npj Regenerative Medicine

Extracellular matrix (ECM) scaffolds induce type 2 immunity to promote repair. Here, we show that immune cells recruited to ECM-treated murine muscle injuries and clinical soft tissue defects express immune checkpoints. Specifically, TH2 cells and regulatory T cells (Tregs) increase LAG3 expression, while macrophages express PDL2. TCR analysis and a triple-reporter strain for interleukin (IL)-13 and Treg fate-mapping suggest that Tregs in ECM-treated wounds transition into TH2-like exTregs that express LAG3. Immune checkpoint inhibition (ICI) significantly stimulated type 2 immunity in ECM-treated wounds, including increased TH2 cells, Treg transition to TH2-like exTregs, and pro-regenerative macrophages. Moreover, ICI enhanced muscle repair and reduced fibrosis in ECM-treated wounds. Collectively, these findings show Treg/TH2 plasticity in wound healing and introduce a novel ICI application to enhance immune-mediated regeneration.

During wounding and material implantation there is a disturbance in tissue homeostasis and release of self-antigen, and regulation between tolerance and auto-inflammation in injury is not well understood. Here, we analyzed antigen-presenting cells in biomaterial-treated muscle injury and found that pro-regenerative materials enrich Batf3-dependent CD103+XCR1+CD301b+ dendritic cells associated with cross-presentation and self-tolerance. Muscle trauma was accompanied by CD8+ iTregs and expansion of CD103+XCR1+CD62L- adaptive immune cells. Up-regulation of E-Cadherin (the ligand for CD103) and XCL-1 in injured tissue suggests a mechanism for cell recruitment to trauma. Without cross-presenting cells T cell activation increases, pro-regenerative macrophage polarization decreases, and muscle healing is impaired. These data describe a regulatory communication network through CD103+XCR1+ immune cells resulting in downstream effects on tissue regeneration.

Achilles tendon rupture is a common sports-related tendon injury. Even with advanced clinical treatments, many patients suffer from long-term pain and reduced function. These unsatisfactory outcomes result primarily from an imbalanced injury response with excessive inflammation and inadequate regeneration. Prior studies showed that extracellular vesicles from inflammation-primed adipose-derived stem cells (iEVs) can attenuate inflammation in the early phase of tendon healing. However, the effect of iEVs on tendon inflammation and regeneration in the later phases of tendon healing and the underlying mechanism remain to be determined. Accordingly, this study investigated the mechanistic roles of iEVs in regulating tendon response to injury using a mouse Achilles tendon injury and repair model in vivo and iEV-macrophage and iEV-tendon cell co-culture models in vitro. Results showed that iEVs promoted tendon anti-inflammatory gene expression and reduced mononuclear cell infiltration in the remodeling phase of tendon healing. iEVs also increased injury site collagen deposition and promoted tendon structural recovery. As such, mice treated with iEVs showed less peritendinous scar formation, much lower incidence of postoperative tendon gap or rupture, and faster functional recovery compared to untreated mice. Further in vitro study revealed that iEVs both inhibited macrophage inflammatory response and increased tendon cell proliferation and collagen production. The iEV effects were partially mediated by miR-147-3p, which blocks the toll-like receptor 4/NF-{kappa}B signaling pathway that activates macrophage M1 polarization. The combined results demonstrated that iEVs are a promising therapeutic agent, which can enhance tendon repair by attenuating inflammation and promoting intrinsic healing. Significance statementUsing a clinically relevant mouse Achilles tendon injury and repair model, this study revealed that iEVs, a biological product generated from inflammation-primed adipose-derived stem cells, can directly target both macrophages and tendon cells and enhance tendon structural and functional recovery by limiting inflammation and promoting intrinsic healing. Results further identified miR-147-3p as one of the active components of iEVs that modulate macrophage inflammatory response by inhibiting toll-like receptor 4/NF-{kappa}B signaling pathway. These promising findings paved the road toward clinical application of iEVs in the treatment of tendon injury and other related disorders.

BackgroundRapid closure of open wound, either temporarily or perpetually, is recognized as the standard of care in patients with thermal burns. Human cadaveric allograft and simple genetically modified porcine xenografts are not able to provide enough durable time for extensively burned patients. A selective germline genome edited pig (SGGEP) skin xenograft, Xeno X skin, would be a valuable candidate to the clinical options. MethodsIn an ongoing investigator-initiated clinical trial in patients with thermal burns, the efficacy and safety of cryopreserved Xeno X skin grafts of SGGEP for burned patients were evaluated. Each patient received surgical grafting with a skin xenotransplant and wild type pig extracellular matrix (wpECM) in a side-by-side manner for in-situ comparison. The primary outcome measures of xeno-skin grafts included Xeno X skin safety and tolerability, as well as the quality and duration of temporary barrier function yielded by Xeno X skin grafts (as determined by Baux score). Seven parameters included in the analysis were vascularization, pigmentation, thickness, relief, pliability, surface area and the overall opinion, with each calculated on an independent 0-10 scale. ResultsA total of 16 burned patients completed the trial. All the patients tolerated Xeno X skin grafts well and no advent events were observed. In all cases, Xeno X skin grafts were vascularized and fully adherent, they also exhibited better overall outcomes than those of wpECM. Xeno X skin grafts survived for at least 25 days without a need of any immunosuppressive drug, well consistent with our earlier preclinical studies in non-human primates. ConclusionXeno X skin grafts of SGGEP did not incur any signs of local and systemic safety issues, and in the meanwhile provided a high quality and long duration of temporary barrier function for burned patients. This is a major milestone in the xenotransplant field, indicating that genome-edited organ xenotransplant has become a clinical reality.

Excess systemic inflammation can often be lethal in septic and trauma patients due to onset of multiple organ dysfunction syndrome (MODS). As of right now, there are no effective immunomodulatory therapeutics that can promote survival within this patient population. Pro-regenerative extracellular matrix (ECM) biomaterials have shown success for treatment of local inflammation but have not been fully explored for treating systemic inflammation. Here, we demonstrate efficacy of an intravenously delivered infusible ECM (iECM) material, which promotes increased survival in a murine model of MODS by decreasing systemic mediators of inflammation. Lung and kidney failure are associated with higher mortality in MODS compared to other organ failures, and we demonstrate that iECM localizes primarily to kidney and lung tissues during systemic inflammation induced by endotoxin. iECM successfully lowered vascular permeability within lung tissue and lowered levels of inflammatory cytokine signaling such as IL-6, verified via ELISA and gene expression analyses. We also demonstrated that immune cell infiltration into lung tissue was modulated with iECM treatment, with an increase in neutrophil retention in the lung and decreases in pro-inflammatory macrophage presence. In summation, iECM improves survival from severe systemic inflammation by decreasing the local and systemic inflammatory signaling pathways that contribute to MODS. These results provide a strong rationale for translational studies of iECM treatment in systemic inflammatory syndromes, including sepsis and trauma.

Decellularized extracellular matrix (ECM) hydrogels present a novel, clinical intervention for a myriad of regenerative medicine applications. The source of ECM is typically the same tissue to which the treatment is applied; however, the need for tissue specific ECM sources has not been rigorously studied. We hypothesized that tissue specific ECM would improve regeneration through preferentially stimulating physiologically relevant processes (e.g. progenitor cell proliferation and differentiation). One of two decellularized hydrogels (tissue specific skeletal muscle or non mesoderm-derived lung) or saline were injected intramuscularly two days after notexin injection in mice (n=7 per time point) and muscle was harvested at days 5 and 14 for histological and gene expression analysis. Both injectable hydrogels were decellularized using the same detergent and were controlled for donor characteristics (i.e. species, age). At day 5, the skeletal muscle ECM hydrogel significantly increased the density of Pax7+ satellite cells in the muscle. Gene expression analysis at day 5 showed that skeletal muscle ECM hydrogels increased expression of genes implicated in muscle contractility. By day 14, skeletal muscle ECM hydrogels improved muscle regeneration over saline and lung ECM hydrogels as shown through a shift in fiber cross sectional area distribution towards larger fibers. This data indicates a potential role for muscle-specific regenerative capacity of decellularized, injectable muscle hydrogels. Further transcriptomic analysis of whole muscle mRNA indicates the mechanism of tissue specific ECM-mediated tissue repair may be immune and metabolism pathway-driven. Taken together, this suggests there is benefit in using tissue specific ECM for regenerative medicine applications.Competing Interest StatementKLC is co-founder, board member, consultant, receives income, and has equity interest in Ventrix, Inc.View Full Text

With age and disease, skeletal muscle is progressively lost and replaced by fibrotic scar and intramuscular adipose tissue (IMAT). While strongly correlated, it remains unclear whether IMAT has a functional impact on muscle. In the present study, we evaluated the effects of IMAT during muscle injury by creating a mouse model where the cellular origin of IMAT, fibro/adipogenic progenitors (FAPs), are prevented from differentiating into adipocytes (FATBLOCK model). We found that blocking IMAT after an adipogenic injury allowed muscle to regenerate more efficiently, resulting in enhanced function. Our data explain why acute muscle injuries featuring IMAT infiltration, such as rotator cuff tears and acute denervation injuries, exhibit poor regeneration and lead to a loss in muscle function. It also demonstrates the therapeutic importance of preventing IMAT formation in acute injuries in order to maximize regeneration and minimize loss in muscle mass and function.

Volumetric muscle loss (VML) represents a critical unmet need in regenerative medicine, with no established standard of care. This study introduces a novel therapeutic strategy using tissue-specific skeletal muscle extracellular matrix (ECM) fibers fabricated using scaffold-free Anchored Cell Sheet Engineering technology. These engineered fibers replicate the native ECM composition and microarchitecture of skeletal muscle, incorporating essential structural and basement membrane proteins. In a rat VML model, engineered ECM fibers demonstrated a promising regenerative capacity compared to commercial porcine-derived small intestine submucosa (SIS) ECM. Over an 8-week period, the engineered fibers preserved muscle volume and weight, regulated inflammatory and fibrotic responses, and promoted vascularization. In contrast, SIS was rapidly degraded by week 4 and associated with excessive fibrotic response. Force recovery in the muscles treated with engineered ECM fibers was lower at the 8-week time point (77% compared to 91% in the control group), but histological and immunohistochemical analyses revealed newly formed, dispersed muscle fibers exclusively within the repaired muscle tissue treated with engineered ECM fibers. Importantly, only in cases where engineered ECM fibers were used, muscle weight was preserved, resulting in similar normalized force-to-weight recovery across all groups (87% in the test group vs. 88% in the control group). The histological analyses further demonstrated ongoing tissue remodeling, indicative of sustained regeneration, in contrast to the premature fibrotic healing observed in the other groups. A novel quantitative image analysis workflow using a custom Python script, enabled objective assessment of spatial tissue heterogeneity through histology and immunohistochemistry images, setting a new standard for tissue regeneration analysis. These findings establish engineered tissue-specific ECM fibers as a transformative approach for VML treatment and lay the groundwork for translation to clinical applications.

Neovascularization is a critical early step toward successful tissue regeneration during wound healing. While vasculature has long been recognized as highly mechanosensitive (to fluid shear, pulsatile luminal pressure, etc.), the effects of extracellular matrix strains on angiogenesis are poorly understood. Previously, we found that dynamic matrix compression in vivo potently regulated neovascular growth during tissue regeneration; however, whether matrix deformations directly regulate00 angiogenesis remained unknown. Here, we tested the effects of load initiation time, strain magnitude, and mode of compressive deformation (uniform compression vs. compressive indentation that also introduced shear stress) on neovascularization and key angiogenic and mechanotransduction signaling pathways by microvascular fragments in vitro. We hypothesized that neovascularization would be enhanced by delayed, moderate compression and inhibited by early, high magnitude compression and by compressive indentation. Consistent with our hypothesis, early, high magnitude loading inhibited vessel growth, while delayed loading enhanced vessel growth. Compressive indentation led to longer, more branched networks than uniform compression - particularly at high strain magnitude. Gene expression was differentially regulated by time of load initiation; genes associated with active angiogenic sprouts were downregulated by early loading but upregulated by delayed loading. Canonical gene targets of the YAP/TAZ mechanotransduction pathway were increased by loading and abrogated by pharmacological YAP inhibition. Together, these data demonstrate that neovascularization is directly responsive to dynamic matrix strain and is particularly sensitive to the timing of load initiation. This work further identifies putative mechanoregulatory angiogenic mechanisms and implicates a critical role for dynamic mechanical cues in vascularized tissue regeneration. Statement of SignificanceMechanical cues influence tissue regeneration, and although vasculature is known to be mechanically sensitive, remarkably little is known about the effects of bulk extracellular matrix deformation on the nascent vessel networks found in healing tissues. Here, we demonstrated that load initiation time, magnitude, and mode all regulate microvascular growth, as well as upstream angiogenic and mechanotransduction signaling pathways. Across all tested magnitudes and modes, microvascular network formation and upstream signaling were powerfully regulated by the timing of load initiation. This work provides a new foundational understanding of how extracellular matrix mechanics regulate angiogenesis and has critical implications for clinical translation of new regenerative medicine therapies and physical rehabilitation strategies designed to enhance revascularization during tissue regeneration.

As of yet, no standard of care incorporates biomaterials to treat traumatic spinal cord injury (SCI). However, intense development of biomaterials for treating SCI has focused on fabricating microscale channels to support the regrowth of axons while minimizing scar formation. We previously demonstrated that plant tissues could be decellularized and processed to form sterile, biocompatible and implantable biomaterials that support cell infiltration and vascularization in vivo. Vascularized plant tissues contain continuous microscale channels with geometries relevant for supporting neural regeneration. We hypothesized that decellularized vascular bundles would support neural regeneration and motor recovery in SCI. Sprague Dawley rats received a complete T8-T9 spinal cord transection and were implanted with acellular plant-derived scaffolds and allowed to recover over 28 weeks. Animals that received the scaffolds alone, with no other therapeutic compounds, demonstrated a significant and stable partial improvement in motor function compared to control animals as early as week 4 post-injury. Hind-limb motor function did not deteriorate over the remaining 28 weeks. Histological analysis revealed minimal astrocyte scarring at the spinal cord - scaffold interface, aligned axonal projection through the scaffolds, populations of serotonergic neurons and Schwann cells, laminin and collagen deposition and the presence of blood vessels. Axonal reconnection via the scaffold was also confirmed by Fluro-gold retrograde tracing. Taken together, our work defines a novel route for building upon naturally occurring plant microarchitectures to support the repair of the spinal cord post-injury. Notably, these results were achieved without the use of growth factors, stem/progenitor cells, or any other interventions.

Vascular graft fibrosis can cause a decrease in cellular infiltration and capillary ingrowth in vascular walls and vascular stiffening. As such, there are still no vascular grafts that can be used in blood vessels where their diameters are less than 6 mm in patients. Although various approaches have been evaluated to mitigate implant-associated fibrosis, effective treatments remain quite limited. In this study, we demonstrated that APOE was significantly increased during vascular regeneration after graft implantation in vivo. APOE knockout (KO) increased compliance of regenerated aortas and reduced extracellular matrix (ECM) deposition in adventitia of the regenerated aortas. Using single cell RNA sequencing (scRNA-seq), a subset of profibrotic macrophages was found to be involved in graft fibrosis and APOE KO limited the formation of profibrotic macrophage formation during vascular regeneration. The interaction between APOE and low-density lipoprotein receptor related protein 1 (LRP1) partially mediated fibrotic differentiation of the macrophages. Profibrotic macrophages promoted graft fibrosis mainly through secretion of insulin-like growth factor-1 (IGF-1) that could support proliferation of fibroblasts. Finally, we showed that APOE knockdown in vivo using adeno-associated virus (AAV) improved the compliance of regenerated aortas and reduced ECM deposited in the adventitial areas by limiting formation of profibrotic macrophages. Collectively, these data indicate that APOE promotes the profibrotic transition of macrophages partially through LRP1, and the profibrotic macrophages increase the proliferation of fibroblasts via IGF-1. Inhibition of APOE by AAV can alleviate graft fibrosis occurring during vascular regeneration.

Normothermic machine perfusion is an emerging preservation technique for kidney allografts to reduce post-transplant complications, including interstitial fibrosis and tubular atrophy. This technique, however, could be improved by adding antifibrotic molecules to perfusion solutions. We established Machine perfusion and Organ slices as a Platform for Ex vivo Drug delivery (MOPED), to explore fibrogenesis suppression strategies. We perfused porcine kidneys ex vivo with galunisertib--a potent inhibitor of the transforming growth factor beta signaling pathway. To determine whether effects persisted, we also cultured precision-cut tissue slices prepared from the respective kidneys. Galunisertib supplementation improved the general viability, without negatively affecting renal function or elevating levels of injury markers or byproducts of oxidative stress. Galunisertib also reduced inflammation and more importantly, strongly suppressed the onset of fibrosis, especially when the treatment was continued in slices. Our results illustrate the value of targeted drug delivery, using isolated organ perfusion, for reducing post-transplant complications. One Sentence SummaryGalunisertib supplementation during normothermic machine perfusion attenuates fibrogenesis without compromising renal function.

Generation of functional engineered myocardial tissue remains a challenge, owing in part to lacking maturity of stem cell-derived cardiomyocytes. Current strategies to mature these cells fall short of achieving in vivo-like physiology. Macrophages, members of the innate immune system, reside in the heart and exert positive effects on cardiomyocyte function. We hypothesized that developmentally informed addition of macrophages to cardiomyocytes would improve their maturity. While some recent studies have added macrophages to stem cell-derived models of the human myocardium, these previous approaches do not replicate the early colonization of the heart. Addition of macrophages to developing cardiomyocytes 8 days after the start of differentiation significantly alters cardiomyocyte behavior. We show that macrophages drive improvements in metabolic capabilities of cardiomyocytes. Developing cardiomyocytes shed lowly polarized mitochondria, adopt a new mitochondria network architecture, and develop more active mitophagy programs after >20 days coculture with macrophages. This interaction is dependent on macrophage MerTK reception of cardiomyocyte-derived mitochondria material. These results inform our understanding of the responsibility of macrophages in the development of the myocardium, and we hope that these interactions can be leveraged to produce more physiologically relevant models of the human myocardium.

Spiny mice (Acomys) can regenerate after injury with minimal fibrosis. Whether Acomys retains the fibrosis-free feature in response to implanted devices is unknown, so we implanted polydimethylsiloxane (PDMS) subcutaneously in Acomys and Mus, a non-regenerative counterpart. In Acomys, we found reduced myeloid cell infiltration, fibroblast activation, and collagen deposition around the PDMS implant. These results suggest that Acomys can regulate FBR and may hold the key to improving implant lifetime and functionality.

The efficacy of cell-based therapeutics is often compromised by host immune recognition of implanted cells and biomaterials, resulting in fibrotic encapsulation and loss of function. Here, we address this challenge with an immunomodulatory cell-based therapy, in which alginate-encapsulated retinal pigment epithelial cells continuously secrete cytokines to locally modulate the implant microenvironment. In a healthy rodent model, the localized production of interleukin-10 (IL-10) or IL-12 from encapsulated cytokine-producing cells prevented host immune rejection and fibrosis of alginate capsules. Mechanistically, treatment was associated with reduced expression of pro-fibrotic genes and immune shifts consistent with macrophage and T-cell regulation, supporting a cytokine-mediated mitigation of foreign body response. In a diabetic murine model (streptozotocin-induced C57BL/6J), co-implantation of human islets with IL-10-producing cells attenuated pericapsular fibrosis, preserved islet viability, and restored normoglycemia for up to 100 days (4.76 times longer than islets alone). Significantly, IL-10-producing cells were also effective in enabling the durability and function of encapsulated cells in a healthy non-human primate, showing translational feasibility. Collectively, these findings suggest that localized cytokine delivery can reduce fibrotic encapsulation and support durable graft function, offering a path to lessen reliance on systemic immunosuppression in islets transplantation and other implantable biomaterial therapies. TeaserEncapsulated IL-10-producing cells locally suppress fibrosis and extend graft function in rodent models and a non-human primate.

Patients born with microtia, the congenital malformation of the external ear, face substantial psychosocial strain. Current reconstruction relies on harvesting rib cartilage, an invasive procedure associated with donor site morbidity and unnaturally stiff ears due to the use of hyaline cartilage. Tissue engineered auricles could overcome these drawbacks by providing patient-specific elastic cartilage without the need for rib harvest. Yet, key challenges such as fibrocartilage formation, inhomogeneous extracellular matrix formation and mechanical inferiority during ex vivo maturation remain, often leading to graft deformation and degradation in vivo. To address this gap, we integrated approaches maintaining the chondrogenic potential with growth factors, promoting elastic cartilage formation through stress-relaxing materials, and achieving homogeneous maturation by culturing grafts on an elevated bioreactor platform that enables uniform nutrient diffusion, using primary human auricular chondrocytes. Together these approaches resulted in the maturation of bioprinted auricular grafts that closely resemble native human auricular cartilage, demonstrated by the uniform distribution of elastin, glycosaminoglycans, and collagen II, and lack of collagen I. RNA sequencing revealed gene expression patterns consistent with the transition from fibrocartilage towards elastic cartilage. On a functional level, grafts achieved a compressive modulus of 1.1{+/-}0.03 MPa, matching that of native human auricular cartilage (1.0{+/-}0.1 MPa) and maintained their structural integrity for 6 weeks in a subcutaneous rat model, where they transitioned towards mature elastic fibers. These grafts represent the closest approximation of native elastic cartilage achieved ex vivo to date, bringing the field closer to a clinically viable, long-term therapy for children affected by microtia. One-sentence summaryBioprinted auricular grafts develop native-like elastic cartilage and advance towards a durable therapy for children with microtia.

Despite numerous advances in our understanding of zebrafish cardiac regeneration, an aspect that remains less studied is how regenerating cardiomyocytes invade, and eventually replace, the collagen-containing fibrotic tissue following injury. Here, we provide an in-depth analysis of the process of cardiomyocyte invasion using live-imaging and histological approaches. We observed close interactions between protruding cardiomyocytes and macrophages at the wound border zone, and macrophage-deficient irf8 mutant zebrafish exhibited defects in extracellular matrix (ECM) remodeling and cardiomyocyte protrusion into the injured area. Using a resident macrophage ablation model, we show that defects in ECM remodeling at the border zone and subsequent cardiomyocyte protrusion can be partly attributed to a population of resident macrophages. Single-cell RNA-sequencing analysis of cells at the wound border revealed a population of cardiomyocytes and macrophages with fibroblast-like gene expression signatures, including the expression of genes encoding ECM structural proteins and ECM-remodeling proteins. The expression of mmp14b, which encodes a membrane-anchored matrix metalloproteinase, was restricted to cells in the border zone, including cardiomyocytes, macrophages, fibroblasts, and endocardial/endothelial cells. Genetic deletion of mmp14b led to a decrease in 1) macrophage recruitment to the border zone, 2) collagen degradation at the border zone, and 3) subsequent cardiomyocyte invasion. Furthermore, cardiomyocyte-specific overexpression of mmp14b was sufficient to enhance cardiomyocyte invasion into the injured tissue and along the apical surface of the wound. Altogether, our data shed important insights into the process of cardiomyocyte invasion of the collagen-containing injured tissue during cardiac regeneration. They further suggest that cardiomyocytes and resident macrophages contribute to ECM remodeling at the border zone to promote cardiomyocyte replenishment of the fibrotic injured tissue.

Adult zebrafish have the ability to perfectly regenerate their skin after injury without leaving a scar behind. Yet, they intermediately form a collagen-rich granulation tissue that later fully regresses. In contrast, adult mammals lose this ability, resulting in persistent tissue fibrosis and scarring. We performed single-cell RNA sequencing to better characterize the dynamics and heterogeneity of involved cell types during different stages of zebrafish cutaneous wound healing, focusing on macrophages and fibroblasts. Macrophage subclusters display pro- and/or anti-inflammatory/repair characteristics, and fibroblast subclusters characteristics of extracellular matrix formation and degradation, which largely co-exist during all stages of wound healing. Strikingly, some fibroblasts display signatures of myofibroblasts, implicated in fibrotic healing in mammals. In addition, zebrafish fibroblasts express multiple genes with described pro-fibrotic effects in mammalian models. One of them is plod2, which encodes lysylhydroxylase 2. In cutaneous mouse wounds, Plod2 is induced in fibroblasts by macrophage-released Resistin-like molecule RELM encoded by the Retlna gene, promoting the formation of DHLNL collagen crosslinks and thereby less resolvable fibrotic tissue. retln genes are absent from the zebrafish genome; nevertheless, plod2 expression is initiated in zebrafish dermal fibroblasts upon wounding, in this case via TGF{beta} signaling, accompanied by increased collagen DHLNL crosslinking. Yet, both transgenic overexpression and genetic knock-out of plod2 do not interfere with granulation tissue formation and regression, pointing to additional pathways assuring the resolution of transient fibrosis in zebrafish skin wounds even in the presence of strong collagen crosslinking.

Immune-mediated necrotizing myopathy (IMNM) is a subgroup of idiopathic inflammatory myopathies associated with anti-signal recognition particle (SRP) or anti-3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) autoantibodies. However, the demonstration of a direct pathogenic effect of IMNM patient autoantibodies on skeletal muscle contractile force, independent of the downstream activation of the complement pathway, has yet to be reported. Thus, the goal of this study was to leverage a custom 3D-human skeletal muscle microtissue (hMMT) culture platform, that enables muscle cell contractile apparatus maturation and the analysis of contractile function, to evaluate the direct effect of total immunoglobulins (IgGs) isolated from IMNM patients with amplification of anti-SRP+ or anti-HMGCR+ autoantibodies. hMMTs capable of force generation were treated with total IgGs, isolated from 3 SRP+ and 3 HMGCR+ patients plasma, and delivered in complement inactivated media for 4 days. hMMT health was then evaluated by quantifying the peak force and contraction kinetics in response to electrical field stimulation and by performing histological analysis of sarcomere and myotube structures. Treating hMMTs with total IgGs from anti-HMGCR+ patients resulted in a decline in tetanus contractile force, though sarcomere Z-line architecture analysis revealed no significant influences on sarcomere organization. hMMT treatment with total IgGs from anti-SRP+ patients induced muscle atrophy, observed via significantly smaller myotube diameter, but this did not translate to a decline in contractile function. This study demonstrates that anti-SRP and anti-HMGCR autoantibodies exert direct, but distinct influences on IMNM-associated skeletal muscle pathogenesis, which may inform IMNM therapy development.

Limb loss remains a significant clinical challenge, but regenerative medicine approaches such as gene therapy offer a promising strategy to trigger endogenous regeneration programs. Optimal vector configurations and molecular targets for appendicular skeletal repair are not well defined. Here, we leveraged insights from species with a high endogenous capacity for appendage regeneration to design an enhancer-directed gene delivery platform that functions during mouse digit regeneration, a well characterized model for partial limb regeneration in mammals. Single-cell RNA sequencing of zebrafish caudal fin regeneration, combined with expression data in regenerating salamander limbs and mouse digit tips, implicated the SP family of transcription factors as conserved, epidermally-expressed mediators of appendage regrowth. Null mutants of Sp8 demonstrated impaired limb regeneration in salamanders, while conditional knockout of Sp6 and/or Sp8 in the mouse basal epidermis resulted in defective bony digit tip regeneration, involving an IL-17 mediated osteoclastogenic program. Spatiotemporally focused expression of FGF8, a known target of SP factors, using a zebrafish-derived tissue regeneration enhancer element via adeno-associated viral vectors, could partially rescue digit tip regeneration in SP knockout mice and accelerate digit regeneration in wildtype mice. Our results demonstrate a contextual gene therapy approach to address limb loss based on genes like SP transcription factors conserved across multiple contexts of appendage regeneration. Significance StatementInstructing regeneration of complex structures in mammals remains an unsolved problem. Gene therapy offers a compelling approach to foster endogenous regeneration by delivering therapeutic gene products to specific cells post injury. We identified a conserved regeneration-linked epidermal transcriptional program in mouse digit regeneration centered on the SP6 and SP8 transcription factors, involving inflammatory responses from osteoclasts. We engineered AAVs harboring a zebrafish tissue regeneration enhancer to direct FGF8 expression in the epidermis after amputation. This enhancer directed delivery partially rescued impaired digit regeneration in Sp6 and Sp8 conditional knockout mice and accelerated regrowth in wildtype digits. Our work links developmental signaling to adult regeneration and establishes a modular, injury site specific gene therapy framework that enables new interventions for limb healing.