display robust regenerative capacity post heart injury. The species-specific traits underlying these differential injury responses are poorly understood. In this review, we will compare the injury induced processes of the mammalian and zebrafish heart, describing where these processes overlap and diverge. Additionally, by examining multiple species across the animal kingdom, we will highlight particular traits that either positively or negatively affect heart regeneration. Last, we will discuss the possibility of overcoming regeneration-limiting traits to induce heart regeneration in mammals.

Navigating the Crossroads of Cell Therapy and Natural Heart Regeneration

Cardiovascular disease remains the leading cause of death worldwide despite significant advances in our understanding of the disease and its treatment. Consequently, the therapeutic potential of cell therapy and induction of natural myocardial regeneration have stimulated a recent surge of research and clinical trials aimed at addressing this challenge. Recent developments in the field have shed new light on the intricate relationship between inflammation and natural regeneration, an intersection that warrants further investigation.

A cross-species analysis of systemic mediators of repair and complex tissue regeneration

Regeneration is an elegant and complex process informed by both local and long-range signals. Many current studies on regeneration are largely limited to investigations of local modulators within a canonical cohort of model organisms. Enhanced genetic tools increasingly enable precise temporal and spatial perturbations within these model regenerators, and these have primarily been applied to cells within the local injury site. Meanwhile, many aspects of broader spatial regulators of regeneration have not yet been examined with the same level of scrutiny. Recent studies have shed important insight into the significant effects of environmental cues and circulating factors on the regenerative process. These observations highlight that consideration of more systemic and possibly more broadly acting cues will also be critical to fully understand complex tissue regeneration. In this review, we explore the ways in which systemic cues and circulating factors affect the initiation of regeneration, the regenerative process, and its outcome. As this is a broad topic, we conceptually divide the factors based on their initial input as either external cues (for example, starvation and light/dark cycle) or internal cues (for example, hormones); however, all of these inputs ultimately lead to internal responses. We consider studies performed in a diverse set of organisms, including vertebrates and invertebrates. Through analysis of systemic mediators of regeneration, we argue that increased investigation of these "systemic factors" could reveal novel insights that may pave the way for a diverse set of therapeutic avenues.

Attenuating ischemia/reperfusion injury in rat cardiac transplantation by intracoronary infusion with siRNA cocktail solution

Tumor necrosis factor-α (TNF-α), caspase-8, and complement component 5a receptor (C5aR) are known to play a crucial role in the myocardial ischemia/reperfusion (I/R) injury in cardiac transplantation. We hypothesized that the intracoronary infusion of TNF-α, caspase-8, and C5aR small interfering RNAs (siRNA) would protect cardiac allograft function and improve graft survival from I/R injury-induced organ failure. I/R injury of cardiac allograft was induced by syngeneic rat cardiac transplantation, in which the transplanted hearts were infused with saline or different amounts of siRNA cocktail solution targeting TNF-α, caspase-8, and C5aR via coronary arteries, and subsequently subjected to 18 h of preservation at 4°C in histidine–tryptophan–ketoglutarate (HTK) solution. The effects of siRNA cocktail solution on prolonged cold I/R injury were determined by assessing graft survival, histopathological changes, myeloperoxidase (MPO) activity, and malondialdehyde (MDA) concentration. The perfused siRNA cocktail solution successfully knocked down the expression of TNF-α, caspase-8, and C5aR in vitro and in vivo. Approximately 91.7% of control hearts that underwent 18 h of cold ischemia ceased their function after transplantation; however, 87.5% of cardiac allografts from the highest dose siRNA cocktail solution-pretreated hearts survived >14 days and exhibited minimal histological changes, with minimal cellular infiltration, interstitial edema, and inflammation and maximal reduced MPO activity and MDA concentration in the cardiac allograft. We demonstrated the feasibility and efficiency of infusion of TNF-α, caspase-8, and C5aR siRNA via the intracoronary route as a promising strategy for gene silencing against I/R injury in cardiac transplantation.

Immunomodulation for optimal cardiac regeneration: insights from comparative analyses

Despite decades of research, regeneration of the infarcted human heart remains an unmet ambition. A significant obstacle facing experimental regenerative therapies is the hostile immune response which arises following a myocardial infarction (MI). Upon cardiac damage, sterile inflammation commences via the release of pro-inflammatory meditators, leading to the migration of neutrophils, eosinophils and monocytes, as well as the activation of local vascular cells and fibroblasts. This response is amplified by components of the adaptive immune system. Moreover, the physical trauma of the infarction and immune-mediated tissue injury provides a supply of autoantigens, perpetuating a cycle of autoreactivity, which further contributes to adverse remodelling. A gradual shift towards an immune-resolving environment follows, culminating in the formation of a collagenous scar, which compromises cardiac function, ultimately driving the development of heart failure. Comparing the human heart with those of animal models that are capable of cardiac regeneration reveals key differences in the innate and adaptive immune responses to MI. By modulating key immune components to better resemble those of regenerative species, a cardiac environment may be established which would, either independently or via the synergistic application of emerging regenerative therapies, improve functional recovery post-MI.

Research progress on myocardial regeneration: what is new?

The regeneration capacity of cardiomyocytes (CMs) is retained in neonatal mouse hearts but is limited in adult mouse hearts. Myocardial infarction (MI) in adult hearts usually leads to the loss of large amounts of cardiac tissue, and then accelerates the process of cardiac remodeling and heart failure. Therefore, it is necessary to explore the potential mechanisms of CM regeneration in the neonates and develop potential therapies aimed at promoting CM regeneration and cardiac repair in adults. Currently, studies indicate that a number of mechanisms are involved in neonatal endogenous myocardial regeneration, including cell cycle regulators, transcription factors, non-coding RNA, signaling pathways, acute inflammation, hypoxia, protein kinases, and others. Understanding the mechanisms of regeneration in neonatal CMs after MI provides theoretical support for the studies related to the promotion of heart repair after MI in adult mammals. However, several difficulties in the study of CM regeneration still need to be overcome. This article reviews the potential mechanisms of endogenous CM regeneration in neonatal mouse hearts and discusses possible therapeutic targets and future research directions.

Unlocking the Secrets of the Regenerating Fish Heart: Comparing Regenerative Models to Shed Light on Successful Regeneration

The adult human heart cannot repair itself after injury and, instead, forms a permanent fibrotic scar that impairs cardiac function and can lead to incurable heart failure. The zebrafish, amongst other organisms, has been extensively studied for its innate capacity to repair its heart after injury. Understanding the signals that govern successful regeneration in models such as the zebrafish will lead to the development of effective therapies that can stimulate endogenous repair in humans. To date, many studies have investigated cardiac regeneration using a reverse genetics candidate gene approach. However, this approach is limited in its ability to unbiasedly identify novel genes and signalling pathways that are essential to successful regeneration. In contrast, drawing comparisons between different models of regeneration enables unbiased screens to be performed, identifying signals that have not previously been linked to regeneration. Here, we will review in detail what has been learnt from the comparative approach, highlighting the techniques used and how these studies have influenced the field. We will also discuss what further comparisons would enhance our knowledge of successful regeneration and scarring. Finally, we focus on the Astyanax mexicanus, an intraspecies comparative fish model that holds great promise for revealing the secrets of the regenerating heart.

A Multi-Network Comparative Analysis of Transcriptome and Translatome Identifies Novel Hub Genes in Cardiac Remodeling

Our understanding of the transition from physiological to pathological cardiac hypertrophy remains elusive and largely based on reductionist hypotheses. Here, we profiled the translatomes of 15 mouse hearts to provide a molecular blueprint of altered gene networks in early cardiac remodeling. Using co-expression analysis, we showed how sub-networks are orchestrated into functional modules associated with pathological phenotypes. We discovered unappreciated hub genes, many undocumented for their role in cardiac hypertrophy, and genes in the transcriptional network that were rewired in the translational network, and associated with semantically different subsets of enriched functional terms, such as Fam210a, a novel musculoskeletal modulator, or Psmd12, implicated in protein quality control. Using their correlation structure, we found that transcriptome networks are only partially reproducible at the translatome level, providing further evidence of post-transcriptional control at the level of translation. Our results provide novel insights into the complexity of the organization of in vivo cardiac regulatory networks.

β-Catenin drives distinct transcriptional networks in proliferative and nonproliferative cardiomyocytes

The inability of the adult mammalian heart to regenerate represents a fundamental barrier in heart failure management. By contrast, the neonatal heart retains a transient regenerative capacity, but the underlying mechanisms for the developmental loss of cardiac regenerative capacity in mammals are not fully understood. Wnt/β-catenin signalling has been proposed as a key cardioregenerative pathway driving cardiomyocyte proliferation. Here, we show that Wnt/β-catenin signalling potentiates neonatal mouse cardiomyocyte proliferation in vivo and immature human pluripotent stem cell-derived cardiomyocyte (hPSC-CM) proliferation in vitro By contrast, Wnt/β-catenin signalling in adult mice is cardioprotective but fails to induce cardiomyocyte proliferation. Transcriptional profiling and chromatin immunoprecipitation sequencing of neonatal mouse and hPSC-CMs revealed a core Wnt/β-catenin-dependent transcriptional network governing cardiomyocyte proliferation. By contrast, β-catenin failed to re-engage this neonatal proliferative gene network in the adult heart despite partial transcriptional re-activation of a neonatal glycolytic gene programme. These findings suggest that β-catenin might be repurposed from regenerative to protective functions in the adult heart in a developmental process dependent on the metabolic status of cardiomyocytes.

Complement activation links inflammation to dental tissue regeneration

OBJECTIVES

Complement is an efficient plasma immune surveillance system. It initiates inflammation by inducing vascular modifications and attracting immune cells expressing Complement receptors. Investigating Complement receptors in non-immune cells pointed out Complement implication in the regeneration of tissue such as liver, skin, or bone. This review will shed the light on Complement implication in the initial steps of dental tissue regeneration.

MATERIALS AND METHODS

Review of literature was conducted on Complement local expression and implication in oral tissue regeneration in vivo and in vitro.

RESULTS

Recent data reported expression of Complement receptors and soluble proteins in dental tissues. Cultured pulp fibroblasts secrete all Complement components. Complement C3b and MAC have been shown to control bacteria growth in the dental pulp while C3a and C5a are involved in the initial steps of pulp regeneration. Indeed, C3a induces pulp stem cell/fibroblast proliferation, and fibroblast recruitment, while C5a induces neurite growth, guides stem cell recruitment, and odontoblastic differentiation. Similarly, cultured periodontal ligament cells produce C5a which induces bone marrow mesenchymal stem cell recruitment.

CONCLUSIONS

Overall, this review highlights that local Complement synthesis in dental tissues plays a major role, not only in eliminating bacteria but also in the initial steps of dental tissue regeneration, thus providing a link between dental tissue inflammation and regeneration.

CLINICAL RELEVANCE

Complement provides an explanation for understanding why inflammation preceeds regeneration. This may also provide a biological rational for understanding the reported success conservative management of mature permanent teeth with carious pulp exposure.

Large-scale variation in single nucleotide polymorphism density within the laboratory axolotl (Ambystoma mexicanum)

BACKGROUND

Recent efforts to assemble and analyze the Ambystoma mexicanum genome have dramatically improved the potential to develop molecular tools and pursue genome-wide analyses of genetic variation.

RESULTS

To better resolve the distribution and origins of genetic variation with A mexicanum, we compared DNA sequence data for two laboratory A mexicanum and one A tigrinum to identify 702 million high confidence polymorphisms distributed across the 32 Gb genome. While the wild-caught A tigrinum was generally more polymorphic in a genome-wide sense, several multi-megabase regions were identified from A mexicanum genomes that were actually more polymorphic than A tigrinum. Analysis of polymorphism and repeat content reveals that these regions likely originated from the intentional hybridization of A mexicanum and A tigrinum that was used to introduce the albino mutation into laboratory stocks.

CONCLUSIONS

Our findings show that axolotl genomes are variable with respect to introgressed DNA from a highly polymorphic species. It seems likely that other divergent regions will be discovered with additional sequencing of A mexicanum. This has practical implications for designing molecular probes and suggests a need to study A mexicanum phenotypic variation and genome evolution across the tiger salamander clade.

Zebrafish as a model for inflammation and drug discovery

Zebrafish is a small teleost (bony) fish used in many areas of pharmacology and toxicology. This animal model has advantages for the discovery of anti-inflammatory drugs, such as the potential for real-time assessment of cell migration mechanisms. Additionally, zebrafish display a repertoire of inflammatory cells, mediators, and receptors that are similar to those in mammals, including humans. Inflammatory disease modeling in either larvae or adult zebrafish represents a promising tool for the screening of new anti-inflammatory compounds, contributing to our understanding of the mechanisms involved in chronic inflammatory conditions. In this review, we provide an overview of the characterization of inflammatory responses in zebrafish, emphasizing its relevance for drug discovery in this research area.

Immunity in salamander regeneration: Where are we standing and where are we headed?

Salamanders exhibit the most extensive regenerative repertoire among vertebrates, being able to accomplish scar-free healing and faithful regeneration of significant parts of the eye, heart, brain, spinal cord, jaws and gills, as well as entire appendages throughout life. The cellular and molecular mechanisms underlying salamander regeneration are currently under extensive examination, with the hope of identifying the key drivers in each context, understanding interspecies differences in regenerative capacity, and harnessing this knowledge in therapeutic settings. The immune system has recently emerged as a potentially critical player in regenerative responses. Components of both innate and adaptive immunity have been found at critical stages of regeneration in a range of salamander tissues. Moreover, functional studies have identified a requirement for macrophages during heart and limb regeneration. However, our knowledge of salamander immunity remains scarce, and a thorough definition of the precise roles played by its members is lacking. Here, we examine the evidence supporting roles for immunity in various salamander regeneration models. We pinpoint observations that need revisiting through modern genetic approaches, uncover knowledge gaps, and highlight insights from various model organisms that could guide future explorations toward an understanding of the functions of immunity in regeneration.

Analysis of mesenchymal stem cell proteomes in situ in the ischemic heart

Rationale: Cell therapy for myocardial infarction is promising but largely unsuccessful in part due to a lack of mechanistic understanding. Techniques enabling identification of stem cell-specific proteomes in situ in the injured heart may shed light on how the administered cells respond to the injured microenvironment and exert reparative effects.

Objective: To identify the proteomes of the transplanted mesenchymal stem cells (MSCs) in the infarcted myocardium, we sought to target a mutant methionyl-tRNA synthetase (MetRS^L274G) in MSCs, which charges azidonorleucine (ANL), a methionine analogue and non-canonical amino acid, to tRNA and subsequently to nascent proteins, permitting isolation of ANL-labeled MSC proteomes from ischemic hearts by ANL-alkyne based click reaction.

Methods and Results: Murine MSCs were transduced with lentivirus MetRS^L274G and supplemented with ANL; the ANL-tagged nascent proteins were visualized by bio-orthogonal non-canonical amino-acid tagging, spanning all molecular weights and by fluorescent non-canonical amino-acid tagging, displaying strong fluorescent signal. Then, the MetRS^L274G-transduced MSCs were administered to the infarcted or Sham heart in mice receiving ANL treatment. The MSC proteomes were isolated from the left ventricular protein lysates by click reaction at days 1, 3, and 7 after cell administration, identified by LC/MS. Among all identified proteins (in Sham and MI hearts, three time-points each), 648 were shared by all 6 groups, accounting for 82±5% of total proteins in each group, and enriched under mitochondrion, extracellular exosomes, oxidation-reduction process and poly(A) RNA binding. Notably, 26, 110 and 65 proteins were significantly up-regulated and 11, 28 and 19 proteins were down-regulated in the infarcted vs. Sham heart at the three time-points, respectively; these proteins are pronounced in the GO terms of extracellular matrix organization, response to stress and regulation of apoptotic process and in the KEGG pathways of complements and coagulation cascades, apoptosis, and regulators of actin cytoskeleton.

Conclusions: MetRS^L274G expression allows successful identification of MSC-specific nascent proteins in the infarcted hearts, which reflect the functional states, adaptive response, and reparative effects of MSCs that may be leveraged to improve cardiac repair.

Single-cell analysis of murine fibroblasts identifies neonatal to adult switching that regulates cardiomyocyte maturation

Cardiac maturation lays the foundation for postnatal heart development and disease, yet little is known about the contributions of the microenvironment to cardiomyocyte maturation. By integrating single-cell RNA-sequencing data of mouse hearts at multiple postnatal stages, we construct cellular interactomes and regulatory signaling networks. Here we report switching of fibroblast subtypes from a neonatal to adult state and this drives cardiomyocyte maturation. Molecular and functional maturation of neonatal mouse cardiomyocytes and human embryonic stem cell-derived cardiomyocytes are considerably enhanced upon co-culture with corresponding adult cardiac fibroblasts. Further, single-cell analysis of in vivo and in vitro cardiomyocyte maturation trajectories identify highly conserved signaling pathways, pharmacological targeting of which substantially delays cardiomyocyte maturation in postnatal hearts, and markedly enhances cardiomyocyte proliferation and improves cardiac function in infarcted hearts. Together, we identify cardiac fibroblasts as a key constituent in the microenvironment promoting cardiomyocyte maturation, providing insights into how the manipulation of cardiomyocyte maturity may impact on disease development and regeneration.

A Systematic Exposition of Methods used for Quantification of Heart Regeneration after Apex Resection in Zebrafish

Myocardial infarction (MI) is a worldwide condition that affects millions of people. This is mainly caused by the adult human heart lacking the ability to regenerate upon injury, whereas zebrafish have the capacity through cardiomyocyte proliferation to fully regenerate the heart following injury such as apex resection (AR). But a systematic overview of the methods used to evidence heart regrowth and regeneration in the zebrafish is lacking. Herein, we conducted a systematical search in Embase and Pubmed for studies on heart regeneration in the zebrafish following injury and identified 47 AR studies meeting the inclusion criteria. Overall, three different methods were used to assess heart regeneration in zebrafish AR hearts. 45 out of 47 studies performed qualitative (37) and quantitative (8) histology, whereas immunohistochemistry for various cell cycle markers combined with cardiomyocyte specific proteins was used in 34 out of 47 studies to determine cardiomyocyte proliferation qualitatively (6 studies) or quantitatively (28 studies). For both methods, analysis was based on selected heart sections and not the whole heart, which may bias interpretations. Likewise, interstudy comparison of reported cardiomyocyte proliferation indexes seems complicated by distinct study designs and reporting manners. Finally, six studies performed functional analysis to determine heart function, a hallmark of human heart injury after MI. In conclusion, our data implies that future studies should consider more quantitative methods eventually taking the 3D of the zebrafish heart into consideration when evidencing myocardial regrowth after AR. Furthermore, standardized guidelines for reporting cardiomyocyte proliferation and sham surgery details may be considered to enable inter study comparisons and robustly determine the effect of given genes on the process of heart regeneration.

Integrative Analysis of Axolotl Gene Expression Data from Regenerative and Wound Healing Limb Tissues

Axolotl (Ambystoma mexicanum) is a urodele amphibian endowed with remarkable regenerative capacities manifested in scarless wound healing and restoration of amputated limbs, which makes it a powerful experimental model for regenerative biology and medicine. Previous studies have utilized microarrays and RNA-Seq technologies for detecting differentially expressed (DE) genes in different phases of the axolotl limb regeneration. However, sufficient consistency may be lacking due to statistical limitations arising from intra-laboratory analyses. This study aims to bridge such gaps by performing an integrative analysis of publicly available microarray and RNA-Seq data from axolotl limb samples having comparable study designs using the "merging" method. A total of 351 genes were found DE in regenerative samples compared to the control in data of both technologies, showing an adjusted p-value < 0.01 and log fold change magnitudes >1. Downstream analyses illustrated consistent correlations of the directionality of DE genes within and between data of both technologies, as well as concordance with the literature on regeneration related biological processes. qRT-PCR analysis validated the observed expression level differences of five of the top DE genes. Future studies may benefit from the utilized concept and approach for enhanced statistical power and robust discovery of biomarkers of regeneration.

Perspectives on Directions and Priorities for Future Preclinical Studies in Regenerative Medicine

The myocardium consists of numerous cell types embedded in organized layers of ECM (extracellular matrix) and requires an intricate network of blood and lymphatic vessels and nerves to provide nutrients and electrical coupling to the cells. Although much of the focus has been on cardiomyocytes, these cells make up <40% of cells within a healthy adult heart. Therefore, repairing or regenerating cardiac tissue by merely reconstituting cardiomyocytes is a simplistic and ineffective approach. In fact, when an injury occurs, cardiac tissue organization is disrupted at the level of the cells, the tissue architecture, and the coordinated interaction among the cells. Thus, reconstitution of a functional tissue must reestablish electrical and mechanical communication between cardiomyocytes and restore their surrounding environment. It is also essential to restore distinctive myocardial features, such as vascular patency and pump function. In this article, we review the current status, challenges, and future priorities in cardiac regenerative or reparative medicine. In the first part, we provide an overview of our current understanding of heart repair and comment on the main contributors and mechanisms involved in innate regeneration. A brief section is dedicated to the novel concept of rejuvenation or regeneration, which we think may impact future development in the field. The last section describes regenerative therapies, where the most advanced and disruptive strategies used for myocardial repair are discussed. Our recommendations for priority areas in studies of cardiac regeneration or repair are summarized in Tables 1 and 2 .