A Model to Study Wound Healing Over Exposed Avascular Structures in Rodents With a 3D-Printed Wound Frame.

BACKGROUND: Soft tissue defects with exposed avascular structures require reconstruction with well-vascularized tissues. Extensive research is ongoing to explore tissue engineered products that provide durable coverage. However, there is a lack of controlled and affordable testbeds in the preclinical setting to reflect this challenging clinical scenario. We aimed to address this gap in the literature and develop a feasible and easily reproducible model in rodents that reflects an avascular structure in the wound bed. METHODS: We created 20 × 20 mm full thickness wounds on the dorsal skin of Lewis rats and secured 0.5-mm-thick silicone sheets of varying sizes to the wound bed. A 3D-printed wound frame was designed to isolate the wound environment. Skin graft and free flap survival along with exposure of the underlying silicone was assessed. Rats were followed for 4 weeks with weekly dressing changes and photography. Samples were retrieved at the endpoint for tissue viability and histologic analysis. RESULTS: The total wound surface area was constant throughout the duration of the experiment in all groups and the wound frames were well tolerated. The portion of the skin graft without underlying silicone demonstrated integration with the underlying fascia and a histologically intact epidermis. Gradual necrosis of the portion of the skin graft overlying the silicone sheet was observed with varying sizes of the silicone sheet. When the size of the silicone sheet was reduced from 50% of the wound surface area, the portion surviving over the silicone sheet increased at the 4-week timepoint. The free flap provided complete coverage over the silicone sheet. CONCLUSION: We developed a novel model of rodent wound healing to maintain the same wound size and isolate the wound environment for up to 4 weeks. This model is clinically relevant to a complex wound with an avascular structure in the wound bed. Skin grafts failed to completely cover increasing sizes of the avascular structure, whereas the free flap was able to provide viable coverage. This cost-effective model will establish an easily reproducible platform to evaluate more complex bioengineered wound coverage solutions.

Biodegradable Nerve Guide with Glial Cell Line-Derived Neurotrophic Factor Improves Recovery After Facial Nerve Injury in Rats.

Background: Bioengineered nerve guides with glial cell line-derived neurotrophic factor (GDNF) support recovery after facial nerve injury by acting as regenerative scaffolds. Objective: To compare functional, electrophysiological, and histological outcomes after repair of rat facial nerve transection in control, empty nerve guide, and nerve guide with GDNF conditions. Methods: Rats underwent transection and primary repair of the buccal branch of the facial nerve and were divided into (1) transection and repair only, (2) transection and repair augmented with empty guide, (3) transection and repair augmented with GDNF-guide groups. Weekly measurements of the whisking movements were recorded. At 12 weeks, compound muscle action potentials (CMAPs) at the whisker pad were assessed, and samples were collected for histomorphometric analysis. Results: Rats in GDNF-guide group displayed the earliest peak in normalized whisking amplitude. CMAPs were significantly higher after GDNF-guide placement. Mean fiber surface area of the target muscle, axonal count of the injured branch, and the number of Schwann cells were highest with GDNF guides. Conclusion: The biodegradable nerve guide containing double-walled GDNF microspheres enhanced recovery after facial nerve transection and primary repair.

Active Vitamin D3 (Calcitriol) Increases Adipose Graft Retention in a Xenograft Model.

BACKGROUND: Autologous fat grafting, although broadly indicated, is limited by unsatisfactory retention and often requires multiple procedures to achieve durable outcomes. Graft survival is strongly influenced by the magnitude and duration of post-engraftment ischemia. Calcitriol is a pleiotropic, safe nutrient with cell-specific influence on viability and metabolic flux. OBJECTIVES: Evaluate the efficacy of activated vitamin D3 (calcitriol) in improving grafting outcomes and examine its mechanisms. METHODS: Lipoaspirate was collected for ex vivo culture (7 unique donors), in vitro bioenergetic analysis (6 unique donors), and in vivo transplantation (5 unique donors). Ex vivo samples were incubated for up to 2 weeks before extraction of the stromal vascular fraction (SVF) for viability or flow cytometry. SVF was collected for Seahorse (Agilent; Santa Clara, CA) analysis of metabolic activity. Human endothelial cell lines were utilized for analyses of endothelial function. In vivo, samples were implanted into athymic mice with calcitriol treatment either (1) once locally or (2) 3 times weekly via intraperitoneal injection. Grafts were assessed photographically, volumetrically, and histologically at 1, 4, and 12 weeks. Hematoxylin and eosin (H&E), Sirius red, perilipin, HIF1α, and CD31 tests were performed. RESULTS: Calcitriol-treated lipoaspirate demonstrated dose-dependent increases in SVF viability and metabolic reserve during hypoxic stress. Calcitriol treatment enhanced endothelial mobility ex vivo and endothelial function in vitro. In vivo, calcitriol enhanced adipocyte viability, reduced fibrosis, and improved vascularity. Continuous calcitriol was sufficient to improve graft retention at 12 weeks (P < .05). CONCLUSIONS: Calcitriol increased fat graft retention in a xenograft model. Calcitriol has potential to be a simple, economical means of increasing fat graft retention and long-term outcomes.

Use of wound edge inversion (epibole) to generate recalcitrant and inflamed diabetic wounds.

Robust and predictive pre-clinical models of recalcitrant diabetic wounds are critical for advancing research efforts toward improving healing. Murine models have logistic and genetic benefits versus larger animals; however, native murine healing inadequately represents clinically recalcitrant wounds in humans. Furthermore, current humanization techniques employing devices, deleterious mutations or chemical agents each carry model-specific limitations. To better replicate human wounds in a mouse, we developed a novel wound-edge inversion (WEI) technique that mimics the architecture of epibole and mitigates contracture, epithelialization, and consequently wound closure. In this study, we evaluated the reliability and durability of the WEI model in wild-type and obese diabetic mice and compared to healing after (i) punch biopsy, (ii) mechanical/silicone stenting or (iii) exogenous oxidative stressors. In wild-type mice, WEI demonstrated favourable closure characteristics compared to both control and stented wounds, however, wounds progressed to closure by 4 weeks. In contrast, diabetic WEI wounds persisted for 6-10 weeks with reduced contracture and epithelialization. In both diabetic and wild-type mice, WEI sites demonstrated persistence of inflammatory populations, absence of epithelialization, and histologic presence of alpha-SMA positive granulation tissue when compared to controls. We conclude that the WEI technique is particularly valuable for modelling recalcitrant diabetic wounds with sustained inflammation and dysfunctional healing.

Comparison of Soluble and Liposome Encapsulated, Sustained Release Latanoprost for Focal Adipose Reduction.

Background: To address the lack of non-cytotoxic, non-surgical options to treat undesirable focal adiposity of the face, we propose use of the anti-glaucoma medication and prostaglandin F2α analogue latanoprost, which has a well-described side effect of periorbital adipose shrinkage. Objective: To evaluate the safety and efficacy of soluble and liposomal latanoprost for focal fat reduction. Approach: To compare efficacy, single administrations of either the FDA-approved cytolytic drug deoxycholic acid (DOCA), latanoprost, or liposomal latanoprost were injected into ob/ob mouse inguinal fat pads. Study outcomes included mouse weight, inguinal fat pad volume, architecture, and cytotoxicity. Results: Both DOCA and soluble latanoprost significantly reduced inguinal fat pad volume whereas liposome encapsulation reduced inguinal fat pad volume insignificantly over the 14-day study period. Hematoxylin and eosin demonstrated effective reduction in adipocyte volume without histologic evidence of cytolysis or inflammation whereas DOCA caused dermal ulcerations, adipocyte lysis, and increased tissue inflammation. Conclusion: Latanoprost reduced fat volume without inducing cell lysis or inflammation.

Evolution of the Push-2-Spin Fat Graft Processing Device: Enhancing Efficiency and Reducing Risk of Contamination.

Background: Small-volume fat graft efficiency is a critical determinant of the cost and material effectiveness of aesthetic fat grafting in the clinical space. Recent development of devices, such as the Push-2-Spin (P2S) system (Pittsburgh, PA), has improved upon the process by yielding a rapid, handheld, multi-use system to minimize operative time and mess. Objectives: In this study, the authors describe further technical innovations on the P2S prototype that improve operative ease of use, time, and safety. Methods: Abdominoplasty samples were obtained as discarded tissue. Lipoaspirate was collected utilizing a 3.0 mm liposuction cannula and processed through centrifugation (Coleman technique), gauze (telfa) rolling, mesh straining, the tabletop P2S device (prototype), or the P2S handheld (P2S-H) device. Operative processing time, spin time, oil fraction, stromal vascular fraction (SVF) yield and viability, and adipocyte viability were assessed to compare the efficacy and viability of each device/technique. Blood agar smears of lipoaspirate were performed to assess for risk of contamination. Results: The P2S-H device outperformed its prior iteration in rotary and processing speed and was significantly faster than each other technique assessed. Furthermore, the use of an inline system offered significant advantages over open-air techniques in terms of resistance to contamination. Serial use characteristics were assessed; under these conditions, oil yield as well as adipocyte and SVF number and viability was similar between all techniques. Conclusions: The technical advancements to the P2S system which enable single-unit, handheld operation significantly improve operative time and minimize space requirements. This operative quality of life improvement comes at no cost to the efficacy of oil extraction, cellular yield, or cell viability.

Tuning Macrophage Phenotype to Mitigate Skeletal Muscle Fibrosis.

Myeloid cells are critical to the development of fibrosis following muscle injury; however, the mechanism of their role in fibrosis formation remains unclear. In this study, we demonstrate that myeloid cell-derived TGF-ß1 signaling is increased in a profibrotic ischemia reperfusion and cardiotoxin muscle injury model. We found that myeloid-specific deletion of Tgfb1 abrogates the fibrotic response in this injury model and reduces fibro/adipogenic progenitor cell proliferation while simultaneously enhancing muscle regeneration, which is abrogated by adaptive transfer of normal macrophages. Similarly, a murine TGFBRII-Fc ligand trap administered after injury significantly reduced muscle fibrosis and improved muscle regeneration. This study ultimately demonstrates that infiltrating myeloid cell TGF-ß1 is responsible for the development of traumatic muscle fibrosis, and its blockade offers a promising therapeutic target for preventing muscle fibrosis after ischemic injury.

Regulation of heterotopic ossification by monocytes in a mouse model of aberrant wound healing.

Heterotopic ossification (HO) is an aberrant regenerative process with ectopic bone induction in response to musculoskeletal trauma, in which mesenchymal stem cells (MSC) differentiate into osteochondrogenic cells instead of myocytes or tenocytes. Despite frequent cases of hospitalized musculoskeletal trauma, the inflammatory responses and cell population dynamics that regulate subsequent wound healing and tissue regeneration are still unclear. Here we examine, using a mouse model of trauma-induced HO, the local microenvironment of the initial post-injury inflammatory response. Single cell transcriptome analyses identify distinct monocyte/macrophage populations at the injury site, with their dynamic changes over time elucidated using trajectory analyses. Mechanistically, transforming growth factor beta-1 (TGFß1)-producing monocytes/macrophages are associated with HO and aberrant chondrogenic progenitor cell differentiation, while CD47-activating peptides that reduce systemic macrophage TGFß levels and help ameliorate HO. Our data thus implicate CD47 activation as a therapeutic approach for modulating monocyte/macrophage phenotypes, MSC differentiation and HO formation during wound healing.

Coordinating Tissue Regeneration Through Transforming Growth Factor-ß Activated Kinase 1 Inactivation and Reactivation.

Aberrant wound healing presents as inappropriate or insufficient tissue formation. Using a model of musculoskeletal injury, we demonstrate that loss of transforming growth factor-ß activated kinase 1 (TAK1) signaling reduces inappropriate tissue formation (heterotopic ossification) through reduced cellular differentiation. Upon identifying increased proliferation with loss of TAK1 signaling, we considered a regenerative approach to address insufficient tissue production through coordinated inactivation of TAK1 to promote cellular proliferation, followed by reactivation to elicit differentiation and extracellular matrix production. Although the current regenerative medicine paradigm is centered on the effects of drug treatment ("drug on"), the impact of drug withdrawal ("drug off") implicit in these regimens is unknown. Because current TAK1 inhibitors are unable to phenocopy genetic Tak1 loss, we introduce the dual-inducible COmbinational Sequential Inversion ENgineering (COSIEN) mouse model. The COSIEN mouse model, which allows us to study the response to targeted drug treatment ("drug on") and subsequent withdrawal ("drug off") through genetic modification, was used here to inactivate and reactivate Tak1 with the purpose of augmenting tissue regeneration in a calvarial defect model. Our study reveals the importance of both the "drug on" (Cre-mediated inactivation) and "drug off" (Flp-mediated reactivation) states during regenerative therapy using a mouse model with broad utility to study targeted therapies for disease. Stem Cells 2019;37:766-778.

Characterizing the Circulating Cell Populations in Traumatic Heterotopic Ossification.

Heterotopic ossification (HO) occurs secondary to trauma, causing pain and functional limitations. Identification of the cells that contribute to HO is critical to the development of therapies. Given that innate immune cells and mesenchymal stem cells are known contributors to HO, we sought to define the contribution of these populations to HO and to identify what, if any, contribution circulating populations have to HO. A shared circulation was obtained using a parabiosis model, established between an enhanced green fluorescent protein-positive/luciferase+ donor and a same-strain nonreporter recipient mouse. The nonreporter mouse received Achilles tendon transection and dorsal burn injury to induce HO formation. Bioluminescence imaging and immunostaining were performed to define the circulatory contribution of immune and mesenchymal cell populations. Histologic analysis showed circulating cells present throughout each stage of the developing HO anlagen. Circulating cells were present at the injury site during the inflammatory phase and proliferative period, with diminished contribution in mature HO. Immunostaining demonstrated that most early circulatory cells were from the innate immune system; only a small population of mesenchymal cells were present in the HO. We demonstrate the time course of the participation of circulatory cells in trauma-induced HO and identify populations of circulating cells present in different stages of HO. These findings further elucidate the relative contribution of local and systemic cell populations to HO.

Strategic Targeting of Multiple BMP Receptors Prevents Trauma-Induced Heterotopic Ossification.

Trauma-induced heterotopic ossification (tHO) is a condition of pathologic wound healing, defined by the progressive formation of ectopic bone in soft tissue following severe burns or trauma. Because previous studies have shown that genetic variants of HO, such as fibrodysplasia ossificans progressiva (FOP), are caused by hyperactivating mutations of the type I bone morphogenetic protein receptor (T1-BMPR) ACVR1/ALK2, studies evaluating therapies for HO have been directed primarily toward drugs for this specific receptor. However, patients with tHO do not carry known T1-BMPR mutations. Here we show that, although BMP signaling is required for tHO, no single T1-BMPR (ACVR1/ALK2, BMPR1a/ALK3, or BMPR1b/ALK6) alone is necessary for this disease, suggesting that these receptors have functional redundancy in the setting of tHO. By utilizing two different classes of BMP signaling inhibitors, we developed a translational approach to treatment, integrating treatment choice with existing diagnostic options. Our treatment paradigm balances either immediate therapy with reduced risk for adverse effects (Alk3-Fc) or delayed therapy with improved patient selection but greater risk for adverse effects (LDN-212854).

Heterotopic Ossification Following Upper Extremity Injury.

Heterotopic ossification (HO) presents a substantial barrier to rehabilitation for patients with severe burns or trauma. Although surgical excision is a mainstay of management for this condition, this is unable to address the chronic sequelae of HO, including chronic pain, joint contractures, nerve dysfunction, and open wounds. Current therapeutic modalities are aimed at excision and the prevention of recurrence using nonsteroidal antiinflammatory drugs (NSAIDs) or radiation therapy. Research is now focused on identifying alternative strategies to prevent the initial occurrence of HO through NSAIDs and novel inhibitors of the bone morphogenetic protein signaling pathway.

Surgical Excision of Heterotopic Ossification Leads to Re-Emergence of Mesenchymal Stem Cell Populations Responsible for Recurrence.

Trauma-induced heterotopic ossification (HO) occurs after severe musculoskeletal injuries and burns, and presents a significant barrier to patient rehabilitation. Interestingly, the incidence of HO significantly increases with repeated operations and after resection of previous HO. Treatment of established heterotopic ossification is challenging because surgical excision is often incomplete, with evidence of persistent heterotopic bone. As a result, patients may continue to report the signs or symptoms of HO, including chronic pain, nonhealing wounds, and joint restriction. In this study, we designed a model of recurrent HO that occurs after surgical excision of mature HO in a mouse model of hind-limb Achilles' tendon transection with dorsal burn injury. We first demonstrated that key signaling mediators of HO, including bone morphogenetic protein signaling, are diminished in mature bone. However, upon surgical excision, we have noted upregulation of downstream mediators of osteogenic differentiation, including pSMAD 1/5. Additionally, surgical excision resulted in re-emergence of a mesenchymal cell population marked by expression of platelet-derived growth factor receptor-α (PDGFRα) and present in the initial developing HO lesion but absent in mature HO. In the recurrent lesion, these PDGFRα+ mesenchymal cells are also highly proliferative, similar to the initial developing HO lesion. These findings indicate that surgical excision of HO results in recurrence through similar mesenchymal cell populations and signaling mechanisms that are present in the initial developing HO lesion. These results are consistent with findings in patients that new foci of ectopic bone can develop in excision sites and are likely related to de novo formation rather than extension of unresected bone. Stem Cells Translational Medicine 2017;6:799-806.

Scleraxis-Lineage Cells Contribute to Ectopic Bone Formation in Muscle and Tendon.

The pathologic development of heterotopic ossification (HO) is well described in patients with extensive trauma or with hyperactivating mutations of the bone morphogenetic protein (BMP) receptor ACVR1. However, identification of progenitor cells contributing to this process remains elusive. Here we show that connective tissue cells contribute to a substantial amount of HO anlagen caused by trauma using postnatal, tamoxifen-inducible, scleraxis-lineage restricted reporter mice (Scx-creERT2/tdTomatofl/fl ). When the scleraxis-lineage is restricted specifically to adults prior to injury marked cells contribute to each stage of the developing HO anlagen and coexpress markers of endochondral ossification (Osterix, SOX9). Furthermore, these adult preinjury restricted cells coexpressed mesenchymal stem cell markers including PDGFRα, Sca1, and S100A4 in HO. When constitutively active ACVR1 (caACVR1) was expressed in scx-cre cells in the absence of injury (Scx-cre/caACVR1fl/fl ), tendons and joints formed HO. Postnatal lineage-restricted, tamoxifen-inducible caACVR1 expression (Scx-creERT2/caACVR1fl/fl ) was sufficient to form HO after directed cardiotoxin-induced muscle injury. These findings suggest that cells expressing scleraxis within muscle or tendon contribute to HO in the setting of both trauma or hyperactive BMP receptor (e.g., caACVR1) activity. Stem Cells 2017;35:705-710.

The role of the adaptive immune system in burn-induced heterotopic ossification and mesenchymal cell osteogenic differentiation.

BACKGROUND: Heterotopic ossification (HO) is the pathologic process of extraskeletal bone formation. Although the exact etiology remains unknown, inflammation appears to catalyze disease progression. The goal of this study is to determine the impact of the adaptive immune system on HO. METHODS: HO was induced in 8-wk-old control C57BL/6 and immunocompromised Rag1tm1Mom (Rag1 KO) male mice deficient in B- and T-lymphocytes via combined Achilles tenotomy and burn injury. Microcomputed tomography quantified the extent of HO formation at the tenotomy site. Adipose-derived mesenchymal stem cells were harvested to evaluate osteogenic differentiation potential. RESULTS: Areas of developing HO demonstrated substantial enrichment of CD45 + leukocytes at 3 wk after injury. HO from Rag1 KO mice was substantially less mature with foci of cartilage and disorganized trabecular bone present 12 wk after injury. Rag1 KO mice formed 60% less bone compared to immunocompetent controls (4.67 ± 1.5 mm versus 7.76 ± 0.65 mm; P = 0.001). Tartrate-resistant acid phosphatase staining and immunofluorescent analysis of osteoprotegerin and nuclear factor kappa-light-chain-enhancer of activated B cells demonstrated no appreciable difference in osteoclast number or activation. Alizarin red staining in vitro demonstrated a significant decrease in osteogenic potential in immunocompromised mice compared to controls (29.1 ± 0.54 mm versus 12.1 ± 0.14 mm; P < 0.001). CONCLUSIONS: We demonstrate a prominent role for the adaptive immune system in the development of HO. In the absence of mature B- and T-lymphocytes, HO growth and development are attenuated. Furthermore, we demonstrate that mesenchymal populations from B- and T-cell deficient mice are inherently less osteogenic. This study identifies a potential therapeutic role for modulation of the adaptive immune system in the treatment of HO.

Local and Circulating Endothelial Cells Undergo Endothelial to Mesenchymal Transition (EndMT) in Response to Musculoskeletal Injury.

Endothelial-to-mesenchymal transition (EndMT) has been implicated in a variety of aberrant wound healing conditions. However, unambiguous evidence of EndMT has been elusive due to limitations of in vitro experimental designs and animal models. In vitro experiments cannot account for the myriad ligands and cells which regulate differentiation, and in vivo tissue injury models may induce lineage-independent endothelial marker expression in mesenchymal cells. By using an inducible Cre model to mark mesenchymal cells (Scx-creERT/tdTomato + ) prior to injury, we demonstrate that musculoskeletal injury induces expression of CD31, VeCadherin, or Tie2 in mesenchymal cells. VeCadherin and Tie2 were expressed in non-endothelial cells (CD31-) present in marrow from uninjured adult mice, thereby limiting the specificity of these markers in inducible models (e.g. VeCadherin- or Tie2-creERT). However, cell transplantation assays confirmed that endothelial cells (ΔVeCadherin/CD31+/CD45-) isolated from uninjured hindlimb muscle tissue undergo in vivo EndMT when transplanted directly into the wound without intervening cell culture using PDGFRα, Osterix (OSX), SOX9, and Aggrecan (ACAN) as mesenchymal markers. These in vivo findings support EndMT in the presence of myriad ligands and cell types, using cell transplantation assays which can be applied for other pathologies implicated in EndMT including tissue fibrosis and atherosclerosis. Additionally, endothelial cell recruitment and trafficking are potential therapeutic targets to prevent EndMT.

Characterization of Cells Isolated from Genetic and Trauma-Induced Heterotopic Ossification.

Heterotopic ossification (HO) is the pathologic formation of bone separate from the normal skeleton. Although several models exist for studying HO, an understanding of the common in vitro properties of cells isolated from these models is lacking. We studied three separate animal models of HO including two models of trauma-induced HO and one model of genetic HO, and human HO specimens, to characterize the properties of cells derived from tissue containing pre-and mature ectopic bone in relation to analogous mesenchymal cell populations or osteoblasts obtained from normal muscle tissue. We found that when cultured in vitro, cells isolated from the trauma sites in two distinct models exhibited increased osteogenic differentiation when compared to cells isolated from uninjured controls. Furthermore, osteoblasts isolated from heterotopic bone in a genetic model of HO also exhibited increased osteogenic differentiation when compared with normal osteoblasts. Finally, osteoblasts derived from mature heterotopic bone obtained from human patients exhibited increased osteogenic differentiation when compared with normal bone from the same patients. These findings demonstrate that across models, cells derived from tissues forming heterotopic ossification exhibit increased osteogenic differentiation when compared with either normal tissues or osteoblasts. These cell types can be used in the future for in vitro investigations for drug screening purposes.