Emerging Insights into the Molecular Basis of Osteoarthritis Pathogenesis and Treatment Strategies

Abstract

Osteoarthritis (OA) is a common and debilitating degenerative joint disease associated with aging and more common among women. OA is a disease that affects many parts of the joint, including cartilage, subchondral bone, and the synovium. Although the exact cause of OA is still under investigation, major factors include dysregulation of inflammatory cytokines and loss of function of mesenchymal stem cells (MSCs). Unfortunately, current treatments for OA are limited to symptomatic management, including nonsteroidal anti-inflammatory drugs (NSAIDs), intra-articular injections such as hyaluronic acid (or cortisol), physical therapy, and surgical intervention, none of which can affect disease progression or provide permanent solutions. Currently there is no FDA approved treatment that can address the molecular basis of OA, although some promising candidates include bone marrow-derived MSC injection, adipose-derived MSC injection, pulsed electromagnetic field (PEMF), TissueGene-C, and CK2.1.

1. Introduction

Osteoarthritis (OA) is a major cause of disability worldwide. In 2020, it was found to affect 595 million individuals globally [1]. It most commonly affects women, accounting for 60% of cases [2] and is strongly correlated with age [3], occurring most frequently after the age of 40 [1] although genetic and environmental factors play a role, as well [4,5]. OA has traditionally been diagnosed by evidence of swelling, limited range of motion, and narrowing of joint space as the articular cartilage degrades [6]. When diagnosed with radiographic evidence, OA is given a score to rank severity on the Kellgren–Lawrence scale based on features of joint space narrowing, bone spurs, sclerosis, and bone deformity. A score of 0 indicates no evidence of OA; 1 is possible narrowing of the joint space; 2 denotes clear osteophyte formation and possible narrowing; 3 indicates multiple osteophytes definite narrowing of the joint space and some sclerosis; and lastly, grade 4 is the most severe. It describes large osteophytes, clear narrowing of joint space, severe sclerosis, and definite bone deformity. Even though OA is usually diagnosed by skeletal changes, osteoarthritis is understood to be a disease of the whole joint, including cartilage, the synovium, and subchondral bone, resulting from the imbalance of metabolic factors and chronic inflammatory signals leading to catabolic activity that degrades the cartilage [7,8,9].

Patients still frequently rely on nonsteroidal anti-inflammatory drugs (NSAIDs), intra-articular injections, such as corticosteroids and hyaluronic acid, with limited effect duration or invasive surgical procedures to control symptoms [10,11]. As such, there is a great need for approval of new molecular or cellular based therapies that could alter or even reverse OA progression. Recently, advances have led to the development of many promising new treatment options. Some of which include MSC injections using cells derived from either adipose tissue [12], or bone marrow, pulsed electromagnetic field (PEMF), TissueGene-C, and CK2.1. This narrative review aims to highlight recent advances in our understanding of the molecular basis of OA pathogenesis and discuss emerging treatment options aimed at addressing them.

2. Osteoarthritis at the Synovial-Joint Scale

Osteoarthritis is the term given to a myriad of possible presentations for degeneration of articular cartilage in joints. As such, two patients can present different symptoms, severity, rate of progression, and may even have distinctly different molecular bases [13]. The underlying factors are complex and still not fully understood, so any general statements about how OA affects any tissue or cell may not apply in every case. Nonetheless, there are certainly patterns that have emerged regarding each tissue’s specific role in the progression of OA.

2.1. The Articular Cartilage in Osteoarthritis

The primary concern of Osteoarthritis (OA) is the breakdown of healthy articular cartilage in the lining of the joints. This type of cartilage is avascular, connective tissue that plays an important role in load distribution, lubrication, and protection of the joint [14]. Healthy, mature articular cartilage is composed mostly of a dense extracellular matrix (ECM) composed of collagen type II and proteoglycans, with the only cell population consisting of chondrocytes. These superficial zone (SZ) chondrocytes are packed tightly and aligned parallel to the articular surface. The middle zone (MZ) contains more mature chondrocytes, still producing ECM. These cells are spherical, and the collagen is organized obliquely. The deep zone (DZ) contains older, columnar chondrocytes, and the collagen fibrils are arranged perpendicular to the joint surface [15].

These chondrocytes maintain the matrix with a balance of catabolic and anabolic activity [15,16], in part regulated by miRNAS. These miRNAs can serve various functions to alter gene expression by transcriptional repression or target degradation. In the context of OA, it has been shown that miRNAs, and their dysregulation, may play a significant role in maintaining the balance of cartilage homeostasis [17]. Several miRNAs have been found in humans that are able to target pathways such as Transforming growth factor beta (TGF-β), BMP (bone morphogenetic protein, associated with anabolic activity and mitogen-activated protein kinase (MAPK) and nuclear factor kappa B (NF-κB) signaling associated with catabolism [17]. It has been shown that abnormal expression can enhance production of inflammatory signals and promote catabolic activity while repressing expression of anabolic indicators [18].

If the cartilage sustains damage, it has some capacity to repair itself; however, it has unique challenges that decrease the efficiency of repair. Firstly, it is avascular; any signals or repair mechanisms would have a difficult time diffusing throughout the affected area, and it is unable to access growth factors present in the blood [19,20]. Secondly, it is a layered tissue consisting of superficial, intermediate, and deep zones, each with its own properties and composition, seen in Figure 1. If the damage is only in the articular cartilage and does not penetrate to the subchondral bone, the damage may result in recruitment of synovial mesenchymal stem cells (MSCs) to differentiate into chondrocytes, temporarily increased chondrocyte proliferation, and ECM production; however, this repair notoriously does not completely “fill the gap” created by the damage [13,20].

If the damage were to penetrate to the subchondral bone, it would then be accessible to bone marrow stem cells and blood cells, which could aid in repair. A fibrin clot would quickly fill in the damage [21] and an inflammatory response would activate, allowing bone mesenchymal stem cells (BMSCs) to migrate to the clot and repair the damage by differentiating into new chondrocytes. However, even in this case, the cartilage that fills in these repairs is fibrous, rather than hyaline, with a decreased mechanical quality than the original tissue [9,22].

2.2. The Synovial Cavity in Osteoarthritis

The synovial cavity is a fluid-filled space between the articulating surface of joints like the knee. It contains the synovial fluid, a viscous fluid secreted by the synovial membrane that encloses the region. Its primary function is to cushion, lubricate, and maintain the ideal environment for the joint tissues, like the cartilage itself. The two major cell types are fibroblast-like synoviocytes (FLSs), along with macrophages, the primary immune cell in the synovial cavity, which are responsible for orchestrating the inflammatory and immune responses within the joint space [23].

In the context of OA progression, the synovium is most strongly associated with inflammation. In an inflammatory environment, chondrocytes and synoviocytes mutually enhance catabolic activity through matrix metalloproteases (MMPs) [24], which cleave essential cartilage proteins like collagen and aggrecan. This cleavage process produces debris like cartilage fragments when they degrade the ECM. These damage-associated molecular patterns (DAMPs) are then sensed by synoviocytes, which interpret these as signs of damage to the cartilage, further enhancing their pro-inflammatory phenotype [24]. Additionally, a physical or mechanical injury can lead to loss of cartilage; however, there is mounting evidence that low-grade persistent inflammation in the synovium (synovitis) can lead to OA symptom progression, and that aging, mitochondrial damage, and senescent cells are highly linked to such inflammation. In an Inflamed joint, both the synoviocytes and chondrocytes shift to the production of MMPs that degrade cartilage [7,8,24].

The chemical landscape of the synovial cavity can change too, contributing to and serving as a marker for OA progression. These changes include an increase in viscosity of the synovial fluid and production of proinflammatory molecules like Follistatin-like protein 1 (FSTL1) or nitric oxide (NO) [8,25]. Many interleukins, including Interleukin-1 beta (Il-1β) are increased in osteoarthritic joints, though varied results have not produced a consensus understanding of the exact role they each play [24].

The synovial space serves as the “signal mediator” of the joint. Any alteration in its composition will ostensibly affect every cell in the joint, hence why abnormal inflammatory signals in the synovium can cause an inflammatory response in the cartilage as well. For this reason, it might be inferred that so long as the synovial space is “pro-inflammatory”, the effectiveness of any OA treatment would diminish drastically, as any chondroprotective effects the treatment might have would be quickly overridden by the constant inflammatory signals. Although cartilage is the primary tissue of concern, the synovium cannot be overlooked for any truly long-term therapy.

2.3. Subchondral Bone in Osteoarthritis

The subchondral bone is the region that lies just below the deepest layer of cartilage in the joint Figure 1. The tissue is innervated and vascular. It supports the cartilage mechanically and by supplying nutrition [26].

Osteocytes (stable bone cells embedded in the matrix) in the subchondral bone can sense aberrant stress and inflammation and respond by signaling for osteoclast (bone-degrading cell) maturation via receptor activator of nuclear factor kappa-B (RANK), leading to bone resorption and the perilacunar remodeling (PLR) process [27]. Osteoblasts (bone producing cells) can generally be thought of as the counter to osteoclasts, as they directly regulate osteogenesis. They can interact either directly or through the secretion of various signals bi-directionally regulating the differentiation and survival of either osteoblasts or osteoclasts [28]. Additionally, endogenic mesenchymal stem cell (MSC) migration, aberrant bone formation, or calcified cartilage can disrupt mechanical homeostasis, causing abnormal mechanical stress leading to chondrocyte apoptosis [26], further elevating the rate of cell turnover in subchondral tissue as cells are subject to abnormal stress and more prone to injury, accelerating OA pathogenesis. It is important to note that subchondral bone is not abnormal in every patient with OA, suggesting it is not sufficient or necessary for disease development on its own [8]. For this reason, subchondral bone could be considered a potential, but not essential, source of inflammatory signals that can lead to OA development. Accordingly, it may not be the most optimal tissue target for treating all presentations of the disease, particularly in the later stages once cartilage has degraded substantially.

2.4. Bone Marrow in Osteoarthritis

Bone marrow is a soft, spongy tissue that occupies most of the internal space of bones and includes the subchondral bone. It is a crucial source of blood cells in the body, as well as mesenchymal stem cells (MSCs) or bone marrow stromal cells (BMSCs), which differentiate into various other cell types that are essential for maintaining joint and bone homeostasis, including chondrocytes, osteoblasts, and osteoclasts [29].

Bone marrow plays a more active role in OA development than previously thought [30]. The health of the marrow can directly impact the health of the MSCs that are crucial to cartilage homeostasis. Furthermore, bone marrow lesions, highly associated with OA, may also lead to angiogenesis, worsening the pain felt by the individual. At least one study reports that these lesions appear earlier than expected, even preceding detectable cartilage damage. Additionally, crosstalk between bone marrow and the cartilage permitted by microcracks or neovascularization allows osteoclast precursors to invade the hypertrophic chondrocyte region to play a role in matrix absorption and ossification [30]. Osteoclasts can also resorb cartilage and calcified cartilage with MMP and cysteine protease-mediated pathways [30]. Bone marrow plays an enormous role in regulating the immune system and is the source of many of the essential cell types in the articular joint. The presence of clear bone marrow lesions before other OA symptoms indicates that bone marrow health could serve as a potential early diagnostic marker for the disease. Additionally, considering how MSCs deplete in number and potency in OA, it may be worth investigating how the depletion of MSCs and formation of lesions might be related [31].

3. Prominent Cell Types Associated with Osteoarthritis

3.1. Articular Chondrocytes in Osteoarthritis

Chondrocytes are the most prominent cell type in mature cartilage. These chondrocytes regulate and maintain the synthesis of the ECM in response to various growth factors such as bone morphogenetic protein 2 (BMP-2), insulin-like growth factor 1 (IGF-1) [32]. Chondrocytes originate as bone marrow mesenchymal stem cells (BMSCs) that aggregate and differentiate into chondroprogenitors. The chondroprogenitors in turn become chondrocytes following stimulation via growth factors like Transforming growth factor beta (TGF-β). These chondrocytes begin in the superficial zone (SFZ) of the cartilage, where they work to produce the major ECM constituents like collagen type II [32]. As the chondrocytes mature, they slow down their production of the major ECM components. Finally, they differentiate into hypertrophic chondrocytes, increasing in size and producing collagen type X [19,32,33]. Finally, these hypertrophic chondrocytes produce catabolic enzymes, namely MMP-13 (an enzyme that cleaves collagen II), to break down/clear cartilage ECM before they eventually undergo apoptosis and are replaced with osteoblasts, which convert the ECM into bone in a process called endochondral ossification. This process of hypertrophy and ossification occurs naturally; however, it can be tied to the progression of OA when chondrocytes hypertrophy during damage repair. If unchecked, this ossification can lead to osteophyte (bone spur) formation, shown in Figure 2, a common symptom of OA [13,14].

Although the role of the chondrocytes can be summarized as simply as “cartilage maintenance”, it is important to remember that these are metabolically active cells that respond to environmental signals to maintain a delicate balance between ECM production and degradation. For example, chondrocytes can cross-regulate with osteoclasts via a variety of mechanisms, such as IL-1, IL-6, PGE2, RANKL, and Vascular endothelial growth factor (VEGF) [30]. In the context of OA, these sorts of signals are usually involved in mediating repair, immune, and inflammatory responses. These signals can tell the chondrocyte when to proliferate, secrete, or degrade ECM, and hypertrophy. If any breakdown of signaling occurs in the chondrocytes or their crosstalk partners in the subchondral bone or synovium, the balance can accelerate towards cartilage degradation, even if otherwise anabolic signals are abundant [34,35,36]. Ultimately, the disruption of chondrocyte homeostasis is “keystone” in the progression of osteoarthritis, as cartilage degradation is the main symptom-causing complication and loss of cartilage can cause aberrant stress leading to inflammation, potentially further worsening the condition of the joint.

3.2. Mesenchymal Stem Cells in Osteoarthritis

Mesenchymal stem cells (MSCs) are multipotent cells that exist in the bone and are capable of differentiating into various cells, including osteoblasts and chondrocytes. Undifferentiated MSCs also play a direct role in regulating the immune response in various mechanisms including paracrine signaling and through direct contact by recruiting dedicated immune cells and suppressing inflammation [8,36]. Mesenchymal stem cells are not directly affected by osteoarthritis; rather, it is the dysregulation and loss of differentiative potential of MSCs that can worsen osteoarthritis. The cells become unable to appropriately differentiate into mature cells that can properly maintain homeostasis in the joint [37]. For this reason, it is of great concern that OA is associated with depletion of local MSCs. Those that remain are less proliferative and have a lesser differentiation capacity [31].

Due to their impactful role in suppressing the inflammatory response, as well as differentiating into new chondrocytes, it is no surprise that they are a subject of great focus in the development of OA treatments. Unfortunately, such a major factor in controlling inflammation can be harmed by those very same inflammatory conditions. Altogether, one might speculate that there is a period of time before OA symptoms emerge where the BMSC population is depleting as they exhaust themselves to fight the “pro-inflammatory loop”.

3.3. Immune Cells Regulate Inflammation in Osteoarthritis with Inflammatory Cytokines

The primary immune cells involved in OA progression are T-cells, macrophages, and MSCs [8]. In the context of OA, these immune system-mediating cells have a massive effect on the joint environment by regulating inflammation and the metabolism of other cells, such as chondrocytes and osteoclasts. Primarily, this regulation is performed through paracrine signaling by releasing pro-inflammatory cytokines like tumor necrosis factor alpha (TNF-α), IL-1β, and IL-6. These three are highly associated with inflammation, cartilage degradation, and pain. There exist numerous other cytokines besides; however, they generally have less clearly defined roles [38]; a full explanation of their effects warrants a review of its own. Instead, the following section describes the some major immune cells and the effects of their associated cytokines focusing on macrophages, chondrocytes, and IL-1β.

Macrophages are abundant in the synovial space in OA and contribute to its progression by releasing inflammatory cytokines, which in turn further stimulate the release of more inflammatory factors from other macrophages in the synovial space [8]. In the subchondral bone, inflammatory cytokines such as IL-7 increase osteoclast production, naturally leading to more bone resorption [8].

Activated T cells produce IL-6, although IL-6’s role in OA is unclear, as it has both positive and negative effects. CD4+ lymphocytes can release IL-17, which promotes inflammation and activates T lymphocytes. IL-17 increases catabolic factors and cartilage degradation [8].

The chondrocytic model, human SW-1353 chondrosarcoma cells stimulated with IL-1β increase expression of a slew of proinflammatory mediators including inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX2), and prostaglandin E2 (PGE2), while also increasing production of reactive oxygen species (ROS), accompanied by higher levels of matric degrading proteases like MMP-13 [39]. In chondrocytes taken from Sprague-Dawley rats, similar results are shown, with IL-1β stimulating MMP-3, MMP-13, and iNOS [40]. iNOS is an enzyme that produces nitric oxide. Essential for immune response, but a double-edged sword as excessive NO can further increase inflammation and cause tissue damage. COX2 converts arachidonic acid into prostaglandins like PGE2, which is a mediator of inflammation, pain, and swelling. All of which can work together to increase MMP production, promote chondrocyte hypertrophy and apoptosis [8]. The increased production of degrading proteases is accompanied by increased expression of inflammatory cytokines, stress/apoptosis markers, i.e., IL-1, IL-6, Runt-related tracsription factor 2 (RUNX2), and SRY-BOC Transcription factor 9 (Sox9) [8]. Additionally, IL-1β can activate nuclear factor kappa B (NF-κB) to inhibit the expression of type II collagen and regulate chondrocyte hypertrophy via SOX9, BMP2, MMPs, and hypoxia-inducible factor 2 alpha (HIF-2α) [8]. It is also worth noting that HIF-2α can induce expression of MMPs and prostaglandins, potentially creating a feedback loop.

It is important to note that the SW-1353 chondrosarcoma cell line for these previous findings is limited in its similarity to actual human chondrocytes. Despite this, it is an established model for osteoarthritic chondrocytes, and similar results have been replicated in Sprague-Dawley rat chondrocytes [40]. Primary human chondrocytes stimulated with IL-1B also show increased expression of COX2 protein and PGE2 release into the cell culture medium [41], and reduced expression of anabolic genes like collagen 2 and aggrecan [42]. The most notable difference is that iNOS production is less clear in these models. However, it is well established that chondrocytes of various types adopt a more catabolic state after IL-1β stimulation [9].

This sort of cascade exemplifies the dense interconnectivity of inflammatory signaling in the joint. A single cytokine acting on a single cell can exponentially expand its signal by inducing other signaling molecules, each with its own potentially context-dependent effects, plausibly causing a positive feedback loop of inflammatory signaling as shown in Figure 3. This is compounded by the fact that the synovial fluid acts as the dispersal system, allowing any of these signals to affect any other cell in the synovium.

4. Molecular Pathways

The molecular pathways involved in the progression of OA are numerous, complex, and still being investigated.

Table 1. Major molecular signaling pathways in osteoarthritis. Major molecular pathways with associated roles, related transcription factors, mechanisms, and treatments. The pathways and treatments in Table 1 are not exhaustive.

Pathway	Role in OA	Relevant Treatments	Related Transcription Factors	Sources
WNT	Promote chondrocyte hypertrophy and differentiation, joint homeostasis [43]	XAV939 (preclinical)	Lymphoid enhancer-binding factor/T-cell Factor (LEF/TCF) family	[43,44]
Bone morphogenetic protein/SMAD	Chondrocyte hypertrophy, MSC differentiation into chondrocytes	CK2.1 (Preclinical), GDF5 (Preclinical), R399E (Preclinical)	SMADs	[45,46,47]
Nuclear factor-kappa B (NF-κB)	Joint inflammation, chondrocyte catabolism	Aspirin, glucocorticoids (interfere with downstream effects of NF-κB)	RUNX2, SOX9, AP-1	[48,49]
Transforming Growth Factor	Chondrocyte homeostasis, differentiation, hypertrophy, cartilage degradation	TissueGene-C (Late clinical trials)	AP-1, SOX9	[50]
Receptor activator of nuclear factor kappa B (RANK) and RANK ligand (RANKL)	Various, the most significant effect seems to be in bone remodeling and inflammation	Denosumab (Ongoing clinical trials)	NF-κB	[51,52]

summarizes some of the most important and well-defined pathways that are known to be particularly relevant to OA.

4.1. WNT Signaling in Osteoarthritis

The WNT signaling pathway is an essential communication path that plays a role in regulating development, homeostasis, and regeneration. WNT signaling can follow a canonical pathway or a non-canonical pathway [43], modeled in Figure 4. The canonical pathway utilizes β-Catenin. Allowing β-Catenin to accumulate in the cytoplasm and translocate into the nucleus. Once it has entered the nucleus, it can interact with the LEF/TCF family of transcription factors (TFs). β-Catenin in the nucleus can also block transcription of certain genes [43]. The non-canonical pathway does not rely on β-Catenin. These can be further divided into the WNT/PCP path that signals through RhoA and JNK, which play a role in cell migration, and WNT/Ca²⁺, which plays a role in intracellular calcium release signals through Protein kinase C (PKC) and Calcium/calmodulin dependentprotein kinase 2 (CaMKII) [43].

WNT signaling is fairly ubiquitous, and its effects and interactions with other pathways are cell-specific and conditional [36] making it a difficult target for potential treatments. Although the Wnt-β-Catenin pathway is essential for chondrocyte proliferation and maintenance, excessive activity increases hypertrophy and cartilage-degrading MMPs as well [43]. Suppression of Wnt can ameliorate OA in mice and promote chondrocyte hypertrophy and differentiation through the canonical pathway [43,44]. Another study found that in an OA mouse model, WNT signaling (both canonical and non-canonical) is excessive in OA development; however the study also found that excessive inhibition caused by the loss of a WNT inhibitor can also drive OA, suggesting that a delicate balance must be maintained to avoid disease progression [53].

WNT signaling plays a multifaceted role in OA progression [43]. Because it is so pervasive, it would probably be pertinent to focus on more specific downstream effects of WNT signaling than on the pathway, as WNT is not specific to OA progression or joint-related tissues. The risk of off-target effects by manipulating such a fundamentally important pathway is immense and may reduce this pathway’s viability as a treatment target.

4.2. NF-κB Signaling in Osteoarthritis

NF-κB is a family of ubiquitous transcription factors that play roles in cell death, immunity, inflammation, and stress response [54]. In the context of osteoarthritis, the NF-κB pathway can be activated by mechanical stress, and when ligands related to cartilage damage, such as inflammatory signals like IL-1β, TNF-α, or fragments of fibronectin, bind to cell-surface receptors on chondrocytes like toll-like receptors (TLRs) [25] shown in Figure 5. This activates one of a number of NF-κB cascades, which generally results in the translocation of the NF-κB complex consisting of P50 and P65 (RELA) from the cytosol into the nucleus where it binds to target DNA sequences. Once bound, it can influence gene transcription to enhance effects like osteoclastogenesis, a hallmark of the dysregulation of catabolism and anabolism in OA [25]. In individuals with OA, NF-κB is abnormally activated, leading to overexpression of the genes related to inflammation and catabolic phenotype [25].

NF-κB seems to regulate several key phenotypes associated with OA progression, including inflammation, osteoclastogenesis, and chondrocyte catabolism, making it an appealing potential target for DMOADs [25]. The caveat is that the NF-κB pathway is involved in many different biological processes beyond inflammation [55]. Additionally, some inflammation is necessary as a part of the natural healing process by driving MSCs to migrate towards the damage site, for example [22]. For these reasons, if NF- κB were to have any success, it would need to be very carefully manipulated to reduce off-target effects. Furthermore, it is already known nonsteroidal anti-inflammatory drugs (NSAIDs), which inhibit cyclooxygenase (COX) enzymes downstream of NF-κB or glucocorticoid injections that inhibit the effects of NF-κB at various points [56] certainly could help a patient feel better by reducing pain and inflammation, but on their own, they are unlikely to address the underlying cause of the disease. There is even evidence to suggest that long-term use of those may potentially worsen the severity of OA [57,58]. Perhaps specific manipulation of the NF-κB pathway in tandem with chondrogenic effects from another source may be a viable option.

4.3. RANK/RANKL Signaling in Osteoarthritis

Receptor activator of NF-κB ligand (RANKL) is the molecular ligand that acts on the receptor RANK [52]. Typically, RANKL binds to RANK on the surface of osteoclast precursors, leading to an increase in osteoclasts and bone resorption. As its name suggests, it is one way to activate the NF-κB pathway; however, it can act outside of the NF-κB pathway as well. For example, it can work synergistically with TNF-α to accelerate the rate of bone resorption via NFATc1 [59].

The exact role that RANK/L plays in OA development is still being discovered; however, it has received significant attention as a potential treatment target, as RANKL expression is significantly increased in OA patients [60,61], originating at least in part, from synovial fibroblasts [62]. RANK/L dysfunction has been closely tied to histological changes in bone and a proinflammatory osteoblast phenotype [8]. The RANK system exerts influence over several key parts of the disease, including cartilage degradation, bone remodeling, and synovial inflammation [51]. The most significant effect may be through bone remodeling by influencing the formation and activity of osteoclasts [51].

Due to the role it seems to play in bone resorption, the RANK/RANKL system has become a promising drug target to modify or treat the disease. RANKL inhibition via the anti-resorptive drug osteoprotegerin (OPG) prevents bone and cartilage degradation in mice [63]. Though these effects of antiresorptive drugs seem modest in humans [51]. Ultimately, the RANK/RANKL system has some promise as a treatment target; however, as its role seems to be more pronounced in bone, rather than the joint and cartilage specifically, one could speculate that it may be a more appropriate target to treat degenerative bone conditions like osteoporosis rather than OA. This may be applied to reduce ectopic bone formation in adults; however, this might be more of a palliative treatment than a curative one. Perhaps a DMOAD that affects the RANK/RANL pathway could be most effective in specific clinical presentations, or in tandem with other strategies.

4.4. TGF-β Signaling in Osteoarthritis

Transforming growth factor beta (TGF-β) is a cytokine with various isoforms and functions. It works by binding to complexes of type I and type II serine/threonine receptors. The type I receptors are activated by the type II, which permits the type I receptor to engage in intracellular signaling by phosphorylating SMAD proteins as seen in Figure 6 [64]. Normally, TGF-β levels are kept low, elevating briefly when under loading. In this situation, TGF-β plays a protective role in the joint by signaling in the SMAD 2/3 pathway. This low-level and intermittent expression pattern is essential for joint health, as interruption will lead to OA [64]. It is worth noting that high and or persistent levels of TGF-β activate different pathways than low levels. This is important because TGF-β is expressed at higher amounts and more persistently in osteoarthritic joints [64]. When TGF-β expression is elevated, it can activate the SMAD1/5/8 pathway, potentially driving chondrocytes toward hypertrophy. TGF-β is also a stimulator of synovial inflammation [64].

As with other pathways, TGF-β has many roles, and it must be very tightly regulated to maintain homeostasis in the joint. Many treatments that affect the signaling of the greater TGF-β pathway would be unappealing due to the risk of unintended side effects. That is not to say that it should be discounted, rather that any drug that acts on the pathway will likely have greater success if it targets specific, more downstream components [65].

4.5. Bone Morphogenetic Protein Signaling in Osteoarthritis

Bone morphogenetic protein (BMP) is a subset of the transforming growth factor β (TGF-β) superfamily. BMPs are a group of signaling proteins that play an important role in bone and cartilage formation, and more recently, there is evidence that BMP signaling is cross-talking with WNT to maintain β-Catenin in the cell [36]. These proteins signal through serine/threonine kinases that work in pairs. Different combinations of the BMP type 1 and type 2 receptors (BMPRI and BMPRII) can determine what kind of signal is transmitted. BMPs are regulated by multiple inhibitors at different levels [66]. Extracellularly, binding proteins like Noggin and Chordin bind to BMPs to prevent them from interacting with a receptor. Canonically, BMPs activate SMAD proteins Figure 5, while inhibitory SMADs prevent signal transduction by interacting with phosphorylated type-I receptors or by competition with partner SMADs [67].

BMP signaling primarily enacts its effects on cartilage and bone by activating SMADS 1/5/8, which are necessary for the terminal differentiation of chondrocytes and bone remodeling via the production of MMPs and osteogenesis [48,68]. Beyond the direct activation of the path, the BMP pathway can be activated non-canonically by other pathways, such as NF-κB [48]. BMPs predominantly regulate bone formation; however, they also play a role in cartilage regeneration and protection by inducing MSCs to differentiate into chondrocytes [36,69].

BMP pathway activation is essential for cartilage and bone formation; however, overactivation of the pathway can also lead to osteoarthritis in mouse models [35]. On the other hand, the inhibition of BMP signaling may serve as an OA treatment option [35]. In humans, there is no single clear way in which BMP signaling is altered in OA, as it seems that any abnormal signaling can play a role; however, in at least one study, elevated levels of BMP2 in the synovial fluid correlated with the severity of OA [70].

One member of the BMP family, morphogenetic protein 2 (BMP-2), is a very promising candidate for OA treatment. BMP-2 has the potential for bone and cartilage repair [71,72]. It enhances the production of both collagen type 2 and aggrecan. It can activate Wnt/β-Catenin signaling, leading to the production of lubricin and LRP5, an essential part of WNT/β-Catenin signaling [36]. BMP-2 can expand chondrogenic phenotype in human chondrocytes, activate the chondrogenic pathway in bovine synovial-derived mesenchymal progenitor cells [69]. The results are even more pronounced and promising in more complex treatments using recombinant or viral vector-induced BMP-2 in tandem with other molecules/treatments such as other BMPs, microfracture, or chondrogenic transcription factors [71,72]. These preclinical trials have shown the potential for cartilage formation, regeneration, and defect/lesion repair. Although encouraging, these effects have not been observed to improve long-term repair in a pony model [69]. Additionally, there is concern for the longevity of such treatment, as these BMP-based treatments are temporary, and work most effectively in tandem with stem cells or anti-inflammatory factors [69]. Despite these concerns, at least one study has shown that selectively activating the BMP pathway in a way that stimulates chondrogenesis without hypertrophy is possible [47]. If this sort of selective activation is possible for the WNT pathway, it may present a more appealing treatment target.

5. Major Transcription Factors Associated with Osteoarthritis

Myriad transcription factors regulate gene expression by controlling inflammatory, hypertrophic, regenerative/apoptotic phenotypes expressed by different tissues within the joint at different stages of OA development.

Individual transcription factors could be activated by various upstream signals; this means that in a complex environment such as OA, blocking one upstream signal may have a miniscule phenotypic effect if the downstream-acting transcription factors are activated by other paths [73]. Recent advances have made it increasingly possible to target TFs with drugs, making the issue of redundancy less of a concern for such options. However, not all transcription factors will have a very neatly defined effect. Some like RUNX2 play a dual role in chondrocyte maturation and degradation depending on the specific context, so any TF-based treatments should be selected carefully [74].

Transcription Factor Key Interactions in Osteoarthritis

Some of the major interactions between the major transcription factors associated with OA include the following:

• SOX9 interacting with: WNT, RUNX2, TGF, SMADS (from Smads), NF-κB.
• RELA/NF-κB: Self, SOX9, HIF-2α, AP1.
• AP1 interacting with: NF-κB, RUNX2, SOX9, MAPK.

TGF-β appears to be an upstream regulator of SOX9. TGF-β signaling via SMAD3 promotes SOX9-mediated transcription [75]. Both are activators of chondrogenesis; however, the exact interplay is still unclear. Additionally, SOX9 can directly inhibit RUNX2 [76], which makes sense as SOX9 is considered the master regulator of chondrogenesis, like RUNX2 is for chondrocyte hypertrophy. Functionally SOX9 seems to have the potential to maintain a productive chondrocyte phenotype. When SOX9 diminishes, RUNX2’s effect takes over and causes hypertrophy. SOX9 has been shown to cross-regulate with the WNT/β-Catenin pathway, particularly in stem cells. Specifically, WNT7B enhances self-renewal and osteogenic differentiation in BMSCs [77].

NF-κB seems to have a dual role in regulating SOX9. It is known that the NF-κB pathway, if stimulated by an inflammatory cytokine, for example, can act as a direct transcription factor to SOX9 by binding to its promoter region. However, the phenotypic effect of this seems to depend on the context. At least one study found that NF-κB activation could inhibit SOX9 [49], and at least one other suggests it activates [78].

Beyond SOX9, NF-κB can also self-stimulate, creating a positive feedback loop of inflammatory signaling in the right conditions [48]. It can also interact with hypoxia-inducible factor 2-α (HIF-2α) and AP-1 to regulate cartilage remodeling and activate catabolic effects like the production of MMP-13 [55]. AP-1 itself is involved in various pathways and can be regulated upstream by an equally universal pathway, MAPK [62,79,80]. These interactions in which NF-κB is involved generally favor the catabolic processes involved in OA rather than the anabolic processes.

Altogether, these looping connections paint a troubling picture and explain the high prevalence of this condition. SOX9 is essential for chondrogenesis and maintenance of the proliferative chondrocyte phenotype, yet when working with AP-1, which can be induced by something as common as mechanical loading, they can work together to activate hypertrophic genes [55,81].

Table 2. Transcription factors in osteoarthritis. Major transcription factors implicated in osteoarthritis progression with associated function, tissue, key interactions, and treatments targeted at the factor.

Transcription Factor	Related Signaling Molecules and Pathways	Function	Relevant Tissue/Cell Type	Related Treatments in Development/Research Relevant to OA
SRY box transcription factor 6 (SOX6)	Bone morphogenetic protein 2 (BMP-2) [82]	Chondrocyte proliferation and differentiation [83]	Articular Cartilage, synovial membrane	-
SRY box transcription factor 9 (SOX9)	BMPs, [84] Transforming growth factor beta (TGF-β) [75]	Chondrogenesis master regulator [73]	Articular cartilage	SOX9 protein delivery, Curcumin (early clinical, based on traditional Chinese medicine) [85]
Activator protein 1 (AP1)	Reactive oxygen species, Interleukin-1β (IL-1β), [86], mechanical stress [81], Mitogen activated protein kinase (MAPK), NF-κB, TGF-β, [87] c-Jun N-terminal kinase (JNK) pathway [88]	Regulates genes involved in cartilage breakdown [89] and inflammation, matrix metalloproteinase (MMP) production, WNT signaling	Chondrocytes, synovial membrane, subchondral bone	T-5224 (Preclinical) [90]
Aryl hydrocarbon receptor (AHR)	Kynurenine pathway [91]	Impairs chondrogenic/chondroprotective effects [92]; regulation of skeletal progenitor cells [91]; immune regulation [92]	Cartilage, Bone	-
E2F transcription factor 1 (E2F1)	Various	Chondrocyte differentiation [93]	Chondrocytes	-
Pituitary homeobox transcription factor 1 (PITX1)	E2F1, Transcription factor Dp-1 (TFDP1) [94]	Inhibit senescence [73]	Cartilage	-
Forkhead box protein M1 (FOXM1)	IL-1β [95]	Activates Janus kinase/signal transducer and activator of transcription (JAK/STAT), regulates genes that regulate cartilage degradation [96], WNT signaling, and apoptosis	Chondrocytes	-
Early growth response 1 (EGR1)	IL-1β [97]	Activates β-Catenin pathway, increases MMPs in response to inflammatory cytokines in OA; accelerates hypertrophy [97]	Cartilage, synovial tissue	-
MYC	Unclear	Unclear, elevated levels associated with worse disease progression [98]	Primarily synovial tissue	-
RELA (p65) key subunit of NF-κB	TNF [99], IL-1β, IL-6, [55] Nuclear factor kappa B (NF-κB) [100]	Subunit of NF-κB, regulates inflammation, catabolic, and pro-inflammatory phenotypes	Chondrocytes, synovial tissue [101]	Multiple drugs that inhibit downstream effects of NF-κB are already available, like aspirin and glucocorticoid injections.
Runtrelated transcription factor 2 (RUNX2)	Fibroblast growth factor 2 (FGF-2) [102], Indirectly activated by SMAD 1/5/8 [73]	Major switch for chondrocyte differentiation into the hypertrophic state. controls the expression of MMPs, aggrecanases [74]	Chondrocytes	Sprifermin [103]
SMADS	TGF-β [104] BMP [104]	Chondrocyte homeostasis, inflammation	Chondrocytes, articular cartilage	TGF-β inhibitors, [105] SMIs, Resveratrol [106], Tenoxicam [107]
Hypoxia-inducible factor 2 alpha (HIF-2α)	IL-1β [108] mechanical stress, [109,110] NF-κB [111]	Promotes catabolic activity, expression of MMP-13 [111]	Chondrocytes	-

contains some of the most significant transcription factors that regulate joint homeostasis. This should not be considered a complete list, but rather a guide to several particularly well-known or impactful transcription factors that are believed to relate to OA progression, favoring those related to the pathways mentioned in this review, as well as any potential drug treatments that relate to these factors.

6. Treatments for Osteoarthritis

Currently available treatments for OA suffer from many shared inadequacies. Namely, they are all symptomatic treatments rather than restorative. The major options are NSAIDs, surgical adjustment or joint replacement, and intra-articular injections to temporarily reduce pain [10,55].

Emerging treatments are designed to modulate inflammation in the joint, protect chondrocytes, and modify signaling pathways to induce regeneration of damaged tissue. Some notable examples include MSC injections, autologous chondrocyte implantation (ACI), platelet-rich plasma (PRP), pulsed electromagnetic field (PEMF), and TissueGene-C (TG-C). Some treatments were considered for treatment but failed to complete clinical trials, like Sprifermin and Lutikizumab.

6.1. Emerging Osteoarthritis Treatments

6.1.1. MSC Injection

MSC injections involve the direct injection of mesenchymal stem cells into the affected joint. There is evidence to suggest that mesenchymal stem cells are able to promote regeneration via paracrine signaling to modulate inflammatory cytokines, immune cell response, and secrete growth factors and other molecules to mediate endogenous repair [112,113]. Currently, four types of mesenchymal stem cells are considered for OA treatment; bone marrow-derived (BM-MSCs), adipose tissue-derived (AD-MSCs), umbilical cord-derived (UC-MSCs), and synovial membrane-derived (SM-MSCs) [114]. Currently, bone-derived is the most predominant option, as they have greater potential for chondrogenic differentiation compared to AD-MSCs or UC-MSCs [113]. BM-MSCs have shown substantial promise. They have been shown to significantly downregulate inflammatory signals like IL-1β, IL-6, and TNF- α while upregulating IL-10 and TGF- β [115] improving knee pain and cartilage quality in clinical studies, in addition they have been shown to improve radiological findings, joint mobility, and reduce inflammation in the synovium. AD-MSCs do not have the same chondrogenic capacity as BM-MSCs; however, they are favored for their ease of acquisition and culturing and have also shown promising results MSCs can produce anti-inflammatory signals like IL-10 and IL-1 and can regulate immune response by inhibiting synovial macrophage activation [115]. AD-MSC injection has been able to alleviate OA progression and prevent cartilage degeneration in a rat model. Some studies even report in plain terms that AD-MSCs are superior to BM-MSCs for pain relief and functional outcome [116]. The other two MSC sources, UC-MSCs and SM-MSCs have shown similarly promising results, however they are less studied than the other options. In brief, there is evidence to suggest that SM-MSCs may have greater chondrogenic potential than other types, and UC-MSCs may have greater immunomodulatory properties than BM-MSCs and have the potential to downregulate the expression of cartilage degrading enzymes [115,117,118].

6.1.2. Autologous Chondrocyte Implantation

Similar to MSC injections, autologous chondrocyte implantation (ACI) does not aim to target any pathway but rather allows the introduction of new healthy cells to the affected joint to perform their functions in regulating the immune system, protecting cartilage from degradation, and stimulating regeneration. ACI involves the implantation of healthy chondrocytes taken from another location on the patient and implanting them into the affected joint [119]. There is at least one case study that suggests a possible improvement in condition when both ACI and MSC injections are combined [120]. One important part to note is that ACI relies on healthy chondrocytes from another part of the patient’s body; as such, it is only considered for specific cases of young patients with full-thickness OA that is not affecting the whole body [119,121,122]. ACI appears to be a step towards more curative treatments for OA; however, is still doubtful that it will be considered for widespread use if there were a more targeted drug treatment available, like the many that are currently in development.

6.1.3. Platelet-Rich Plasma

Platelet-rich plasma (PRP) is another treatment based on introducing healthy tissue into the joint affected by OA. In this case, however, rather than introducing tissue native to the joint, a concentration of platelets and plasma proteins taken from the patient’s own blood is injected. The cartilage is normally avascular; as such, it does not readily have access to the blood cells and proteins that can typically stimulate regeneration and growth. Research is ongoing, but results are promising with many patients reporting reduced pain, enhanced joint function, and range of motion [123].

A major strength of cell-based treatments is that they can compensate for any current lack of understanding by allowing the cells to do what they do best naturally. Even if the specific molecular basis of the patient’s condition is not understood, the healthy cells could be able to respond to it positively, ideally with minimal immune response to the patient’s own tissue. Unfortunately, this also means that it is unlikely to ever be a permanent solution in its current state; the injected cells are not infinite, and the very process of harvesting them from another tissue in the body is at least modestly invasive. The success of the treatment may also depend on the genetic makeup of the patient. It does not seem convincing that these treatments would be as effective in the long term, especially for individuals with more severe osteoarthritis.

Same as with MSC injection and ACI. PRP has the potential to help at the time of writing; however, it seems that the success and approval of successful DMOADs could potentially overshadow these cell-based therapies.

6.1.4. Pulsed Electromagnetic Field

Pulsed electromagnetic field (PEMF) is a more unorthodox potential method of treatment. It is non-invasive and involves specifically exposing the affected area to pulses of electromagnetic fields at particular frequencies. There is some evidence to suggest that patients who receive this therapy show signs of preserved cartilage and bone structure [124]. The results of PEMF treatment may be inconclusive because PEMF acts selectively enough that only certain subtypes of OA may be affected fully. This method is still in need of validation; however, its potential as a non-invasive intervention to slow the progression of OA should not be ignored.

6.1.5. Sprifermin

Sprifermin is a recombinant human fibroblast growth factor rhFGF18 analog. FGF-18 binds to fibroblast growth factor 3 receptor (FGFR3) to enact its effects, and sprifermin is about five times more effective at binding to this receptor [103]. Once bound, FGF18 uses the MAPK pathway to regulate RUNX2. Studies have shown that FGF-18 can stimulate chondrocyte proliferation and the production of the ECM. When injected, its effects address many of the prime causes for OA development. It can promote glycosoaminoglycan and collagen type 2 synthesis, while simultaneously inhibiting the expression of MMP-13. It has reduced the degeneration of cartilage and promoted its regeneration and repair. OARSI scores reflect this with positive improvements in the cartilage quality. Sprifermin has shown its effects both in vitro and in vivo.

Sprifermin stimulates chondrocytes in the ECM to begin degradation of the matrix, allowing for space for more chondrocytes to grow, ultimately producing more ECM and chondrogenesis [103]. Counter-intuitively, the controlled degradation of cartilage would generate a chondrogenic response that could counter the initial degradation.

Sprifermin is a promising candidate for OA treatment that addresses the molecular mechanisms behind OA. Perhaps most importantly, though Sprifermin seems to counter the molecular and cellular irregularities in OA, patients did not report significant condition improvement in phase II clinical trials [103]. Since then, development has stalled. In its current state, the odds of Sprifermin ever being widely adopted as a treatment for humans are relatively low, although its mechanism of inducing chondrogenesis by controlled cartilage degradation may still be a promising strategy.

6.1.6. CK2.1

CK2.1 is another molecular-based drug that shows promise for treating OA. CK2.1 is a memetic peptide of ALK3 that significantly increased production of collagen II and Collagen IX, stimulating cartilage repair, crucially without stimulating chondrocyte hypertrophy in a mouse model [47]. CK2.1 acts by activating BMP receptor type 1A (BMPRIa) in the absence of the natural ligand, which activates the BMP pathway in a very selective manner [125]. Unfortunately, the actual mechanism of action is still being elucidated, and its effects have only been demonstrated in mouse models. A major promise of this treatment is that it does not induce hypertrophy [47]. Understanding the mechanism of action of CK2.1 may offer insights into the complex biochemical pathways involved in OA that may allow growth stimulation without leading to the natural hypertrophy and ossification. The ability to separate chondrocyte growth and differentiation from hypertrophy may be crucial for long-term OA treatments in the future. Beyond this, CK2.1 was applied in a slow-release hydrogel injection [47].

6.2. Additional Emerging Osteoarthritis Treatments

Table 3. Additional restorative treatment strategies considered for osteoarthritis, including the target pathway, the supposed mechanism of action, the general effect after application, and the development status.

Drug/Treatment	Target Pathway	Proposed Mechanism	Effect	Development Status	Sources
Mesenchymal stem cell (MSC) injections	Immune/inflammatory modulation Anti-apoptosis	Suppress interleukin-1 beta (IL-1β), Interleukin-6 (IL-6) Differentiation into different cells in the joint Secrete B-cell lymphoma 2 (BCL-2) and Insulin-like growth factor 1 (IGF-1)	Seems to have a generally positive effect on regeneration potential. Supplies healthier MSCs to support chondrogenesis and immune regulation	Late clinical trials	[120,126,127]
Autologous Chondrocyte Implantation (ACI)	Does not directly target the pathway	Implant the patient’s own healthy chondrocytes into the damaged joint	Reduced pain, improved joint function	FDA approved for specific cases in younger patients usually following acute injury. Not generally considered a primary treatment for osteoarthritis (OA)	[119]
Amniotic fluid injection	Does not directly target the pathway	Immunomodulation	Mixed, inconclusive results	Mid-stage clinical trials, labelled as “investigational” by the FDA	[128]
Plasma PRP	Does not directly target the pathway	Introduce various growth factors to induce regeneration that would otherwise not be accessible to the avascular tissue.	Seems to slow disease progression	Mid–late clinical trials labeled as “investigational” by the FDA. Legally can be applied “off label” because the FDA does not have the authority to regulate a person’s own bodily products.	[123]
Pulsed electromagnetic frequency (PEMF)	Tumor necrosis factor-alpha (TNF-α), possibly Bone morphogenetic protein (BMP)	Increases adenosine receptors A2A, A3 expression, suppresses pro-inflammatory cytokine release, and increases Transforming growth factor-beta (TGF-β) secretion. Increases, express anti-apoptotic proteins	Results are mixed. Some sources claim it suppresses cartilage degeneration and enhances chondrocyte differentiation	Inconclusive. Several devices are approved for specific applications, not generally linked to OA.	[124,129,130]
Nerve growth factor (NGF) Blockade	Neurotrophin signaling	Blocking the effect of NGF, which stimulates nerve growth and is connected to acute and chronic pain	Promising symptom reduction, however, with the risk of inducing rapid progression of the disease	Clinical development stalled due to concerns of causing the rapid progression of OA	[131]
CK2.1	BMP	Unknown	Stimulates chondrogenesis and cartilage repair, without hypertrophy	Preclinical	[132]
R399E 1	BMP	Induction of aggrecan and SRY-box transcription factor 9 (SOX9); reduced collagen X; lower cartilage hypertrophy and osteogenic activity; cartilage repair	Cartilage structure improvement	Preclinical	[45,133]
GDF5 2	BMP	Induction of aggrecan and SOX9; reduced collagen X; cartilage repair. Can also form osteophytes	Maintain cartilage homeostasis	Preclinical	[46,133]
Sprifermin	FGF-18 homologue	Binds to FGF3R, signals through Mitogen activated protein kinase (MAPK) to regulate Runt-related transcription factor 2 (RUNX2)	Sustained anabolic effect on cartilage. Structural improvements	Completed phase 2 clinical trials. Inconsistent symptom relief in phase 2 has stalled any phase 3 trials	[103,134]
Tissue Gene-C (TG-C)	TGF-β1	Inject chondrocytes and cells that overexpress TGF-β 1. Synthesize cartilage components	Improved OA metric scores	Late clinical, preparing for completion	[134,135]
Lutikizumab	Block interleukin 1α and 1β	Binds to inflammatory cytokines IL-1α and 1β	Improved pain score, did not affect the structural score	Discontinued for the treatment of OA after phase 2 clinical, now being considered for other inflammatory conditions	[136]
Denosumab	Receptor activator of NF-kappa B (RANK) and ligand of RANK (RANKL) (Related to NF-κB)	Bands to RANKL, preventing its activation of the RANK receptor	Reduces chondrocyte apoptosis, bone resorption. Reduces bone and cartilage degradation	Phase 2 ongoing	[51]
T-5224	Upstream regulation of proteases and inflammatory cytokines	C-Fos/Activator protein 1 (AP-1) inhibitor	Protects cartilage, reduces osteophyte formation	Preclinical	[90]
XAV-939	WNT inhibition	Inhibition of WNT by anticatabolic effects on chondrocytes, antifibrotic effects on synovial fibroblasts	Reduced OA severity	Preclinical	[44,137]
UBX0101	DNA damage response, proapoptotic pathway	Inhibit p53/Murine duble minute 2 (MDM2) interaction, senolytic, remove senescent cells to reverse OA pathology	Promising animal model results failed to meet efficacy goals in human trials	Discontinued in phase 2	[138,139]
AntagomiR-10a-5p	Homeobox gene HoxA1	Interferes with the silencing of HOXA1 by miR-10a-5p	Suppresses IL-1β induced apoptosis	Preclinical	[140]
miR-18A3p	NF-κB	Binds to and negatively regulated Pyruvate Dehydrogenase Phosphatase 1 (PDP1) expression.	Improves matrix remodeling and suppresses inflammation	Preclinical	[141]
miR-365	IL-1β	Possibly binds to Hypoxia-inducible factor 2 alpha (HIF-2α), suppresses IL-1β-induced expression of HIF-2α	Prevents MMP-13 expression in primary cultured chondrocytes, protecting cartilage from degradation in OA microenvironment	Preclinical	[142]
Antagomir of miR-375	ATG-2B, a key autophagy gene	The antagomir inhibits mIR-375 from inhibiting ATG-2B, and thus it reduces apoptosis, promotes autophagy	Reduces apoptosis, promotes autophagy in mouse model	Preclinical	[143]

¹ Study funded by Merck; ² authors are employed by Johnson and Johnson and Bolder BioPATH.

summarizes several developing osteoarthritis treatments that have the potential to form the basis of restorative treatment strategies that target the disease at its source more so than current widely accepted options. The treatments in Table 3 exist in various stages of development. Table 3 should not be considered exhaustive, but rather representative of the many diverse strategies that are being considered.

7. Conclusions

Osteoarthritis is a debilitating condition that involves all parts of the joint, including articular cartilage, the synovial cavity, subchondral bone, and bone marrow. It is characterized by degradation of articular cartilage abnormal bone formation, and inflammation [7,9]. Recently OA has been reclassified from a “wear-and-tear” disease to an inflammatory condition primarily mediated by the imbalance of catabolic and anabolic factors in the joint, leading to chronic, abnormal cartilage degradation. The molecular basis of osteoarthritis is still under investigation; however, dysregulation of inflammatory mediators in the joint [7] associated with damage repair and chondrocyte growth, as well as the depletion and impotency of MSCs [30,31], seems to play a significant role in disease progression.

In response to the relatively recent shift in understanding of disease progression [7], many promising cell-based therapies and disease-modifying drug candidates to address the causes of OA have begun development. These treatments are typically aimed at reducing inflammation, modulating the immune response, and protecting and regenerating chondrocytes [127], and include candidates such as MSC injection, PEMF, TG-C, and CK2.1. It is important to recognize that these treatments are still in need of more research to understand the most effective method of application, long-term effects, and safety.

A review of emerging OA treatment strategies leads us to conclude that all facets of the disease should be considered when designing a treatment strategy. For this reason, we believe that efforts should be made to implement more precise and distinct diagnosis of OA, and that personalized treatments, or combined strategies that affect multiple aspects of disease progression should be considered.

Some limitations of this study are that there are mixed and inconclusive results for many treatments, like PEMF, and some sources describing the effectiveness of treatments have declared conflicts of interest, study designs are inconsistent, leading to some difficulty in accurately comparing results. This review was aimed at describing emerging treatments that are in various stages of development, such as preclinical, clinical trials, and limited FDA approval. As such, any descriptions of a treatment’s potential may be at varying levels of certainty.