Formation and maturation of the murine meniscus

Exploration of bone metabolism status in the distal femur of mice at different growth stages

The mouse femur, particularly the distal femur, is commonly utilized in orthopedic research. Despite its significance, little is known about the key events involved in the postnatal development of the distal femur. Therefore, investigating the development process of the mouse distal femur is of great importance. In this study, distal femurs of CD-1 mice aged 1, 2, 4, 6, and 8 weeks were examined. We found that the width and height of the distal femur continued to increase till the 4th week, followed with stabilization. Notably, the width to height ratio remained relatively consistent with age. Micro computed tomography analysis demonstrated gradual increases in bone volume/tissue volume, trabecular number, and trabecular thickness from 1 to 6 weeks, alongside a gradual decrease in trabecular separation. Histological analysis further indicated the appearance of the secondary ossification center at approximately 2 weeks, with ossification mostly completed by 4 weeks, leading to the formation of a prototype epiphyseal plate. Subsequently, the epiphyseal plate gradually narrowed at 6 and 8 weeks. Moreover, the thickness and maturity of the bone cortex surrounding the epiphyseal plate increased over time, reaching peak cortical bone density at 8 weeks. In conclusion, to enhance model stability and operational ease, we recommend constructing conventional mouse models of the distal femur between 4 and 8 weeks old.

Temporal progression of subchondral bone alterations in OA models involving induction of compromised meniscus integrity in mice and rats: A scoping review

OBJECTIVE

To categorize the temporal progression of subchondral bone alterations induced by compromising meniscus integrity in mouse and rat models of knee osteoarthritis (OA).

METHOD

Scoping review of investigations reporting subchondral bone changes with appropriate negative controls in the different mouse and rat models of OA induced by compromising meniscus integrity.

RESULTS

The available literature provides appropriate temporal detail on subchondral changes in these models, covering the entire spectrum of OA with an emphasis on early and mid-term time points. Microstructural changes of the subarticular spongiosa are comprehensively described; those of the subchondral bone plate are not. In mouse models, global subchondral bone alterations are unidirectional, involving an advancing sclerosis of the trabecular structure over time. In rats, biphasic subchondral bone alterations begin with an osteopenic degeneration and loss of subchondral trabeculae, progressing to a late sclerosis of the entire subchondral bone. Rat models, independently from the applied technique, relatively faithfully mirror the early bone loss detected in larger animals, and the late subchondral bone sclerosis observed in human advanced OA.

CONCLUSION

Mice and rats allow us to study the microstructural consequences of compromising meniscus integrity at high temporal detail. Thickening of the subchondral bone plate, an early loss of thinner subarticular trabecular elements, followed by a subsequent sclerosis of the entire subchondral bone are all important and reliable hallmarks that occur in parallel with the advancing articular cartilage degeneration. Thoughtful decisions on the study design, laterality, selection of controls and volumes of interest are crucial to obtain meaningful data.

Revealing Early Spatial Patterns of Cellular Responsivity in Fiber-Reinforced Microenvironments

Fiber-reinforcement approaches have been used to replace aligned tissues with engineered constructs after injury or surgical resection, strengthening soft biomaterial scaffolds and replicating anisotropic, load-bearing properties. However, most studies focus on the macroscale aspects of these scaffolds, rarely considering the cell-biomaterial interactions that govern remodeling and extracellular matrix organization toward aligned neo-tissues. As initial cell-biomaterial responses within fiber-reinforced microenvironments likely influence the long-term efficacy of repair and regeneration strategies, here we elucidate the roles of spatial orientation, substrate stiffness, and matrix remodeling on early cell-fiber interactions. Bovine mesenchymal stromal cells (MSCs) were cultured in soft fibrin gels reinforced with a stiff 100 µm polyglycolide-co-caprolactone fiber. Gel stiffness and remodeling capacity were modulated by fibrinogen concentration and aprotinin treatment, respectively. MSCs were imaged at 3 days and evaluated for morphology, mechanoresponsiveness (nuclear Yes-associated protein [YAP] localization), and spatial features including distance and angle deviation from fiber. Within these constructs, morphological conformity decreased as a function of distance from fiber. However, these correlations were weak (R2 = 0.01043 for conformity and R2 = 0.05542 for nuclear YAP localization), illustrating cellular heterogeneity within fiber-enforced microenvironments. To better assess cell-fiber interactions, we applied machine-learning strategies to our heterogeneous dataset of cell-shape and mechanoresponsive parameters. Principal component analysis (PCA) was used to project 23 input parameters (not including distance) onto 5 principal components (PCs), followed by agglomerative hierarchical clustering to classify cells into 3 groups. These clusters exhibited distinct levels of morpho-mechanoresponse (combination of morphological conformity and YAP signaling) and were classified as high response (HR), medium response (MR), and low response (LR) clusters. Cluster distribution varied spatially, with most cells (61%) closest to the fiber (0-75 µm) belonging to the HR cluster, and most cells (55%) furthest from the fiber (225-300 µm) belonging to the LR cluster. Modulation of gel stiffness and fibrin remodeling showed differential effects for HR cells, with stiffness influencing the level of mechanoresponse and remodeling capacity influencing the location of responding cells. Together, these novel findings demonstrate early trends in cellular patterning of the fiber-reinforced microenvironment, showing how spatial orientation, substrate biophysical properties, and matrix remodeling may guide the amplitude and localization of cellular mechanoresponses. These trends may guide approaches to optimize the design of microscale scaffold architecture and substrate properties for enhancing organized tissue assembly at the macroscale.

The common marmoset as a translational model of age-related osteoarthritis

Age-related osteoarthritis (OA) is a degenerative joint disease characterized by pathological changes in nearly every intra- and peri-articular tissue that contributes to disability in older adults. Studying the etiology of age-related OA in humans is difficult due to an unpredictable onset and insidious nature. A barrier in developing OA modifying therapies is the lack of translational models that replicate human joint anatomy and age-related OA progression. The purpose of this study was to determine whether the common marmoset is a faithful model of human age-related knee OA. Semi-quantitative microCT scoring revealed greater radiographic OA in geriatric versus adult marmosets, and the age-related increase in OA prevalence was similar between marmosets and humans. Quantitative assessments indicate greater medial tibial cortical and trabecular bone thickness and heterogeneity in geriatric versus adult marmosets which is consistent with an age-related increase in focal subchondral bone sclerosis. Additionally, marmosets displayed an age-associated increase in synovitis and calcification of the meniscus and patella. Histological OA pathology in the medial tibial plateau was greater in geriatric versus adult marmosets driven by articular cartilage damage, proteoglycan loss, and altered chondrocyte cellularity. The age-associated increase in medial tibial cartilage OA pathology and meniscal calcification was greater in female versus male geriatric marmosets. Overall, marmosets largely replicate human OA as evident by similar 1) cartilage and skeletal morphology, 2) age-related progression in OA pathology, and 3) sex differences in OA pathology with increasing age. Collectively, these data suggest that the common marmoset is a highly translatable model of the naturally occurring, age-related OA seen in humans.

Reduction in postnatal weight-bearing does not alter the trajectory of murine meniscus growth and maturation

The early postnatal period represents a critical window for the maturation and development of orthopedic tissues, including those within the knee joint. To understand how mechanical loading impacts the maturational trajectory of the meniscus and other tissues of the hindlimb, perturbation of postnatal weight bearing was achieved through surgical resection of the sciatic nerve in neonatal mice at 1 or 14 days old. Sciatic nerve resection (SNR) produced significant and persistent disruptions in gait, leading to reduced tibial length and reductions in Achilles tendon mechanical properties. However, SNR resulted in minimal disruptions in morphometric parameters of the menisci and other structures in the knee joint, with no detectable differences in Col1a1-YFP or Col2a1-CFP expressing cells within the menisci. Furthermore, micromechanical properties of the meniscus and cartilage (as assessed by atomic force microscopy-based nanoindentation testing) were not different between experimental groups. In contrast to our initial hypothesis, reduced hindlimb weight bearing via neonatal SNR did not significantly impact the growth and development of the knee meniscus. This unexpected finding demonstrates that the input mechanical threshold required to sustain meniscus development may be lower than previously hypothesized, though future studies incorporating skeletal kinematic models coupled with force plate measurements will be required to calculate the loads passing through the affected hindlimb and precisely define these thresholds. Collectively, these results provide insight into the mechanobiological responses of the meniscus to alterations in load, and contribute to our understanding of the factors that influence normal postnatal development.

TLR4 antagonism provides short-term but not long-term clinical benefit in a full-depth cartilage defect mouse model

PURPOSE/AIM

Cartilage injury and subsequent osteoarthritis (OA) are debilitating conditions affecting millions worldwide. As there are no cures for these ailments, novel therapies are needed to suppress disease pathogenesis. Given that joint injuries are known to produce damage-associated molecular patterns (DAMPs), our central premise is that the Toll-like receptor 4 (TLR4) pathway is a principal driver in the early response to cartilage damage and subsequent pathology. We postulate that TLR4 activation is initiated/perpetuated by DAMPs released following joint damage. Thus, antagonism of the TLR4 pathway immediately after injury may suppress the development of joint surface defects.

MATERIALS AND METHODS

Two groups were utilized: (1) 8-week-old, male C57BL6 mice treated systemically with a known TLR4 antagonist and (2) mice injected with vehicle control. A full-depth cartilage lesion on the midline of the patellofemoral groove was created in the right knee of each mouse. The left knee was used as a sham surgery control. Gait changes were evaluated over 4 weeks using a quantitative gait analysis system. At harvest, knee joints were processed for pathologic assessment, Nanostring® transcript expression, and immunohistochemistry (IHC).

RESULTS

Short-term treatment with a TLR4 antagonist at 14-days significantly improved relevant gait parameters; improved cartilage metrics and modified Mankin scores were also seen. Additionally, mRNA expression and IHC showed reduced expression of inflammatory mediators in animals treated with the TLR4 antagonist.

CONCLUSIONS

Collectively, this work demonstrates that systemic treatment with a TLR4 antagonist is protective to further cartilage damage 14-days post-injury in a murine model of induced disease.

Lack of skeletal muscle contraction disrupts fibrous tissue morphogenesis in the developing murine knee

Externally applied forces, such as those generated through skeletal muscle contraction, are important to embryonic joint formation, and their loss can result in gross morphologic defects including joint fusion. While the absence of muscle contraction in the developing chick embryo leads to dissociation of dense connective tissue structures of the knee and ultimately joint fusion, the central knee joint cavitates whereas the patellofemoral joint does not in murine models lacking skeletal muscle contraction, suggesting a milder phenotype. These differential results suggest that muscle contraction may not have as prominent of a role in the growth and development of dense connective tissues of the knee. To explore this question, we investigated the formation of the menisci, tendon, and ligaments of the developing knee in two murine models that lack muscle contraction. We found that while the knee joint does cavitate, there were multiple abnormalities in the menisci, patellar tendon, and cruciate ligaments. The initial cellular condensation of the menisci was disrupted and dissociation was observed at later embryonic stages. The initial cell condensation of the tendon and ligaments were less affected than the meniscus, but these tissues contained cells with hyper-elongated nuclei and displayed diminished growth. Interestingly, lack of muscle contraction led to the formation of an ectopic ligamentous structure in the anterior region of the joint as well. These results indicate that muscle forces are essential for the continued growth and maturation of these structures during this embryonic period.

Postnatal morpho-functional development of a dog's meniscus

This study evaluates the morpho-functional modifications that characterize meniscal development from neonatal to adult dogs. Even if menisci are recognized as essential structures for the knee joint, poor information is available about their morphogenesis, in particular in dog models. Menisci from a group of Dobermann Pinchers aged 0, 10, 30 days, and 4 years (T0, T10, T30, adult, respectively) were analyzed by SEM, histochemistry (Safranin O and Picro Sirius Red Staining analyzed under a polarized light microscope), immunofluorescences (collagen type I and II), biomechanical (compression) and biochemical analyses (glycosaminoglycans, GAGs, and DNA content). SEM analyses revealed that the T0 meniscus is a bulgy structure that during growth tends to flatten, firstly in the inner zone (T10) and then even in the outer zone (T30), until the achievement of the completely smooth adult final shape. These results were further supported by the histochemistry analyses in which the deposition of GAGs started from T30, and the presence of type I birefringent collagen fibers was observed from T0 to T30, while poorly refringent type III collagen fibers were observed in the adult dogs. Double immunofluorescence analyses also evidenced that the neonatal meniscus contains mainly type I collagen fibers, as well as the T10 meniscus, and demonstrated a more evident regionalization and crimping in the T30 and adult meniscus. Young's elastic modulus of the meniscus in T0 and T10 animals was lower than the T30 animals, and this last group was also lower than adult ones (T0-T10 vs T30 vs adult). Biochemical analysis confirmed that cellularity decreases over time from neonatal to adult (p < 0.01). The same decreasing trend was observed in GAGs deposition. These results may suggest that the postnatal development of canine meniscus may be related to the progressive functional locomotory development: after birth, the meniscus acquires its functionality over time, through movement, load, and growth itself.

Silencing of NOTCH3 Signaling in Meniscus Smooth Muscle Cells Inhibits Fibrosis and Exacerbates Degeneration in a HEYL-Dependent Manner

The mechanisms of meniscus fibrosis and novel ways to enhance fibrosis is unclear. This work reveals human meniscus fibrosis initiated at E24 weeks. Smooth muscle cell cluster is identified in embryonic meniscus, and the combined analysis with previous data suggests smooth muscle cell in embryonic meniscus as precursors of progenitor cells in the mature meniscus. NOTCH3 is constantly expressed in smooth muscle cells throughout embryogenesis to adulthood. Inhibition of NOTCH3 signaling in vivo inhibits meniscus fibrosis and exacerbates degeneration. Continuous histological sections show that HEYL, NOTCH3 downstream target gene, is expressed consistently with NOTCH3. HEYL knockdown in meniscus cells attenuated the COL1A1 upregulation by CTGF and TGF-β stimulation. Thus, this study discovers the existence of smooth muscle cells and fibers in the meniscus. Inhibition of NOTCH3 signaling in meniscus smooth muscle cells in a HEYL-dependent manner prevented meniscus fibrosis and exacerbated degeneration. Therefore, NOTCH3/HEYL signaling might be a potential therapeutic target for meniscus fibrosis.

Histologically Confirmed Recellularization is a Key Factor that Affects Meniscal Healing in Immature and Mature Meniscal Tears

Healing outcomes of meniscal repair are better in younger than in older. However, exact mechanisms underlying superior healing potential in younger remain unclear from a histological perspective. This study included 24 immature rabbits and 24 mature rabbits. Tears were created in the anterior horn of medial meniscus of right knee in each rabbit. Animals were sacrificed at 1, 3, 6, and 12 weeks postoperatively. We performed macroscopic and histological evaluations of post-meniscal repair specimens. Cells were counted within a region of interest to confirm cellularization at tear site in immature menisci. The width of cell death zone was measured to determine the region of cell death in mature menisci. Apoptosis was evaluated by TUNEL assay. Vascularization was assessed by CD31 immunofluorescence. The glycosaminoglycans and the types 1 and 2 collagen content was evaluated by calculating average optical density of corresponding histological specimens. Cartilage degeneration was also evaluated. Healing outcomes following untreated meniscal tears were superior in immature group. Recellularization with meniscus-like cell morphology was observed at tear edge in immature menisci. Superior recellularization was observed at meniscal sites close to joint capsule than at sites distant from the capsule. Recellularization did not occur at tear site in mature group; however, we observed gradual enlargement of cell death zone. Apoptosis was presented at 1, 3, 6, 12 weeks in immature and mature menisci after untreated meniscal tears. Vascularization was investigated along the tear edges in immature menisci. Glycosaminoglycans and type 2 collagen deposition were negatively affected in immature menisci. We observed glycosaminoglycan degradation in mature menisci and cartilage degeneration, specifically in immature cartilage of the femoral condyle. In conclusion, compared with mature rabbits, immature rabbits showed more robust healing response after untreated meniscal tears. Vascularization contributed to the recellularization after meniscal tears in immature menisci. Meniscal injury fundamentally alters extracellular matrix deposition.

Intrinsic and growth-mediated cell and matrix specialization during murine meniscus tissue assembly

The incredible mechanical strength and durability of mature fibrous tissues and their extremely limited turnover and regenerative capacity underscores the importance of proper matrix assembly during early postnatal growth. In tissues with composite extracellular matrix (ECM) structures, such as the adult knee meniscus, fibrous (Collagen-I rich), and cartilaginous (Collagen-II, proteoglycan-rich) matrix components are regionally segregated to the outer and inner portions of the tissue, respectively. While this spatial variation in composition is appreciated to be functionally important for resisting complex mechanical loads associated with gait, the establishment of these specialized zones is poorly understood. To address this issue, the following study tracked the growth of the murine meniscus from its embryonic formation through its first month of growth, encompassing the critical time-window during which animals begin to ambulate and weight bear. Using histological analysis, region specific high-throughput qPCR, and Col-1, and Col-2 fluorescent reporter mice, we found that matrix and cellular features defining specific tissue zones were already present at birth, before continuous weight-bearing had occurred. These differences in meniscus zones were further refined with postnatal growth and maturation, resulting in specialization of mature tissue regions. Taken together, this work establishes a detailed timeline of the concurrent spatiotemporal changes that occur at both the cellular and matrix level throughout meniscus maturation. The findings of this study provide a framework for investigating the reciprocal feedback between cells and their evolving microenvironments during assembly of a mechanically robust fibrocartilage tissue, thus providing insight into mechanisms of tissue degeneration and effective regenerative strategies.

The critical role of Hedgehog-responsive mesenchymal progenitors in meniscus development and injury repair

Meniscal tears are associated with a high risk of osteoarthritis but currently have no disease-modifying therapies. Using a Gli1 reporter line, we found that Gli1+ cells contribute to the development of meniscus horns from 2 weeks of age. In adult mice, Gli1+ cells resided at the superficial layer of meniscus and expressed known mesenchymal progenitor markers. In culture, meniscal Gli1+ cells possessed high progenitor activities under the control of Hh signal. Meniscus injury at the anterior horn induced a quick expansion of Gli1-lineage cells. Normally, meniscal tissue healed slowly, leading to cartilage degeneration. Ablation of Gli1+ cells further hindered this repair process. Strikingly, intra-articular injection of Gli1+ meniscal cells or an Hh agonist right after injury accelerated the bridging of the interrupted ends and attenuated signs of osteoarthritis. Taken together, our work identified a novel progenitor population in meniscus and proposes a new treatment for repairing injured meniscus and preventing osteoarthritis.

Intra-Articular Administration of Cramp into Mouse Knee Joint Exacerbates Experimental Osteoarthritis Progression

Osteoarthritis (OA) is the most common type of arthritis and is associated with wear and tear, aging, and inflammation. Previous studies revealed that several antimicrobial peptides are up-regulated in the knee synovium of patients with OA or rheumatoid arthritis. Here, we investigated the functional effects of cathelicidin-related antimicrobial peptide (Cramp) on OA pathogenesis. We found that Cramp is highly induced by IL-1β via the NF-κB signaling pathway in mouse primary chondrocytes. Elevated Cramp was also detected in the cartilage and synovium of mice suffering from OA cartilage destruction. The treatment of chondrocytes with Cramp stimulated the expression of catabolic factors, and the knockdown of Cramp by small interfering RNA reduced chondrocyte catabolism mediated by IL-1β. Moreover, intra-articular injection of Cramp into mouse knee joints at a low dose accelerated traumatic OA progression. At high doses, Cramp affected meniscal ossification and tears, leading to cartilage degeneration. These findings demonstrate that Cramp is associated with OA pathophysiology.

Comparative anatomy and morphology of the knee in translational models for articular cartilage disorders. Part I: Large animals

BACKGROUND

The human knee is a complex joint, and affected by a variety of articular cartilage disorders. Large animal models are critical to model the complex disease mechanisms affecting a functional joint. Species-dependent differences highly affect the results of a pre-clinical study and need to be considered, necessitating specific knowledge not only of macroscopic and microscopic anatomical and pathological aspects, but also characteristics of their individual gait and joint movements.

METHODS

Literature search in Pubmed.

RESULTS AND DISCUSSION

This narrative review summarizes the most relevant anatomical structural and functional characteristics of the knee (stifle) joints of the major translational large animal species, comprising dogs, (mini)pigs, sheep, goats, and horses in comparison with humans. Specific characteristics of each species, including kinematical gait parameters are provided. Considering these multifactorial dimensions will allow to select the appropriate model for answering the research questions in a clinically relevant fashion.

Comparative anatomy and morphology of the knee in translational models for articular cartilage disorders. Part II: Small animals

BACKGROUND

Small animal models are critical to model the complex disease mechanisms affecting a functional joint leading to articular cartilage disorders. They are advantageous for several reasons and significantly contributed to the understanding of the mechanisms of cartilage diseases among which osteoarthritis.

METHODS

Literature search in Pubmed.

RESULTS AND DISCUSSION

This narrative review summarizes the most relevant anatomical structural and functional characteristics of the knee (stifle) joints of the major small animal species, including mice, rats, guinea pigs, and rabbits compared with humans. Specific characteristics of each species, including kinematical gait parameters are provided and compared with the human situation. When placed in a proper context respecting their challenges and limitations, small animal models are important and appropriate models for articular cartilage disorders.

Molecular biology of meniscus pathology: Lessons learned from translational studies and mouse models

Injury to any individual structure in the knee interrupts the overall function of the joint and initiates a cascade of biological and biomechanical changes whose endpoint is often osteoarthritis (OA). The knee meniscus is an integral component of knee biomechanics and may also contribute to the biological homeostasis of the joint. Meniscus injury altering knee function is associated with a high risk of OA progression, and may also be involved in the initiation of OA. As the relationship between meniscus injury and OA is very complex; despite the availability of transcript level data on human meniscus injury and meniscus mediated OA, mechanistic studies are lacking, and available human data are difficult to validate in the absence of patient-matched noninjured control tissues. As similarities exist between human and mouse knee joint structure and function, investigators have begun to use cutting-edge genetic and genomic tools to examine the usefulness of the mouse as a model to study the intricate relationship between meniscus injury and OA. In this review, we use evidence from human meniscus research to identify critical barriers hampering our understanding of meniscus injury induced OA and discuss strategies to overcome these barriers, including those that can be examined in a mouse model of injury-mediated OA.

Meniscal and ligament modifications in spontaneous and post-traumatic mouse models of osteoarthritis

BACKGROUND

Osteoarthritis (OA) is a whole joint disease that affects all joint tissues, with changes in the articular cartilage (AC), subchondral bone and synovium. Pathologies in menisci and ligaments, however, are rarely analysed, although both are known to play vital roles in the mechanical stability of the joint. The aim of our study was to describe the pathological changes in menisci and ligament during disease development in murine spontaneous and post-traumatic surgically induced OA and to quantify tissue mineralisation in the joint space using micro-computed tomography (μCT) imaging during OA progression.

METHODS

Knees of Str/ort mice (spontaneous OA model; 26-40 weeks) and C57CBA F1 mice following destabilisation of medial meniscus (DMM) surgery (post-traumatic OA model; 8 weeks after DMM), were used to assess histological meniscal and ligament pathologies. Joint space mineralised tissue volume was quantified by μCT.

RESULTS

Meniscal pathological changes in Str/ort mouse knees were associated with articular cartilage lesion severity. These meniscal changes included ossification, hyperplasia, cell hypertrophy, collagen type II deposition and Sox9 expression in the fibrous region near the attachment to the knee joint capsule. Anterior cruciate ligaments exhibited extracellular matrix changes and chondrogenesis particularly at the tibial attachment site, and ossification was seen in collateral ligaments. Similar changes were confirmed in the post-traumatic DMM model. μCT analysis showed increased joint space mineralised tissue volume with OA progression in both the post-traumatic and spontaneous OA models.

CONCLUSIONS

Modifications in meniscal and ligament mineralisation and chondrogenesis are seen with overt AC degeneration in murine OA. Although the aetiology and the consequences of such changes remain unknown, they will influence stability and load transmission of the joint and may therefore contribute to OA progression. In addition, these changes may have important roles in movement restriction and pain, which represent major human clinical symptoms of OA. Description of such soft tissue changes, in addition to AC degradation, should be an important aspect of future studies in mouse models in order to furnish a more complete understanding of OA pathogenesis.

Biological and Mechanical Predictors of Meniscus Function: Basic Science to Clinical Translation

Progressive knee joint degeneration occurs following removal of a torn meniscus. However, there is significant variability in the rate of development of post-meniscectomy osteoarthritis (OA). While there is no current consensus on the risk factors for development of knee OA in patients with meniscus tears, it is likely that both biological and biomechanical factors play critical roles. In this perspective paper, we review the mechanical and the biological predictors of the response of the knee to partial meniscectomy. We review the role of patient-based studies, in vivo animal models, cadaveric models, bioreactor systems, and statistically augmented computational models for the study of meniscus function and post-meniscectomy OA, providing insight into the important interplay between biomechanical and biologic factors. We then discuss the clinical translation of these concepts for "biologic augmentation" of meniscus healing and meniscus replacement. Ultimately, collaborative studies between engineers, biologists, and clinicians is the optimal way to improve our understanding of meniscus pathology and response to injury and/or disease, and to facilitate effective translation of laboratory findings to improved treatments for our patients. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:937-945, 2020.

FOXO1 and FOXO3 transcription factors have unique functions in meniscus development and homeostasis during aging and osteoarthritis

The objective of this study was to examine FoxO expression and FoxO function in meniscus. In menisci from human knee joints with osteoarthritis (OA), FoxO1 and 3 expression were significantly reduced compared with normal menisci from young and old normal donors. The expression of FoxO1 and 3 was also significantly reduced in mouse menisci during aging and OA induced by surgical meniscus destabilization or mechanical overuse. Deletion of FoxO1 and combined FoxO1, 3, and 4 deletions induced abnormal postnatal meniscus development in mice and these mutant mice spontaneously displayed meniscus pathology at 6 mo. Mice with Col2Cre-mediated deletion of FoxO3 or FoxO4 had normal meniscus development but had more severe aging-related damage. In mature AcanCreERT2 mice, the deletion of FoxO1, 3, and 4 aggravated meniscus lesions in all experimental OA models. FoxO deletion suppressed autophagy and antioxidant defense genes and altered several meniscus-specific genes. Expression of these genes was modulated by adenoviral FoxO1 in cultured human meniscus cells. These results suggest that FoxO1 plays a key role in meniscus development and maturation, and both FoxO1 and 3 support homeostasis and protect against meniscus damage in response to mechanical overuse and during aging and OA.

FOXO1 and FOXO3 transcription factors have unique functions in meniscus development and homeostasis during aging and osteoarthritis

The objective of this study was to examine FoxO expression and FoxO function in meniscus. In menisci from human knee joints with osteoarthritis (OA), FoxO1 and 3 expression were significantly reduced compared with normal menisci from young and old normal donors. The expression of FoxO1 and 3 was also significantly reduced in mouse menisci during aging and OA induced by surgical meniscus destabilization or mechanical overuse. Deletion of FoxO1 and combined FoxO1, 3, and 4 deletions induced abnormal postnatal meniscus development in mice and these mutant mice spontaneously displayed meniscus pathology at 6 mo. Mice with Col2Cre-mediated deletion of FoxO3 or FoxO4 had normal meniscus development but had more severe aging-related damage. In mature AcanCreERT2 mice, the deletion of FoxO1, 3, and 4 aggravated meniscus lesions in all experimental OA models. FoxO deletion suppressed autophagy and antioxidant defense genes and altered several meniscus-specific genes. Expression of these genes was modulated by adenoviral FoxO1 in cultured human meniscus cells. These results suggest that FoxO1 plays a key role in meniscus development and maturation, and both FoxO1 and 3 support homeostasis and protect against meniscus damage in response to mechanical overuse and during aging and OA.

Cell sources of inflammatory mediators present in bone marrow areas inside the meniscus

Purpose

To demonstrate the production of inflammatory mediators by cells located in bone marrow spaces inside rodent menisci.

Methods

Mice subjected to transection of the medial collateral and anterior cruciate ligaments and meniscotomy (osteoarthritis model) or to a sham procedure, as well as non-operated (naive) mice and rats, had knee joints excised. Tissues were stained with hematoxylin-eosin and tartrate-resistant acid phosphatase (TRAP). CD68⁺ cells, inducible nitric oxide synthase (iNOS), interleukin (IL)-1β, and tumor necrosis factor (TNF) expression were detected using immunohistochemistry.

Results

Lamellar ossified areas, bone-entrapped osteocytes and bone marrow spaces were found inside menisci of one week up to 6 months-old naïve mice, regardless of gender. Menisci from naive rats also showed the same pattern with bone marrow areas. CD68⁺ cells were identified in bone marrow areas inside the meniscus of mice. TRAP⁺ osteoclasts, and hematogenous precursors expressing IL-1β, TNF, and iNOS were identified inside bone marrow areas in meniscal samples from both naïve and sham operated mice. Quantitative immunoexpression of IL-1 β, TNF and iNOS was more intense, P = 0.0194, 0.0293, 0.0124, respectively, in mouse knees from mice sacrificed 49 days after being subjected to an osteoarthritis (OA) model as compared to sham operated animals.

Conclusion

We provide novel data showing that rodent menisci display bone marrow areas with cells able to produce inflammatory mediators. Immunoexpression of inflammatory mediators in those bone marrow areas is significantly more pronounced in mice subjected to experimental OA

Characterization and Comparison of Postnatal Rat Meniscus Stem Cells at Different Developmental Stages

Meniscus‐derived stem cells (MeSCs) are a potential cell source for meniscus tissue engineering. The stark morphological and structural changes of meniscus tissue during development indicate the complexity of MeSCs at different tissue regions and stages of development. In this study, we characterized and compared postnatal rat meniscus tissue and MeSCs at different tissue regions and stages of development. We observed that the rat meniscus tissue exhibited marked changes in tissue morphology during development, with day 7 being the most representative time point of different developmental stages. All rat MeSCs displayed typical stem cell characteristics. Rat MeSCs derived from day 7 inner meniscus tissue exhibited the highest self‐renewal capacity, cell proliferation, differentiation potential toward various mesenchymal lineage and the highest expression levels of chondrogenic genes and proteins. Transplantation of rat MeSCs derived from day 7 inner meniscus tissue promoted neo‐tissue formation and effectively protected joint surface cartilage in vivo. Our results demonstrated for the first time that rat MeSCs are not necessarily better at earlier developmental stages, and that rat MeSCs derived from day 7 inner meniscus tissue may be a superior cell source for effective meniscus regeneration and articular cartilage protection. This information could make a significant contribution to human meniscus tissue engineering in the future. stem cells translational medicine2019;8:1318&1329

(A): Meniscus tissue at different tissue regions and stages of development. (B): MeSCs at different tissue regions and stages of development. (C): Intra‐articular injection of MeSCs for meniscus regeneration and OA suppression. *Significant difference between two groups at p < .05. **Significant difference between two groups at p < .01. ***Significant difference between two groups at p < .001. ****Significant difference between two groups at p < .0001. N.S., No significant difference between two groups at p ≥ .05.

The Role of Bmp2 in the Maturation and Maintenance of the Murine Knee Joint

Bone morphogenetic proteins (BMPs) are key regulators of skeletal development, growth, and repair. Although BMP signaling is required for synovial joint formation and is also involved in preserving joint function after birth, the role of specific BMP ligands in adult joint homeostasis remains unclear. The purpose of this study was to define the role of Bmp2 in the morphogenesis and maintenance of the knee joint. To do this, we first created Bmp2-LacZ and Gdf5-LacZ knock-in mice and compared their expression patterns in the developing and postnatal murine knee joint. We then generated a knockout mouse model using the Gdf5-cre transgene to specifically delete Bmp2 within synovial joint-forming cells. Joint formation, maturation, and homeostasis were analyzed using histology, immunohistochemistry, qRT-PCR, and atomic force microscopy (AFM)-based nanoindentation to assess the cellular, molecular, and biomechanical changes in meniscus and articular cartilage. Bmp2 is expressed in the articular cartilage and meniscus of the embryonic and adult mouse knee in a pattern distinct from Gdf5. The knee joints of the Bmp2 knockout mice form normally but fail to mature properly. In the absence of Bmp2, the extracellular matrix and shape of the meniscus are altered, resulting in functional deficits in the meniscus and articular cartilage that lead to a progressive osteoarthritis (OA) like knee pathology as the animals age. These findings demonstrate that BMP activity provided by Bmp2 is required for the maturation and maintenance of the murine knee joint and reveal a unique role for Bmp2 that is distinct from Gdf5 in knee joint biology. © 2018 American Society for Bone and Mineral Research.

Postnatal deletion of Alk5 gene in meniscal cartilage accelerates age-dependent meniscal degeneration in mice

Activation of transforming growth factor-β (TGF-β) signaling has been used to enhance healing of meniscal degeneration in several models. However, the exact role and molecular mechanism of TGF-β signaling in meniscus maintenance and degeneration are still not understood due to the absence of in vivo evidence. In this study, we found that the expression of activin receptor-like kinases 5 (ALK5) in the meniscus was decreased with the progression of age and/or osteoarthritis induced meniscal degeneration. Col2α1 positive cells were found to be specifically distributed in the superficial and inner zones of the anterior horn, as well as the inner zone of the posterior horn in mice, indicating that Col2α1-CreER^T2 mice can be a used for studying gene function in menisci. Furthermore, we deleted Alk5 in Col2α1 positive cells in meniscus by administering tamoxifen. Alterations in the menisci structure were evaluated histologically. The expression levels of genes and proteins associated with meniscus homeostasis and TGF-β signaling were analyzed by quantitative real-time PCR analysis (qRT-PCR) and immunohistochemistry (IHC). Our results revealed severe and progressive meniscal degeneration phenotype in 3- and 6-month-old Alk5 cKO mice compared with Cre-negative control, including aberrantly increased hypertrophic meniscal cells, severe fibrillation, and structure disruption of meniscus. qRT-PCR and IHC results showed that disruption of anabolic and catabolic homeostasis of chondrocytes may contribute to the meniscal degeneration phenotype observed in Alk5 cKO mice. Thus, TGF-β/ALK5 signaling plays a chondro-protective role in menisci homeostasis, in part, by inhibiting matrix degradation and maintaining extracellular matrix proteins levels in meniscal tissues.

Impacts of maturation on the micromechanics of the meniscus extracellular matrix

To elucidate how maturation impacts the structure and mechanics of meniscus extracellular matrix (ECM) at the length scale of collagen fibrils and fibers, we tested the micromechanical properties of fetal and adult bovine menisci via atomic force microscopy (AFM)-nanoindentation. For circumferential fibers, we detected significant increase in the effective indentation modulus, E_ind, with age. Such impact is in agreement with the increase in collagen fibril diameter and alignment during maturation, and is more pronounced in the outer zone, where collagen fibrils are more aligned and packed. Meanwhile, maturation also markedly increases the E_ind of radial tie fibers, but not those of intact surface or superficial layer. These results provide new insights into the effect of maturation on the assembly of meniscus ECM, and enable the design of new meniscus repair strategies by modulating local ECM structure and mechanical behaviors.