Toward a comprehensive understanding of the molecular biology of meniscus pathology: lessons learned from translational studies and mouse models

Abstract

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 alters 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 true patient-matched non-injured 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 induced OA, and discuss strategies to overcome these barriers, including those that can be examined in a mouse model of injury mediated OA.

Introduction

The knees are the largest synovial joints in the human body and are among the joints most commonly affected by the highly prevalent degenerative disease known as osteoarthritis (OA). Despite extensive research, the etiology and pathogenesis of primary idiopathic OA are not fully understood. Although the increasing prevalence of OA has been attributed, at least in part, to an increase in life expectancy and the obesity epidemic^{1; 2}, a recent study has, however, reported that the high prevalence of OA cannot be explained by increases in longevity and obesity alone, necessitating the identification of additional independent risk factors³. Moreover, a number of genetic components have been linked to OA⁴, and it is likely that identification of additional genetic susceptibility variants will augment our understanding of individual susceptibility to primary knee OA. In contrast to primary OA, secondary OA occurs after an injury, and injury to any individual knee structure(s) disrupts the normal function of the knee and initiates a cascade of biological and biomechanical changes, which often terminate in post-traumatic OA⁵.

The meniscus, a fibrocartilaginous structure found in the knee (and also in the jaws), is an integral part of the complex biomechanics of the knee joint and may also contribute to the biological homeostasis of other knee joint structures, including the articular cartilages of the femur and the tibia. Meniscal injury often alters knee functions, and often leads to progressive degenerative disease of the joint. Traumatic meniscal tears are common in young, active individuals and as a result, approximately 690,000 partial meniscectomies and almost one million additional knee arthroscopies are performed every year in the United States^{6; 7}. Meniscal resection for traumatic or degenerative meniscal tears is reported to be associated with a 2 to 7 fold risk of development of symptomatic knee OA^{8; 9}. While altered tibiofemoral contact force and joint destabilization resulting from injured meniscus appear to be the primary drivers for OA initiation, a clear mechanistic understanding of how meniscus injury/degeneration initiates a cascade of molecular events leading to OA remains elusive.

In this review, we use evidence from human meniscus research to discuss critical barriers hampering our understanding of the pathogenic mechanisms of meniscus induced OA. These barriers include the heterogeneous nature of meniscus, both in terms of structure and function; the influence of both intrinsic and extrinsic confounders (risk factors); lack of understanding of the capacity of the meniscus to regenerate due to limited success in enhancing meniscus healing with available treatments; and, lack of availability of patient-matched non-injured controls. These significant obstacles currently preclude development of a mechanistic understanding of meniscus mediated OA that would not only provide important insights into the pathogenesis of OA but also help identify targets for therapeutic intervention. Finally, we discuss strategies to overcome the aforementioned barriers, including the utility of a mouse model predictive of meniscus injury mediated OA.

The meniscus is a complex structure that performs diverse functions required for optimal knee joint function

Human menisci are C-shaped structures located between the femoral condyle and tibial plateau within the knee. Composed primarily of fibrocartilage endowed with high water content, the menisci are strong, flexible and resilient¹⁰. Menisci perform diverse functions critical to normal knee physiology. Physically, they distribute loads across the knee and provide a low friction surface for joint movement¹¹. Biologically, menisci contribute to the composition of synovial fluid^{12; 13} and communicate, via production of both biomechanical and biological signals (such as inflammatory pathways, anabolic and catabolic markers, and immune system pathways) with the adjacent articular surfaces of the tibia and femur, and perhaps the synovium and other tissues in the knee joint as well¹⁴. Often underappreciated is the structural and cellular complexity of the meniscus that allows for these functions. When injured, impaired meniscus function has a huge impact on joint physiology, increasing the risk for development of OA, and contributing to poor joint health and disability^{15; 16}.

Structural heterogeneity of meniscus

Each meniscus can be divided into an anterior horn, body and posterior horn, and can also be divided into an outer region, middle region and inner region¹⁷. Anatomically, the outer edges and ends of each meniscus are attached to the tibia by ligaments. The inner edges lack attachments, allowing for frequent adaptation in meniscus shape with joint movements. The vascularity of the meniscus is described as consisting of three zones. The outer red-red zone is essentially at the meniscus-capsule junction and has excellent vascularity and potential for healing of meniscal repairs. Moving centrally, the middle red-white zone has intermediate vascularity and healing potential. The inner white-white zone has essentially no vasculature and minimal healing potential. It is important to note that the healing capacity of the meniscus varies by region; meniscus outer regions that contain ample vasculature and neuronal signals are more likely to initiate a regenerative response than the meniscus inner region, a more cartilage-like structure¹⁸. It is thought that the lack of vasculature in the inner and middle regions of the meniscus is responsible for the low reparative and regenerative capability that occurs after injury in this region¹⁸.

Cellular heterogeneity of the meniscus

Unlike articular cartilage which consists of only one cell type, the meniscus is comprised of several distinct cell populations with unknown lineage relationships to one another¹⁹. The main cell type in the inner and middle regions of the meniscus is the fibrochondrocyte, a round or oval shaped cell surrounded by abundant extracellular matrix²⁰. The major fibrillar collagen produced by meniscus fibrochondrocytes is type II collagen²¹. Meniscus fibrochondrocytes stain negatively for cluster of differentiation (CD) 34, a surrogate marker for hematopoietic stem and progenitor cells. In addition to fibrochondrocytes, the inner avascular region of the meniscus also harbors a unique progenitor cell population that exhibits multilineage differentiation potential and migratory activity²². In contrast, the outer region of the meniscus consists predominantly of spindle-shaped fibroblast-like cells within a dense connective tissue matrix²⁰ composed mainly of type I collagen²³. These cells exhibit positive staining for CD34 but are negative for CD31, a marker for platelet-endothelial cells. It is suggested that this cell type contributes to the healing capability of the outer meniscus since the presence of the vasculature is an important factor in this as well²⁴.

Current treatment of meniscus injuries is only partially successful

There is now increased realization that surgeries aimed at saving the meniscus are preferable to meniscus removal²⁵. Currently, three types of surgical managements are in practice: meniscus repair; arthroscopic partial meniscectomy; and meniscus replacement or transplant²⁶. The structural diversity of the meniscus allows for tailored approaches for treatment of meniscus injuries based on their location. Repair is most often performed in tears occurring in posterior medial region of the meniscus²⁷ as the tears within this zone are the most amenable to interventions²⁸. Recently, because of increased awareness of the significance of meniscus preservation and associated degenerative (osteoarthritic) changes due to meniscectomy²⁵, there has been an attempt to expand repair to include tears in more locations and with different configurations. Partial meniscectomy is a procedure in which damaged fragments of the meniscus are debrided while preserving as much meniscus tissue as possible. While both meniscus repair and partial meniscectomy may be acceptable treatment options for certain types of meniscus tears, meniscal repairs have a higher reoperation rate than partial meniscectomies, although repair is associated with better long-term outcomes^{29; 30}. Meniscus repair techniques are considered to be a significant challenge for lesions found in the inner avascular region, primarily due to the lack of blood supply and perhaps in part because this inner avascular zone appears devoid of stem cell populations²⁴. As such, evolving techniques for meniscus reconstruction employing implantation of meniscus substitutes with or without exogenous stem cells and/or growth factors are being explored in this setting³¹. When a substantial portion of the meniscus is damaged or deficient, meniscus transplantation of fresh or frozen meniscal allografts is the last remaining treatment option³². The biological and synthetic substitutes must possess the anatomical, biological and mechanical characteristics of meniscus, making their utility limited by availability.

Multitude of factors influence meniscus pathology

A multitude of (risk) factors can influence meniscus biology, injury, and repair. Biomechanics obviously play an important role in terms of how meniscal resection impacts the joint overall but the focus of this review is on other factors that impact the biology of the underlying meniscus. Each factor may work independently or in concert with other factors, further highlighting the complexity of meniscus biology and pathology. Some of the most important factors are discussed here and summarized in Table 1.

Table 1:

Factors that contribute to meniscus injury and pathology

Factor	Functional attributes	Reference
Age	Increased risk for meniscus tear Heightened risk for complications after meniscus surgery Increased risk for meniscus degeneration	33
		35
		34
Sex	Female sex is associated with lower risk for meniscus tear	33
Genetics	Genetics increases predisposition to meniscus damage	46
Obesity	Increased risk of first hospitalization due to meniscus lesions Decreased knee flexion after arthroscopic partial meniscectomy Increased incidence of medial meniscus body extrusion	47
		48
		49
Activity level	High activity levels e.g. service in Armed Forces is associated with increased incidence of meniscus injury Participation in ball playing sports, gymnastics and jogging predict the highest risk of meniscus lesions. A gradual increase in activity (e.g. frequency, duration, intensity) may be warranted prior to returning to activities that involve running	33
		47
		69
Time from injury	Duration of complaint is associated with heightened risk of meniscectomy Shorter duration of symptoms prior to partial meniscectomy is associated with a greater improvement in clinical symptoms after surgery	61
		62
Prior knee injury and surgery	Prior ACL tears is associated with a decreased risk of meniscectomy Young people with ACL injuries have a very high associated incidence of meniscal disease at the time of corrective surgery	61
		63
Tear pattern	Root tear is associated with increased pain Young patients with ACL tears have a very high prevalence of meniscal pathology Strong association between ipsilateral meniscus tear and medial meniscus body extrusion No predictable relationship between meniscal damage and meniscal symptoms	68
		63
		49
		68
Status of the articular cartilage	A weak evidence exists to establish a link between early degenerative changes in the cartilage and gene expression in meniscus	40

ACL = anterior cruciate ligament

Age

Like any musculoskeletal tissue, the biology of meniscus is impacted by aging. It has been reported that increasing age is associated with an increased risk for meniscus injury³³ and the prevalence of degenerative meniscal lesions frequently increases with age³⁴. Further, it has been reported that aging heightens risk for complications after meniscus surgery³⁵. However, there is some discrepancy in the literature with regard to the impact of age on meniscus repair. While some authors have reported that young patients are the best candidates for a meniscal repair because of their greater healing potential^{36; 37}, others have reported that patients, under the age of 30 years, experience significantly higher rates of subsequent meniscectomies than older patients and that meniscal repairs healed equally well in younger and older patients^{38; 39}. Microarray analysis of injured menisci has revealed that gene signatures associated with the immune response, inflammation, cell cycle, and cellular proliferation are increased with age, whereas genes associated with cartilage and skeletal development and extracellular matrix synthesis are repressed with age, suggesting a distinct phenotype of meniscus degeneration with aging⁴⁰. From a mechanistic perspective, autophagy, a natural cellular homeostasis mechanism that facilitates normal cell function and survival by removing unnecessary or dysfunctional components and allowing the orderly degradation and recycling of cellular components⁴¹, is also significantly changed in the meniscus during aging. Aging and injury can lower basal levels of autophagy and result in meniscus degeneration and development of OA⁴². Conversely, induction of autophagy has been shown to significantly reduce the severity of OA⁴³.

Sex

There is a consensus, and available data strongly supports the observation that females are more affected and burdened by knee OA than males⁴⁴. Rates of anterior cruciate ligament (ACL) tears are also higher in women than men when engaged in similar activities⁴⁵. Men, however, have a greater prevalence of meniscus injury than women³³. There are relatively small differences observed in gene expression of meniscus tissue when segregated by sex, with a need for more investigation in this area.

Genetics

Interestingly, it has been noted that meniscus damage also occurs in the absence of knee trauma³⁴, suggesting that factors other than acute injury could contribute to this damage. One study provided evidence for genetic predisposition for meniscus damage⁴⁶. In this study, authors used the Framingham cohort to determine whether radiographic hand OA is associated with meniscus damage by examining the correlation of finger OA on hand radiographs, and the number of fingers affected by OA, with the presence of meniscus damage. The prevalence of meniscus damage was 24.9% to 47.2% and was positively associated with the number of fingers affected by OA. This observation supports the concept of a common systemic/genetic predisposition and/or a common environmental risk factor for radiographic hand OA and meniscus damage.

Obesity

Obesity has a strong impact on meniscus health; it is associated with a high risk of first hospitalization due to meniscus lesions⁴⁷, and with decreased knee flexion after arthroscopic partial meniscectomy⁴⁸. Medial meniscus body extrusion is more common in patients with elevated body mass index (BMI)⁴⁹. Transcriptome profiling of fragments of injured meniscus showed that higher BMI is negatively associated with extracellular matrix gene transcripts in human injured meniscus⁵⁰. Interestingly, greater gene expression differences exist between obese and overweight patients than obese and lean patients or between lean and overweight patients. Gene ontology analysis revealed that biological processes related to oxygen transport, calcium binding, extracellular matrix, metal ion binding, and metabolic process were enriched in the obese and lean or overweight comparison. In contrast, no significant biological processes other than axon guidance were enriched for the lean and overweight comparison. This finding suggests that there is a weight-threshold at which injured meniscus has a magnified response to increased body weight. Additionally, obesity-related changes in gene expression present a plausible explanation for how the biology of the meniscus may contribute to the elevated risk of OA in obese patients. Adipose tissues are a major source of cytokines, chemokines, and metabolically active mediators called adipokines (or adipocytokines) including leptin, resistin, adiponectin, and visfatin¹. These adipokines play important roles in metabolic homeostasis through autocrine, paracrine, and endocrine signaling⁵¹ and provide a critical non-biomechanical link between obesity and OA⁵². A detailed description of how adipokines impact metabolic homeostasis and OA is reviewed in Rai and Sandell¹. While adipokines exert catabolic and anti-anabolic effects both on meniscus and articular cartilage, meniscus has been shown to be more susceptible to adipokine-stimulated catabolism than articular cartilage⁵³. Additionally, specific adipokines play different roles in meniscus homeostasis and injury. For instance, visfatin, but not leptin, exerts catabolic effects on meniscus⁵⁴, while adiponectin and resistin have been shown to be higher in meniscus tear patients with early degenerative changes on radiographs⁵⁵.

Level of activity

The activity level of the individual is a very important factor in determining the rate of meniscus damage. It has been reported that members of the armed services (Army and Marine Corps) have increased incidence of meniscus injury³³. Participation in ball related sports, gymnastics and jogging predict a higher risk of meniscus lesions⁴⁷. Moreover, occupational hazard such as kneeling has also been reported to influence meniscus degeneration as degenerative tears were reported to be more prevalent in individuals with increased kneeling such as floor layers compared to those with less kneeling such as graphic designers⁵⁶.

Time from injury

While there is a fair amount of literature on the significance of time of intervention versus time from injury in ACL tears^57–60, little is known about the impact of time from injury on intervention after meniscal tears. There is one report suggesting that duration of symptoms is associated with heightened risk of meniscectomy⁶¹ but meniscal symptoms may not be obvious after meniscus injury. Clinical data suggest that a shorter duration of symptoms prior to partial meniscectomy is associated with a greater improvement in clinical symptoms after surgery⁶².

Concomitant knee injury and surgery

A prior knee injury is associated with a decreased risk of meniscectomy⁶¹. However, it has been reported that young people with ACL injuries have a very high associated incidence of meniscal disease at the time of surgery⁶³. A comparison of gene expression signatures in isolated meniscus tears versus meniscus tears with concomitant ACL tears found increased inflammation and depressed matrix synthesis in combined injuries, suggesting that the meniscus appears to undergo more substantial molecular changes as a result of combined injury⁶⁴.

Tear pattern

Meniscus tear is a known predisposing factor for the development of post-traumatic OA and meniscus tears account for 12% of all knee OA⁶⁵. Approximately 50% of people with meniscal tears have radiographic evidence for OA 6 to 20 years after injury^{66; 67} suggesting that meniscus injury represents a sentinel event for early stage OA. While meniscus tears can present with a wide range of patterns, they are generally classified as traumatic and degenerative.

A recent study, found that the gene expression signatures in injured meniscus vary based on the nature of tear (degenerative vs. traumatic), a finding that may have important implications for both the varying potential for healing following meniscus repair and for the differential risk for developing OA⁶³. Traumatic (vertical) tears expressed higher levels of chemokines and matrix metalloproteases (MMPs) and lower levels of collagen than degenerative (complex, horizontal, or flap) tears. However, to date, no predictable relationship between meniscal tear patterns and meniscal symptoms has been established⁶⁸.

Relationship between meniscus injury and OA is not fully elucidated

Current patient data suggest a strong link between meniscus injury and knee OA^{37; 70–76}, but also suggests that this linkage is complex (Fig. 1). The role of the meniscus in the initiation and progression of OA is thought to be minimal unless injured^{66; 77}. Once injured, emerging evidence suggests that meniscus injury disrupts a number of key biological processes and signaling pathways leading to joint degeneration (Table 2). Meniscus cells secrete inflammatory cytokines and degradative enzymes in response to injury or altered loading^{78; 79}; these signals then affect meniscal cell biology, limiting the native repair potential of meniscus lesions in vitro and likely in vivo⁸⁰. Deleterious effects of injury and/or aging on the meniscus can also affect the homeostasis of other joint tissues, including the synovium and articular cartilage^{12; 79; 81–85}, likely via secreted MMP activity provided by injured meniscus cells^85–89. Additionally, studies comparing torn meniscus from knees undergoing partial meniscectomy to OA knees revealed that menisci undergoing partial meniscectomy exhibited a repair phenotype compared to the inflammatory profile seen in the OA meniscus^{70; 71}, a finding that suggests that the meniscus gene expression profile changes both with injury and degeneration and that these profiles are distinct from one another^{90; 91}. Taken together, current information clearly suggests a biological role for the meniscus in knee OA. One plausible scenario is that meniscal damage induces mechanical instability and increases loading stress in the knee; this in turn stimulates the production of catabolic proteins such as cytokines, chemokines, and matrix‐degrading enzymes by joint tissues and over time results in OA⁷⁹. It is also possible that the initial response of the meniscus to injury could be a repair response, which interacts with the other tissues in the knee in a manner that switches this repair potential into catabolic signals that lead to tissue breakdown.

Fig. 1: Simple diagram showing complex interaction between meniscus tear, degeneration and OA.

Meniscus tear and degeneration both often lead to OA over a course of years. New evidence suggests that OA also contributes to meniscus degeneration and likely increases the susceptibility to meniscus tear. Meniscus tear may precede meniscus degeneration or vice versa. Moreover, the presence of a number of confounding factors (listed in Table 1) further complicates this interaction.

Table 2:

Biological processes and pathways altered in meniscus injury and degeneration

Process/Pathway	Gene/protein involved	Comparison	Tissue	Method	Condition	Reference
Inflammation	↑ IL-6, TNFα	Injured vs. non-injured sites	Human meniscus	ELISA	APM	82
	↑ IL8, CXCL6, MMP3; ↓ COL1A1	Traumatic vs. degenerative	Human meniscus	Real-time PCR	APM	83
	↑ IL-10, IL-6, TNFα, IL-8	Chondral changes vs. control	Human synovial fluid	ELISA	APM	12
Adipokines	↑ ADIPOQ, RETN	Degenerative changes on X-rays	Human meniscus	Real-time PCR	APM	55
Autophagy	↓ ATG-5, LC3	Meniscus degeneration	Murine meniscus	IHC	DMM	42
	↑ BECN1, CASP3	Medial vs. lateral meniscus	Lapine meniscus	Real-time PCR	Arthrotomy	93

IL = interleukin; TNF = tumor necrosis factor; CXCL6 = C-X-C motif chemokine ligand 6; MMP = matrix metalloproteinase; COL1A1 = collagen type I alpha 1; ADIPOQ = adiponectin; RETN = resistin; ATG −5 = autophagy related 5; LC3 = light chain 3; BECN1 = beclin 1; CASP3 = caspase 3; ELISA = enzyme linked immunosorbent assay; IHC = immunohistochemistry; PCR = polymerase chain reaction; APM = arthroscopic partial meniscectomy; DMM = destabilization of medial meniscus

Unfortunately, developing a mechanistic understanding of meniscus-mediated OA is very difficult to establish in human studies, particularly since obtaining suitable control knees may be practically impossible and/or unethical. It is also unlikely that tissue will be available immediately after meniscal injury. There have been attempts to study meniscus tears from clinically asymptomatic patients; however, there are obvious practical and ethical challenges in collecting meniscus tissue from asymptomatic individuals, even though meniscus tears are common in asymptomatic individuals^{34; 75; 92}. Ideally, a comparison between normal uninjured meniscus and a torn or degenerated meniscus is required to determine if the biology of menisci truly differs between the two conditions. However, harvesting of ‘intact’ meniscus from an asymptomatic knee is obviously not possible for ethical reasons. Normal meniscus may be accessible from deceased donors, but a variety of factors such as the nature of injury/disease causing death, the presence or absence of systemic inflammatory or autoimmune disorders, and acute or chronic exposure to medications make it difficult to be sure whether this tissue is a reliable surrogate for a truly intact healthy meniscus.

Strategies to overcome barriers in human meniscus research are emerging

More clinical and translational studies are needed to better understand the circumstances under which meniscal injury leads to the initiation and or furthers the progression of knee OA. While there have been a number of studies recently that attempt to inform this question^{40; 50; 55; 64; 83; 94; 95}, two challenges make it particularly difficult. First, a wide range of factors such as age, BMI, activity level, and concomitant injury can influence meniscal injury biology and the subsequent development of OA, making it necessary to recruit large numbers of patients to account for these covariates. Second, OA takes years if not decades to develop following meniscal injury, requiring relatively long follow up to document the development and progression of OA. The first barrier can be overcome with collaborative efforts to recruit larger cohorts of patients across different providers and locations. The second barrier requires more sensitive yet noninvasive means of detecting early joint degeneration such as cartilage sensitive magnetic resonance imaging or more specific serum biomarkers. Advances in both of these areas may facilitate future studies aimed at more precisely understanding the impact of meniscal injury on the knee.

What might we learn from using mouse models to study the role of meniscal injury in the development of knee OA?

Currently missing from our understanding of the impact of meniscus injury on OA is knowledge of the events set in motion by meniscus trauma that drive subsequent articular cartilage degeneration⁵. The similarities of joint structure and biology, combined with a vast array of genetic and proteomic tools, have led investigators to use the mouse to study joint injuries and examine articular cartilage pathogenesis⁹⁶. Mice have become an increasingly popular model system for the study of post-traumatic OA. Current mouse models of meniscus injury that progress to knee OA include destabilization of the medial meniscus (DMM), meniscal ligament injury (MLI), and medial meniscus injury (MMT). In each, disruption of meniscus function directly affects knee biomechanics, compromising multiple joint tissues, most prominent of which is the knee articular cartilage. It is important to note that mouse knee biomechanics are not identical to those of humans and the differences that exist may affect the pathophysiological processes leading to OA. This is also true for the tissue components making up the knee joint; mouse articular cartilage is much thinner than human articular cartilage and the mouse meniscus undergoes ossification during tissue maturation, unlike the human meniscus. Last, but not least, mouse meniscus biology (in health and disease) does not necessarily represent human meniscus biology (in health and disease) as animal findings do not always translate to humans. These differences should be considered when interpreting data obtained from meniscus injury post-traumatic OA models and balanced against the ability to perform mechanistic studies through genetic manipulation in mice.

Relationship of biomechanical and biological changes in the development of OA

Structure function studies have clearly established that disruption of the complex meniscus extracellular matrix network negatively affects joint biomechanics and joint function. However, analyses of meniscal injuries in humans and large animals strongly indicate that the incomplete meniscal tears that are common in patients do not significantly alter meniscus biomechanics at the time they occur, but often result in the development of knee OA decades later. Injured meniscus secretes a variety of factors including activated MMPs, and damage-associated molecular patterns (DAMPs)⁸¹. Brophy et al⁸³ reported that meniscal tissues from traumatic tears show much greater expression of chemokines and degradative enzyme including interleukin 8, C-X-C motif chemokine ligand 6, MMP-1, and MMP-3 that have been implicated in articular cartilage degradation. However, it is difficult to establish a direct cause and effect relationship due to the variable nature of the meniscus injury, the differences in time from injury to when the sample is taken, the specific site within the meniscus where the specimen was taken from and the overall health status of each individual with an injured meniscus. Availability of a mouse model of meniscus injury that progresses to knee OA would allow for the study the complex relationship between meniscus injury and OA without having to determine how differences in age, sex, injury site, time post-injury and health status influence data interpretation.

Why meniscal injuries exhibit failed healing responses

How tissue regeneration programs are triggered after injury is an emerging research focus due to the availability of new technologies that allow for interrogation of injury on a genome-wide level⁹⁷. In mammals, tissue regeneration ability is organ specific and age-dependent. The use of mouse models has been transformative in resolving key mechanisms of tissue repair allowing investigators to ablate specific cell types, map cell lineages, track cell progeny after tissue injury, and perform bulk and single cell RNA-sequencing⁹⁸. Recent results in mice suggest the number of genes that are exclusively dedicated to tissue regeneration will be limited, and large-scale repurposing of genes with functions in embryonic development will drive the regeneration response after injury⁹⁸. This paradigm can be observed during bone repair where the regeneration process mimics many aspects of skeletal development and many developmental gene are reactivated upon injury in the same temporal and spatial patterns⁹⁹. With respect to meniscus regeneration, no information in the current literature focuses on the genes that direct meniscal morphogenesis in any species other than the mouse, leaving us unable to compare meniscus injury events to meniscus development events in a species other than mice¹⁰⁰.

While mice are considered to be a great resource for OA research due to the ability to study the function of single gene, and mouse meniscus injury models that induce OA such as DMM, MLI and MMT are popular among the scientific community, direct comparison between human and mouse meniscus has limitations, as described above. One area where mouse meniscal injury models will be valuable is the comparison between tissue morphogenesis during development and tissue regeneration after injury.

Studies that exist for mouse meniscus morphogenesis are largely incomplete. Once completed, they should provide us with the knowledge of the genes that are activated during meniscal development, allowing us to ask whether these genes are reactivated after injury, to uncover the signaling pathways that initiate this process, and to investigate the ability of therapeutic agents to enhance signaling pathways associated with meniscal morphogenesis thereby enhancing meniscus healing.

Conclusion

The meniscus is a key tissue in the knee, performing a diverse range of functions necessary to maintain optimal knee joint function and homeostasis. The functional diversity of the meniscus is derived from its structural and cellular diversity, and our lack of understanding of the intricacies of meniscus biology are major reasons why current treatment of meniscus injuries is only partially successful. A number of intrinsic (age, sex, genetics, body weight) and extrinsic (nature, time, and pattern of injury and degeneration) factors influence meniscus homeostasis, repair and injury response. As meniscus injuries often lead to OA over the long term, evaluating the relationship between meniscus injury and OA is difficult. Although efforts have now been directed toward the study of molecular and cellular level changes that occur with meniscus injury, degeneration and regeneration, identification of specific molecular pathways that underlie the meniscus injury response and the cellular mechanisms they influence are missing in the current literature. While transcript-level data on meniscus injury, degeneration and regeneration exist, these are difficult to validate in the absence of patient-matched non-injured 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.