Gene expression profiles of the meniscus avascular phenotype: A guide for meniscus tissue engineering

Current advances in engineering meniscal tissues: insights into 3D printing, injectable hydrogels and physical stimulation based strategies

The knee meniscus is the cushioning fibro-cartilage tissue present in between the femoral condyles and tibial plateau of the knee joint. It is largely avascular in nature and suffers from a wide range of tears and injuries caused by accidents, trauma, active lifestyle of the populace and old age of individuals. Healing of the meniscus is especially difficult due to its avascularity and hence requires invasive arthroscopic approaches such as surgical resection, suturing or implantation. Though various tissue engineering approaches are proposed for the treatment of meniscus tears, three-dimensional (3D) printing/bioprinting, injectable hydrogels and physical stimulation involving modalities are gaining forefront in the past decade. A plethora of new printing approaches such as direct light photopolymerization and volumetric printing, injectable biomaterials loaded with growth factors and physical stimulation such as low-intensity ultrasound approaches are being added to the treatment portfolio along with the contemporary tear mitigation measures. This review discusses on the necessary design considerations, approaches for 3D modeling and design practices for meniscal tear treatments within the scope of tissue engineering and regeneration. Also, the suitable materials, cell sources, growth factors, fixation and lubrication strategies, mechanical stimulation approaches, 3D printing strategies and injectable hydrogels for meniscal tear management have been elaborated. We have also summarized potential technologies and the potential framework that could be the herald of the future of meniscus tissue engineering and repair approaches.

"Slow walk" mimetic tensile loading maintains human meniscus tissue resident progenitor cells homeostasis in photocrosslinked gelatin hydrogel

Meniscus, the cushion in knee joint, is a load-bearing tissue that transfers mechanical forces to extracellular matrix (ECM) and tissue resident cells. The mechanoresponse of human tissue resident stem/progenitor cells in meniscus (hMeSPCs) is significant to tissue homeostasis and regeneration but is not well understood. This study reports that a mild cyclic tensile loading regimen of ∼1800 loads/day on hMeSPCs seeded in 3-dimensional (3D) photocrosslinked gelatin methacryloyl (GelMA) hydrogel is critical in maintaining cellular homeostasis. Experimentally, a "slow walk" biomimetic cyclic loading regimen (10% tensile strain, 0.5 Hz, 1 h/day, up to 15 days) is applied to hMeSPCs encapsulated in GelMA hydrogel with a magnetic force-controlled loading actuator. The loading significantly increases cell differentiation and fibrocartilage-like ECM deposition without affecting cell viability. Transcriptomic analysis reveals 332 mechanoresponsive genes, clustered into cell senescence, mechanical sensitivity, and ECM dynamics, associated with interleukins, integrins, and collagens/matrix metalloproteinase pathways. The cell-GelMA constructs show active ECM remodeling, traced using a green fluorescence tagged (GFT)-GelMA hydrogel. Loading enhances nascent pericellular matrix production by the encapsulated hMeSPCs, which gradually compensates for the hydrogel loss in the cultures. These findings demonstrate the strong tissue-forming ability of hMeSPCs, and the importance of mechanical factors in maintaining meniscus homeostasis.

Formulation and evaluation of a bioink composed of alginate, gelatin, and nanocellulose for meniscal tissue engineering

1The necessity to preserve meniscal function prompts the research and development of novel treatment options, like three-dimensional (3D) bioprinting. However, bioinks for meniscal 3D bioprinting have not been extensively explored. Therefore, in this study, a bioink composed of alginate, gelatin, and carboxymethylated cellulose nanocrystal (CCNC) was formulated and evaluated. Firstly, bioinks with varying concentrations of the aforementioned components were subjected to rheological analysis (amplitude sweep test, temperature sweep test, and rotation). The optimal bioink formulation of 4.0% gelatin, 0.75% alginate, and 1.4% CCNC dissolved in 4.6% D-mannitol was further used for printing accuracy analysis, followed by 3D bioprinting with normal human knee articular chondrocytes (NHAC-kn). The encapsulated cells' viability was > 98%, and collagen II expression was stimulated by the bioink. The formulated bioink is printable, stable under cell culture conditions, biocompatible, and able to maintain the native phenotype of chondrocytes. Aside from meniscal tissue bioprinting, it is believed that this bioink could serve as a basis for the development of bioinks for various tissues.

Pneumatospinning Biomimetic Scaffolds for Meniscus Tissue Engineering

Nanofibrous scaffolds fabricated via electrospinning have been proposed for meniscus tissue regeneration. However, the electrospinning process is slow, and can only generate scaffolds of limited thickness with densely packed fibers, which limits cell distribution within the scaffold. In this study, we explored whether pneumatospinning could produce thicker collagen type I fibrous scaffolds with higher porosity, that can support cell infiltration and neo-fibrocartilage tissue formation for meniscus tissue engineering. We pneumatospun scaffolds with solutions of collagen type I with thicknesses of approximately 1 mm in 2 h. Scanning electron microscopy revealed a mix of fiber sizes with diameters ranging from 1 to 30 µm. The collagen scaffold porosity was approximately 48% with pores ranging from 7.4 to 100.7 µm. The elastic modulus of glutaraldehyde crosslinked collagen scaffolds was approximately 45 MPa, when dry, which reduced after hydration to 0.1 MPa. Mesenchymal stem cells obtained from the infrapatellar fat pad were seeded in the scaffold with high viability (>70%). Scaffolds seeded with adipose-derived stem cells and cultured for 3 weeks exhibited a fibrocartilage meniscus-like phenotype (expressing COL1A1, COL2A1 and COMP). Ex vivo implantation in healthy bovine and arthritic human meniscal explants resulted in the development of fibrocartilage-like neotissues that integrated with the host tissue with deposition of glycosaminoglycans and collagens type I and II. Our proof-of-concept study indicates that pneumatospinning is a promising approach to produce thicker biomimetic scaffolds more efficiently that electrospinning, and with a porosity that supports cell growth and neo-tissue formation using a clinically relevant cell source.

Histologic and molecular features in pathologic human menisci from knees with and without osteoarthritis

The objective of this study was to evaluate histologic and molecular features of meniscus degeneration in cohorts of patients with and without osteoarthritis (OA) of the knee. Menisci were obtained from patients undergoing total knee arthroplasty for OA (TKA) or arthroscopic partial meniscectomy (APM) for a torn knee meniscus. Degenerative meniscal tears were among the most common tear type in the APM group based on the pattern. Using an integrative workflow for molecular evaluation of formalin-fixed and paraffin-embedded tissues, human menisci underwent blinded histologic evaluation and NanoString gene expression analyses. Histology revealed increased proteoglycan content in TKA menisci compared to APM menisci, but otherwise no significant differences in the total pathology score or sub-scores between patients based on age or cohort. NanoString analyses revealed differential expression of genes primarily associated with the PI3K-AKT signaling pathway, cell cycle, and apoptosis. These data provide new insights into histological and molecular features of meniscus degeneration in patients with and without knee OA. Histologic assessment of menisci showed similar severity of overall degeneration between cohorts, but there were differences at the molecular level. The dysregulated pathways identified in this study could contribute to early-onset meniscus degeneration, or to a predisposition to meniscus tears and subsequent knee OA. Further studies that validate genes and pathways uncovered in this study will allow us to evaluate novel approaches to assess and treat meniscal degeneration.

Collagen fibrous scaffolds for sustained delivery of growth factors for meniscal tissue engineering

Aim: To mimic the ultrastructural morphology of the meniscus with nanofiber scaffolds coupled with controlled growth factor delivery to modulate cellular performance for tissue engineering of menisci. Methods: The authors functionalized collagen nanofibers by conjugating heparin to the following growth factors for sustained release: PDGF-BB, TGF-β1 and CTGF. Results: Incorporating growth factors increased human meniscal and synovial cell viability, proliferation and infiltration in vitro, ex vivo and in vivo; upregulated key genes involved in meniscal extracellular matrix synthesis and enhanced generation of meniscus-like tissue. Conclusion: The authors' results indicate that functionalizing collagen nanofibers can create a cell-favorable micro- and nanoenvironment and can serve as a system for sustained release of bioactive factors.

Advances in electrospun scaffolds for meniscus tissue engineering and regeneration

The meniscus plays a critical role in maintaining the homeostasis, biomechanics, and structural stability of the knee joint. Unfortunately, it is predisposed to damages either from sports-related trauma or age-related degeneration. The meniscus has an inherently limited capacity for tissue regeneration. Self-healing of injured adult menisci only occurs in the peripheral vascularized portion, while the spontaneous repair of the inner avascular region seems never happens. Repair, replacement, and regeneration of menisci through tissue engineering strategies are promising to address this problem. Recently, many scaffolds for meniscus tissue engineering have been proposed for both experimental and preclinical investigations. Electrospinning is a feasible and versatile technique to produce nano- to micro-scale fibers that mimic the microarchitecture of native extracellular matrix and is an effective approach to prepare nanofibrous scaffolds for constructing engineered meniscus. Electrospun scaffolds are reported to be capable of inducing colonization of meniscus cells by modulating local extracellular density and stimulating endogenous regeneration by driving reprogramming of meniscus wound microenvironment. Electrospun nanofibrous scaffolds with tunable mechanical properties, controllable anisotropy, and various porosities have shown promises for meniscus repair and regeneration and will undoubtedly inspire more efforts in exploring effective therapeutic approaches towards clinical applications. In this article, we review the current advances in the use of electrospun nanofibrous scaffolds for meniscus tissue engineering and repair and discuss prospects for future studies.

Meniscus cell regional phenotypes: Dedifferentiation and reversal by biomaterial embedding

Meniscus injuries are common and a major cause of long-term joint degeneration and disability. Current treatment options are limited, so novel regenerative therapies or tissue engineering strategies are urgently needed. The development of new therapies is hindered by a lack of knowledge regarding the cellular biology of the meniscus and a lack of well-established methods for studying meniscus cells in vitro. The goals of this study were to (1) establish baseline expression profiles and dedifferentiation patterns of inner and outer zone primary meniscus cells, and (2) evaluate the utility of poly(ethylene glycol) diacrylate (PEGDA) and gelatin methacrylate (GelMA) polymer hydrogels to reverse dedifferentiation trends for long-term meniscus cell culture. Using reverse transcription-quantitative polymerase chain reaction, we measured expression levels of putative meniscus phenotype marker genes in freshly isolated meniscus tissue, tissue explant culture, and monolayer culture of inner and outer zone meniscus cells from porcine knees to establish baseline dedifferentiation characteristics, and then compared these expression levels to PEGDA/GelMA embedded passaged meniscus cells. COL1A1 showed robust upregulation, while CHAD, CILP, and COMP showed downregulation with monolayer culture. Expression levels of COL2A1, ACAN, and SOX9 were surprisingly similar between inner and outer zone tissue and were found to be less sensitive as markers of dedifferentiation. When embedded in PEGDA/GelMA hydrogels, expression levels of meniscus cell phenotype genes were significantly modulated by varying the ratio of polymer components, allowing these materials to be tuned for phenotype restoration, meniscus cell culture, and tissue engineering applications.

Gene expression profiling of primary fibrochondrocyte cultures in traumatic and degenerative meniscus lesions

PURPOSE

This study aimed to investigate how fibroblastic and chondrocytic properties of human meniscal fibrochondrocytes are affected in culture conditions according to the type of meniscal pathology and localization, and to provide basic information for tissue-engineering studies.

METHODS

Primary fibrochondrocyte cultures were prepared from meniscus samples of patients who had either traumatic tear or degeneration due to osteoarthritis. Cultures were compared in terms of mRNA expression levels of COL1A1, COL2A1, COMP1, HIF1A, HIF2A, and SOX9 and secreted total collagen and sulfated sGAG levels according to the type of meniscal pathology, anatomical localization, and the number of subcultures.

RESULTS

mRNA expression levels of COL1A1, COMP1, HIF1A, HIF2A, and SOX9 were found to be increased in subsequent subcultures in all specimens. COL1A1 mRNA expression levels of both lateral and medial menisci of patients with traumatic tear were significantly higher than in patients with degenerative pathology, indicating a more fibroblastic character. P1 subculture of lateral and P3 or further subculture of medial meniscus showed more fibroblastic characteristics in patients with degenerative pathology. Furthermore, in patients with degenerative pathology, the subcultures of the lateral meniscus (especially on the inner part) presented more chondrocytic characteristics than did those of medial meniscus.

CONCLUSIONS

The mRNA expression levels of the cultures showed significant differences according to the anatomical localization and pathology of the meniscus, indicating distinct chondrocytic and fibroblastic features. This fundamental knowledge would help researchers to choose more efficient cell sources for cell-seeding of a meniscus scaffold, and to generate a construct resembling the original meniscus tissue.

Three-dimensional (3D) hydrogel serves as a platform to identify potential markers of chondrocyte dedifferentiation by combining RNA sequencing

Dedifferentiation of chondrocyte greatly restricts its function and application, however, it is poorly understood except a small number of canonical markers. The non-cell-adhesive property endows polysaccharide hydrogel with the ability to maintain chondrocyte phenotype, which can serve as a platform to identify new molecular markers and therapeutic targets of chondrocyte dedifferentiation. In this study, the high-throughput RNA sequencing (RNA-seq) was first performed on articular chondrocytes at primary (P0) and passage 1 (P1) stages to explore the global alteration of gene expression along with chondrocyte dedifferentiation. Significantly, several potential marker genes, such as PFKFB3, KDM6B, had been identified via comparatively analyzing their expression in P0 and P1 chondrocytes as well as in 3D constructs (i.e. chondrocyte-laden alginate hydrogel and HA-MA hydrogel) at both mRNA and protein level. Besides, the changes in cellular morphology and enriched pathway of differentially expressed genes during chondrocyte dedifferentiation was studied in detail. This study developed the use of hydrogel as a platform to investigate chondrocyte dedifferentiation; the results provided new molecular markers and potential therapeutic targets of chondrocyte dedifferentiation.

Human engineered meniscus transcriptome after short-term combined hypoxia and dynamic compression

This study investigates the transcriptome response of meniscus fibrochondrocytes (MFCs) to the low oxygen and mechanical loading signals experienced in the knee joint using a model system. We hypothesized that short term exposure to the combined treatment would promote a matrix-forming phenotype supportive of inner meniscus tissue formation. Human MFCs on a collagen scaffold were stimulated to form fibrocartilage over 6 weeks under normoxic (NRX, 20% O₂) conditions with supplemented TGF-β3. Tissues experienced a delayed 24h hypoxia treatment (HYP, 3% O₂) and then 5 min of dynamic compression (DC) between 30 and 40% strain. Delayed HYP induced an anabolic and anti-catabolic expression profile for hyaline cartilage matrix markers, while DC induced an inflammatory matrix remodeling response along with upregulation of both SOX9 and COL1A1. There were 41 genes regulated by both HYP and DC. Overall, the combined treatment supported a unique gene expression profile favouring the hyaline cartilage aspect of inner meniscus matrix and matrix remodeling.

Assessing the Resident Progenitor Cell Population and the Vascularity of the Adult Human Meniscus

PURPOSE

To identify, characterize, and compare the resident progenitor cell populations within the red-red, red-white, and white-white (WW) zones of freshly harvested human cadaver menisci and to characterize the vascularity of human menisci using immunofluorescence and 3-dimensional (3D) imaging.

METHODS

Fresh adult human menisci were harvested from healthy donors. Menisci were enzymatically digested, mononuclear cells isolated, and characterized using flow cytometry with antibodies against mesenchymal stem cell surface markers (CD105, CD90, CD44, and CD29). Cells were expanded in culture, characterized, and compared with bone marrow-derived mesenchymal stem cells. Trilineage differentiation potential of cultured cells was determined. Vasculature of menisci was mapped in 3D using a modified uDisco clearing and immunofluorescence against vascular markers CD31, lectin, and alpha smooth muscle actin.

RESULTS

There were no significant differences in the clonogenicity of isolated cells between the 3 zones. Flow cytometry showed presence of CD44⁺CD105⁺CD29⁺CD90⁺ cells in all 3 zones with high prevalence in the WW zone. Progenitors from all zones were found to be potent to differentiate to mesenchymal lineages. Larger vessels in the red-red zone of meniscus were observed spanning toward red-white, sprouting to smaller arterioles and venules. CD31⁺ cells were identified in all zones using the 3D imaging and co-localization of additional markers of vasculature (lectin and alpha smooth muscle actin) was observed.

CONCLUSIONS

The presence of resident mesenchymal progenitors was evident in all 3 meniscal zones of healthy adult donors without injury. In addition, our results demonstrate the presence of vascularization in the WW zone.

CLINICAL RELEVANCE

The existence of progenitors and presence of microvasculature in the WW zone of the meniscus suggests the potential for repair and biologic augmentation strategies in that zone of the meniscus in young healthy adults. Further research is necessary to fully define the functionality of the meniscal blood supply and its implications for repair.

Meniscal tissue repair with nanofibers: future perspectives

The knee menisci are critical to the long-term health of the knee joint. Because of the high incidence of injury and degeneration, replacing damaged or lost meniscal tissue is extremely clinically relevant. The multiscale architecture of the meniscus results in unique biomechanical properties. Nanofibrous scaffolds are extremely attractive to replicate the biochemical composition and ultrastructural features in engineered meniscus tissue. We review recent advances in electrospinning to generate nanofibrous scaffolds and the current state-of-the-art of electrospun materials for meniscal regeneration. We discuss the importance of cellular function for meniscal tissue engineering and the application of cells derived from multiple sources. We compare experimental models necessary for proof of concept and to support translation. Finally, we discuss future directions and potential for technological innovations.

Implementation of Endogenous and Exogenous Mesenchymal Progenitor Cells for Skeletal Tissue Regeneration and Repair

Harnessing adult mesenchymal stem/progenitor cells to stimulate skeletal tissue repair is a strategy that is being actively investigated. While scientists continue to develop creative and thoughtful ways to utilize these cells for tissue repair, the vast majority of these methodologies can ultimately be categorized into two main approaches: (1) Facilitating the recruitment of endogenous host cells to the injury site; and (2) physically administering into the injury site cells themselves, exogenously, either by autologous or allogeneic implantation. The aim of this paper is to comprehensively review recent key literature on the use of these two approaches in stimulating healing and repair of different skeletal tissues. As expected, each of the two strategies have their own advantages and limitations (which we describe), especially when considering the diverse microenvironments of different skeletal tissues like bone, tendon/ligament, and cartilage/fibrocartilage. This paper also discusses stem/progenitor cells commonly used for repairing different skeletal tissues, and it lists ongoing clinical trials that have risen from the implementation of these cells and strategies. Lastly, we discuss our own thoughts on where the field is headed in the near future.

Pharmacological blockade of PCAF ameliorates osteoarthritis development via dual inhibition of TNF-α-driven inflammation and ER stress

Background

Epigenetic mechanisms have been reported to play key roles in osteoarthritis (OA) development. P300/CBP-associated factor (PCAF) is a member of the histone acetyltransferases, which exhibits a strong relationship with endoplasmic reticulum (ER) stress and transcription factor nuclear factor kappa B (NF-κB) signals. Salidroside, a natural histone acetylation inhibitor, showed its anti-inflammatory and anti-apoptotic effects in lipopolysaccharide (LPS)-stimulated microglia cells in our previous study. However, whether Sal has a protective effect against OA remains unknown, and its relationships to PCAF, NF-κB, and the ER stress pathway should be explored further.

Methods

We identified the role of PCAF in the pathogenesis of OA and determined the chondroprotective effect of Sal on both tumor necrosis factor alpha (TNF-α)-treated human chondrocytes and a destabilized medial meniscus (DMM) mouse OA model.

Findings

We found increased PCAF expression in human OA cartilage and TNF-α-driven chondrocytes. Meanwhile, silencing of PCAF attenuated nuclear p65 and C/EBP homologous protein levels in chondrocytes upon TNF-α stimulation. Furthermore, Sal was found to specifically bind to the inhibitory site of the PCAF protein structure, which subsequently reversed the TNF-α-induced activation of NF-κB signal and ER stress-related apoptosis in chondrocytes. In addition, the protective effect of Sal and its inhibitory effects on PCAF as well as inflammatory- and ER stress-related markers were also observed in the mouse DMM model.

Interpretation

Pharmacological blockade of PCAF by Sal ameliorates OA development via inhibition of inflammation and ER stress, which makes Sal a promising therapeutic agents for the treatment of OA.

Core-Shell Nanofibrous Scaffolds for Repair of Meniscus Tears

Electrospinning is an attractive method of fabricating nanofibers that replicate the ultrastructure of the human meniscus. However, it is challenging to approximate the mechanical properties of meniscal tissue while maintaining the biocompatibility of collagen fibers. Our objective was to determine if functionalizing polylactic acid (PLA) nanofibers with collagen would enhance their biocompatibility. We therefore used coaxial electrospinning to generate core-shell nanofibers with a core of PLA for mechanical strength and a shell of collagen to enhance cell attachment and matrix synthesis. We characterized the nanostructure of the engineered scaffolds and measured the hydrophilic and mechanical properties. We assessed the performance of human meniscal cells seeded on coaxial electrospun scaffolds to produce meniscal tissue by gene expression and histology. Finally, we investigated whether these cell-seeded scaffolds could repair surgical tears created ex vivo in avascular meniscal explants. Histology, immunohistochemistry, and mechanical testing of ex vivo repair provided evidence of neotissue that was significantly better integrated with the native tissue than with the acellular coaxial electrospun scaffolds. Human meniscal cell-seeded coaxial electrospun scaffolds may have potential in enhancing repair of avascular meniscus tears. Impact Statement The success of any tissue-engineered meniscus graft relies on its ability to mimic native three-dimensional microstructure, support cell growth, produce tissue-specific matrix, and enhance graft integration into the repair site. Polylactic acid scaffolds possess the desired mechanical properties, whereas collagen scaffolds induce better cell attachment and enhanced tissue regeneration. We therefore fabricated nanofibrous scaffolds that combined the properties of two biomaterials. These novel coaxial scaffolds more closely emulated the structure, mechanical properties, and biochemical composition of native meniscal tissue. Our findings of meniscogenic tissue generation and integration in meniscus defects have the potential to be translated to clinical use.