Emerging ideas: Evaluation of stem cells genetically modified with scleraxis to improve rotator cuff healing

Tenascin C deletion impairs tendon healing and functional recovery after rotator cuff repair

The biological factors that affect healing after rotator cuff repair (RCR) are not well understood. Genetic variants in the extracellular matrix protein Tenascin C (TNC) are associated with impaired tendon healing and it is expressed in rotator cuff tendon tissue after injury, suggesting it may have a role in the repair process. The purpose of the current study was to determine the role of TNC on tendon healing after RCR in a murine model. The supraspinatus tendon was transected and repaired on the left shoulder of wild-type (WT-RCR), Tenascin C null (Tnc--RCR) and Tnc heterozygous (Tnc+/--RCR) mice. Controls included the unoperated, contralateral shoulder of WT-RCR, Tnc-RCR, Tnc+/--RCR mice and unoperated shoulders from age and genotype matched controls. We performed histologic, activity testing, bulk RNA-seq, and biomechanical analyses. At 8-weeks post-RCR, Tnc- and Tnc+/- mice had severe bone and tendon defects following RCR. Tnc--RCR mice had reduced activity after RCR including reduced wheel rotations, wheel duration, and wheel episode average velocity compared with WT-RCR. Loss of Tnc following RCR altered gene expression in the shoulder, including upregulation of sex hormone and WNT pathways and a downregulation of inflammation and cell cycle pathways. Tnc- mice had similar biomechanical properties after repair as WT. Further research is required to evaluate tissue specific alterations of Tnc, the interactions of Tnc and sex hormone and inflammation pathways as well as possible adjuvants to improve enthesis healing in the setting of reduced TNC function.

Tenascin C Deletion Impairs Tendon Healing and Functional Recovery After Rotator Cuff Repair

The biological factors that affect healing after rotator cuff repair (RCR) are not well understood. Genetic variants in the extracellular matrix protein Tenascin C (TNC) are associated with impaired tendon healing and it is expressed in rotator cuff tendon tissue after injury, suggesting it may have a role in the repair process. The purpose of the current study was to determine the role of TNC on tendon healing after RCR in a murine model. The supraspinatus tendon was transected and repaired on the left shoulder of Wild-Type (WT-RCR), Tenascin C null (Tnc --RCR) and Tnc heterozygous (Tnc +/--RCR) mice. Controls included the unoperated, contralateral shoulder of WT-RCR, Tnc - RCR, Tnc +/--RCR mice and unoperated shoulders from age and genotype matched controls. We performed histologic, activity testing, RNA-seq, and biomechanical analyses. At 8-weeks post-RCR, Tnc - and Tnc +/- mice had severe bone and tendon defects following rotator cuff repair. Tnc --RCR mice had reduced activity after rotator cuff repair including reduced wheel rotations, wheel duration, and wheel episode average velocity compared with WT-RCR. Loss of Tnc following RCR altered gene expression in the shoulder, including upregulation of sex hormone and WNT pathways and a downregulation of inflammation and cell cycle pathways. Tnc - mice had similar biomechanical properties after repair as WT. Further research is required to evaluate tissue specific alterations of Tnc, the interactions of Tnc and sex hormone and inflammation pathways as well as possible adjuvants to improve enthesis healing in the setting of reduced TNC function.

Tendon and Ligament Healing and Current Approaches to Tendon and Ligament Regeneration

Ligament and tendon injuries are common problems in orthopedics. There is a need for treatments that can expedite nonoperative healing or improve the efficacy of surgical repair or reconstruction of ligaments and tendons. Successful biologically-based attempts at repair and reconstruction would require a thorough understanding of normal tendon and ligament healing. The inflammatory, proliferative, and remodeling phases, and the cells involved in tendon and ligament healing will be reviewed. Then, current research efforts focusing on biologically-based treatments of ligament and tendon injuries will be summarized, with a focus on stem cells endogenous to tendons and ligaments. Statement of clinical significance: This paper details mechanisms of ligament and tendon healing, as well as attempts to apply stem cells to ligament and tendon healing. Understanding of these topics could lead to more efficacious therapies to treat ligament and tendon injuries. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:7-12, 2020.

Regenerative Engineering of the Rotator Cuff of the Shoulder

Rotator cuff tears often heal poorly, leading to re-tears after repair. This is in part attributed to the low proliferative ability of the resident cells (tendon fibroblasts and tendon-stem cells) upon injury to the rotator cuff tissue and the low vascularity of the tendon insertion. In addition, surgical outcomes of current techniques used in clinical settings are often suboptimal, leading to the formation of neo-tissue with poor biomechanics and structural characteristics, which results in re-tears. This has prompted interest in a new approach, which we term as "Regenerative Engineering", for regenerating rotator cuff tendons. In the Regenerative Engineering paradigm, roles played by stem cells, scaffolds, growth factors/small molecules, the use of local physical forces, and morphogenesis interplayed with clinical surgery techniques may synchronously act, leading to synergistic effects and resulting in successful tissue regeneration. In this regard, various cell sources such as tendon fibroblasts and adult tissue-derived stem cells have been isolated, characterized, and investigated for regenerating rotator cuff tendons. Likewise, numerous scaffolds with varying architecture, geometry, and mechanical characteristics of biologic and synthetic origin have been developed. Furthermore, these scaffolds have been also fabricated with biochemical cues (growth factors and small molecules), facilitating tissue regeneration. In this Review, various strategies to regenerate rotator cuff tendons using stem cells, advanced materials, and factors in the setting of physical forces under the Regenerative Engineering paradigm are described.

Tissue-specific endothelial cells: a promising approach for augmentation of soft tissue repair in orthopedics

Biologics are playing an increasingly significant role in the practice of modern medicine and surgery in general and orthopedics in particular. Cell-based approaches are among the most important and widely used modalities in orthopedic biologics, with mesenchymal stem cells and other multi/pluripotent cells undergoing evaluation in numerous preclinical and clinical studies. On the other hand, fully differentiated endothelial cells (ECs) have been found to perform critical roles in homeostasis of visceral tissues through production of an adaptive panel of so-called "angiocrine factors." This newly discovered function of ECs renders them excellent candidates for novel approaches in cell-based biologics. Here, we present a review of the role of ECs and angiocrine factors in some visceral tissues, followed by an overview of current cell-based approaches and a discussion of the potential applications of ECs in soft tissue repair.

Therapeutic potential of hAECs for early Achilles tendon defect repair through regeneration

Cell-based therapy holds great promise for tendon disorders, a widespread debilitating musculoskeletal condition. Even if the cell line remains to be defined, preliminary evidences have proven that amniotic-derived cells possess in vitro and in vivo a great tenogenic potential. This study investigated the efficacy of transplanted human amniotic epithelial cells (hAECs) by testing their early regenerative properties and mechanisms involved on a validated ovine Achilles tendon partial defect performed on 29 animals. The injured tendons treated with hAECs recovered rapidly, in 28 days, structural and biomechanical properties undertaking a programmed tissue regeneration, differently from the spontaneous healing tissues. hAECs remained viable within the host tendons establishing with the endogenous progenitor cells an active dialogue. Through the secretion of modulatory factors, hAECs inhibited the inflammatory cells infiltration, activated the M2 macrophage subpopulation early recruitment, and accelerated blood vessel as well as extracellular matrix remodelling. In parallel, some in situ differentiated hAECs displayed a tenocytelike phenotype. Both paracrine and direct hAECs stimulatory effects were confirmed analysing their genome profile before and after transplantation. The 49 human up-regulated transcripts recorded in transplanted hAECs belonged to tendon lineage differentiation (epithelial-mesenchymal transition, connective specific matrix components, and skeleton or muscle system development-related transcripts), as well as the in situ activation of paracrine signalling involved in inflammatory and immunomodulatory response. Altogether, these evidences support the hypothesis that hAECs are a practicable and efficient strategy for the acute treatment of tendinopathy, reinforcing the idea of a concrete use of amniotic epithelial cells towards the clinical practice.

Cell-based approaches for augmentation of tendon repair

Cell-based approaches are among the principal interventions in orthobiologics to improve tendon and ligament healing and to combat degenerative processes. The number of options available for investigation are expanding rapidly and investigators have an increasing number of cell types to choose from for research purposes. However, in part due to the current regulatory environment, the list of available cells at clinicians' disposal for therapeutic purposes is still rather limited. In this review, we present an overview of the main cellular categories in current use. Notable recent developments in cell-based approaches include the introduction of diverse sources of mesenchymal stem cells, pluripotent cells of extra-embryonic origin, and the emerging popularity of fully differentiated cells such as tenocytes and endothelial cells. Delivery strategies are discussed and a succinct discussion of the current regulatory environment in the United States is presented.

Regulation of fibronectin gene expression in cardiac fibroblasts by scleraxis

The glycoprotein fibronectin is a key component of the extracellular matrix. By interacting with numerous matrix and cell surface proteins, fibronectin plays important roles in cell adhesion, migration and intracellular signaling. Up-regulation of fibronectin occurs in tissue fibrosis, and previous studies have identified the pro-fibrotic factor TGFβ as an inducer of fibronectin expression, although the mechanism responsible remains unknown. We have previously shown that a key downstream effector of TGFβ signaling in cardiac fibroblasts is the transcription factor scleraxis, which in turn regulates the expression of a wide variety of extracellular matrix genes. We noted that fibronectin expression tracked closely with scleraxis expression, but it was unclear whether scleraxis directly regulated the fibronectin gene. Here, we report that scleraxis acts via two E-box binding sites in the proximal human fibronectin promoter to govern fibronectin expression, with the second E-box being both sufficient and necessary for scleraxis-mediated fibronectin expression to occur. A combination of electrophoretic mobility shift and chromatin immunoprecipitation assays indicated that scleraxis interacted to a greater degree with the second E-box. Over-expression or knockdown of scleraxis resulted in increased or decreased fibronectin expression, respectively, and scleraxis null mice presented with dramatically decreased immunolabeling for fibronectin in cardiac tissue sections compared to wild-type controls. Furthermore, scleraxis was required for TGFβ-induced fibronectin expression: TGFβ lost its ability to induce fibronectin expression following scleraxis knockdown. Together, these results demonstrate a novel and required role for scleraxis in the regulation of cardiac fibroblast fibronectin gene expression basally or in response to TGFβ.

The role of an octacalcium phosphate in the re-formation of infraspinatus tendon insertion

BACKGROUND

To improve the success rate of rotator cuff repair, we investigated whether octacalcium phosphate (OCP) with gelatin (Gel) vehicle had a positive effect on tendon-to-bone healing.

METHODS

We assessed the histologic characteristics of the tendon-to-bone healing using the rabbit rotator cuff repair model. We divided the shoulders into 3 groups: control (without OCP/Gel composite), OCP/Gel composite (OCP+group), and Gel alone without OCP (Gel group) to evaluate the effectiveness of gelatin.

RESULTS

Both the number of newly formed tendon fibers and the Sharpey fibers at the repair site increased in the OCP+group compared with those in the other 2 groups on hematoxylin-eosin staining (P < .05). On immunohistochemical evaluation, both the bone and the fibers in the OCP+group demonstrated that type I collagen was picked up, whereas the newly formed tendon fibers and Sharpey fibers revealed type III collagen.

CONCLUSION

Treatment with OCP made collagen fibers and the Sharpey fibers, constituted by type I and type III collagens, increase at the tendon-to-bone insertion. It might be beneficial for the healing of rotator cuff tendon to bone.

Repairing damaged tendon and muscle: are mesenchymal stem cells and scaffolds the answer?

Mesenchymal stem cells (MSCs) have become an area of intense interest in the treatment of musculoskeletal conditions, such as muscle and tendon injury, as various animal and human trials have demonstrated that implantation with MSCs leads to improved healing and function. However, these trials have usually been relatively small scale and lacking in adequate controls. Additionally, the optimum source of these cells has yet to be determined, partly due to a lack of understanding as to how MSCs produce their beneficial effects when implanted. Scaffolds have been shown to improve tissue-engineering repairs but require further work to optimize their interactions with both native tissue and implanted MSCs. Robust, well-controlled trials are therefore required to determine the usefulness of MSCs in musculoskeletal tissue repair.

Crucial transcription factors in tendon development and differentiation: their potential for tendon regeneration

Tendons that connect muscles to bone are often the targets of sports injuries. The currently unsatisfactory state of tendon repair is largely attributable to the limited understanding of basic tendon biology. A number of tendon lineage-related transcription factors have recently been uncovered and provide clues for the better understanding of tendon development. Scleraxis and Mohawk have been identified as critical transcription factors in tendon development and differentiation. Other transcription factors, such as Sox9 and Egr1/2, have also been recently reported to be involved in tendon development. However, the molecular mechanisms and application of these transcription factors remain largely unclear and this prohibits their use in tendon therapy. Here, we systematically review and analyze recent findings and our own data concerning tendon transcription factors and tendon regeneration. Based on these findings, we provide interaction and temporal programming maps of transcription factors, as a basis for future tendon therapy. Finally, we discuss future directions for tendon regeneration with differentiation and trans-differentiation approaches based on transcription factors.

Composite growth factor supplementation strategies to enhance tenocyte bioactivity in aligned collagen-GAG scaffolds

Biomolecular environments encountered in vivo are complex and dynamic, with combinations of biomolecules presented in both freely diffusible (liquid-phase) and sequestered (bound to the extracellular matrix) states. Strategies for integrating multiple biomolecular signals into a biomimetic scaffold provide a platform to simultaneously control multiple cell activities, such as motility, proliferation, phenotype, and regenerative potential. Here we describe an investigation elucidating the influence of the dose and mode of presentation (soluble, sequestered) of five biomolecules (stromal cell-derived factor 1α [SDF-1α], platelet-derived growth factor BB [PDGF-BB], insulin-like growth factor 1 [IGF-1], basic fibroblast growth factor [bFGF], and growth/differentiation factor 5 [GDF-5]) on the recruitment, proliferation, collagen synthesis, and genomic stability of equine tenocytes within an anisotropic collagen-GAG scaffold for tendon regeneration applications. Critically, we found that single factors led to a dose-dependent trade-off between driving tenocyte proliferation (PDGF-BB, IGF-1) versus maintenance of a tenocyte phenotype (GDF-5, bFGF). We identified supplementation schemes using factor pairs (IGF-1, GDF-5) to rescue the tenocyte phenotype and gene expression profiles while simultaneously driving proliferation. These results suggest coincident application of multi-biomolecule cocktails has a significant value in regenerative medicine applications where control of cell proliferation and phenotype are required. Finally, we demonstrated an immobilization strategy that allows efficient sequestration of bioactive levels of these factors within the scaffold network. We showed sequestration can lead to a greater sustained bioactivity than soluble supplementation, making this approach particularly amenable to in vivo translation where diffusive loss is a concern and continuous biomolecule supplementation is not feasible.

Common threads in cardiac fibrosis, infarct scar formation, and wound healing

Wound healing, cardiac fibrosis, and infarct scar development, while possessing distinct features, share a number of key functional similarities, including extracellular matrix synthesis and remodeling by fibroblasts and myofibroblasts. Understanding the underlying mechanisms that are common to these processes may suggest novel therapeutic approaches for pathologic situations such as fibrosis, or defective wound healing such as hypertrophic scarring or keloid formation. This manuscript will briefly review the major steps of wound healing, and will contrast this process with how cardiac infarct scar formation or interstitial fibrosis occurs. The feasibility of targeting common pro-fibrotic growth factor signaling pathways will be discussed. Finally, the potential exploitation of novel regulators of wound healing and fibrosis (ski and scleraxis), will be examined.

Mesenchymal stem cell applications to tendon healing

Tendons are often subject to age related degenerative changes that coincide with a diminished regenerative capacity. Torn tendons often heal by forming scar tissue that is structurally weaker than healthy native tendon tissue, predisposing to mechanical failure. There is increasing interest in providing biological stimuli to increase the tendon reparative response. Stem cells in particular are an exciting and promising prospect as they have the potential to provide appropriate cellular signals to encourage neotendon formation during repair rather than scar tissue. Currently, a number of issues need to be investigated further before it can be determined whether stem cells are an effective and safe therapeutic option for encouraging tendon repair. This review explores the in-vitro and invivo evidence assessing the effect of stem cells on tendon healing, as well as the potential clinical applications.

Cell- and gene-based approaches to tendon regeneration

Repair of rotator cuff tears in experimental models has been significantly improved by the use of enhanced biologic approaches, including platelet-rich plasma, bone marrow aspirate, growth factor supplements, and cell- and gene-modified cell therapy. Despite added complexity, cell-based therapies form an important part of enhanced repair, and combinations of carrier vehicles, growth factors, and implanted cells provide the best opportunity for robust repair. Bone marrow-derived mesenchymal stem cells provide a stimulus for repair in flexor tendons, but application in rotator cuff repair has not shown universally positive results. The use of scaffolds such as platelet-rich plasma, fibrin, and synthetic vehicles and the use of gene priming for stem cell differentiation and local anabolic and anti-inflammatory impact have both provided essential components for enhanced tendon and tendon-to-bone repair in rotator cuff disruption. Application of these research techniques in human rotator cuff injury has generally been limited to autologous platelet-rich plasma, bone marrow concentrate, or bone marrow aspirates combined with scaffold materials. Cultured mesenchymal progenitor therapy and gene-enhanced function have not yet reached clinical trials in humans. Research in several animal species indicates that the concept of gene-primed stem cells, particularly embryonic stem cells, combined with effective culture conditions, transduction with long-term integrating vectors carrying anabolic growth factors, and development of cells conditioned by use of RNA interference gene therapy to resist matrix metalloproteinase degradation, may constitute potential advances in rotator cuff repair. This review summarizes cell- and gene-enhanced cell research for tendon repair and provides future directions for rotator cuff repair using biologic composites.

Lessons we can learn from gene expression patterns in rotator cuff tears and tendinopathies

Persistently high failure rates that are reported after rotator cuff repairs have encouraged greater understanding of the pathophysiology that underlies rotator cuff tears. Biologic changes that contribute to the pathogenesis of rotator cuff tears and tendinopathies, as well as adaptation after these changes, have been well described. A subset of patients with a genetic predisposition to early onset of rotator cuff tears and earlier symptom and disease progression have been identified. Many biologic changes occurring at the gene level have been identified. Pathways that are believed to contribute to rotator cuff tendinopathies include extracellular matrix remodeling, angiogenesis, changes in metabolism, apoptosis, and stress-related genes. Metaplasia of rotator cuff cells is contributed to by changes in gene expression. Modification of these gene changes may be possible through mechanical loading, drugs, or cellular manipulation. Gene changes may offer greater insight into why certain tears fail to heal and help to identify therapeutic targets.