Obesity is a significant risk factor for osteoarthritis in both weight-bearing and non-weight bearing joints (1
). Here, we report that older mice lacking leptin signaling due to non-functional circulating leptin (ob/ob
) or non-functional leptin receptors (db/db
) develop extreme obese phenotypes without increased incidence of knee osteoarthritis. These findings suggest that body fat alone may not be a risk factor for joint degeneration, but rather, other local and systemic factors are responsible for the relationship between obesity and osteoarthritis. In weight-bearing joints, such as the knee, much progress has been made in identifying relationships among mechanical factors and osteoarthritis onset and progression (22
). Yet mechanical factors seem less likely to explain the increased risk of osteoarthritis in non-weight bearing joints, implicating the involvement of a systemic factor. Recent studies suggest that leptin may be that obesity-linked systemic factor because of leptin’s pro-inflammatory effects in cartilage and its elevated levels in osteoarthritic joints (9
It is surprising that the incidence of knee osteoarthritis is unchanged in ob/ob
mice given their dramatic obese phenotype—they weigh more than three times as much and have approximately 10-fold more body fat compared with age-matched controls (). Studies using diet-induced models of obesity show that feeding C57BL mice a high-fat diet increases the incidence of osteoarthritis in the knee despite a much less severe obese phenotype (16
). Thus, given the severity of obesity in the leptin-impaired mice, it seems likely that both local (e.g., mechanical) and systemic (e.g., metabolic or inflammatory) factors would promote the development of knee osteoarthritis.
The fact that knee osteoarthritis is not increased is consistent with the interpretation that leptin influences the pathogenesis of knee osteoarthritis directly rather than being correlated with obesity. This interpretation is also supported by two additional observations. First, hyperphagia-induced obesity, which is caused by administration of aurothioglucose and results in reduces hypothalamic leptin signaling (24
), does not increase the incidence of osteoarthritis (25
). Second, we find that heterozygosity for the ob
gene, which results in reduced plasma leptin concentrations, increased body mass, and increased fat mass compared to WT
mice () (26
), does not increase knee osteoarthritis. Intriguingly, though, cartilage structural changes in ob/+
mice are reduced relative to ob/ob
mice, suggesting that potential chondroprotective effects of reduced leptin signaling can be modified by other factors.
Discerning a direct role for leptin in the pathogenesis of osteoarthritis is difficult because non-specific disruption of leptin signaling produces phenotypes that may be primary, secondary, or tertiary to the interruption in brain and peripheral tissue signaling pathways that regulate energy homeostasis (27
). Nevertheless, the resultant phenotype provides a model for interpreting the relationship between obesity-related pathologies and osteoarthritis with and without intact leptin signaling. Furthermore, the severity of weight gain in leptin-impaired mice may provide a model of altered joint loading associated with morbid obesity.
Body mass and body mass index are associated with changes in the magnitude and orientation of joint loading and the subsequent development of knee osteoarthritis in humans (22
). We observed medial-lateral differences in subchondral bone thinning between leptin-impaired and leptin-intact mice—specifically decreased subchondral bone thickness in the lateral, but not medial, compartments in leptin-impaired mice. This pattern of an increased ratio of medial:lateral subchondral bone thickness in leptin-impaired mice () is consistent with observations in overweight humans (29
) and may be related to altered joint loading patterns.
As in obese humans (30
), leptin-impaired mice load their joints less frequently and likely generate joint stresses that are much less than proportional to body weight compared to controls. The ob/ob
mice have significantly reduced levels of spontaneous activity (32
), and muscular forces, which contribute significantly to joint stresses, are likely reduced in ob/ob
mice due to reduced skeletal muscle contractile dynamics and mass (33
). Furthermore, their large abdominal fat deposits may unweight the limbs by providing significant bodyweight support (unpublished observations). It is not clear to what extent these changes in musculoskeletal loading affect the pathogenesis of osteoarthritis in leptin-impaired mice. A reduction in loading may seem to be protective; however, increased physical activity in mice and humans does not necessarily increase the incidence of osteoarthritis (34
We investigated the circulating serum levels of pro- and anti-inflammatory cytokines to determine how systemic inflammatory status was affected by excessive adiposity in ob/ob
mice. Adipose tissue is a potent source of pro- and anti-inflammatory cytokine production (4
), which may promote catabolic processes that link obesity with osteoarthritis (14
). Leptin-impaired mice, however, were not in a generalized inflammatory state, as indicated by comparable serum levels of pro-inflammatory cytokines in leptin-impaired and intact mice (). Only KC, a C-X-C chemokine and human IL-8 analog that functions as an inflammatory chemoattractant (37
), was elevated in leptin-impaired mice. Furthermore, IL-2 was reduced in leptin-impaired mice, similar to that seen in obese humans (38
). Additionally, impaired leptin signaling did not dramatically affect serum levels of anti-inflammatory cytokines, except for IL-4, which was elevated in db/db
The lack of association between obesity and inflammation in ob/ob
mice, while perhaps surprising given their extreme adiposity, is consonant with research showing that leptin deficiency modulates immune function (39
). Leptin deficiency increases sensitivity to innate (i.e., monocyte/macrophage-activating) immune responses (40
); whereas, it decreases sensitivity to acquired (i.e., T cell-mediated) immune response (41
). These effects of leptin on the immune response have been demonstrated by two recent studies targeting innate versus acquired immune-mediated arthritis in ob/ob
). The overall similarity in serum cytokines levels and knee osteoarthritis scores in leptin-impaired and control mice is consistent with a hypothesized relationship between systemic inflammation and obesity-associated osteoarthritis. Future studies are needed to determine the relationship between systemic and local (i.e., intra-articular) inflammation.
Leptin may also mediate the development of osteoarthritis via central and peripheral mechanisms that regulate bone mass (6
). In ob/ob
mice, leptin deficiency results in a mosaic bone mass phenotype with bone mass being increased in the axial skeleton and decreased in the appendicular skeleton (7
). While the relationship linking altered bone remodeling to osteoarthritis is complex, being dependent on both osteoarthritis model and degree of disease progression (45
), osteoarthritis is typically associated with increased subchondral bone mass (i.e., sclerosis) as well as osteophyte growth in the joint periphery. We found that leptin deficiency produced a mosaic bone phenotype in the joint with respect to subchondral cortical versus trabecular bone ().
Leptin-impaired mice showed regional subchondral bone thinning without changes in the overall relative cortical bone volume or density in the proximal tibial epiphysis. In contrast, the trabecular bone volume was increased in the proximal tibial epiphysis. Thus, unlike the femoral neck region in which there is reduced cortical bone thickness and trabecular bone volume (7
), the proximal tibial epiphysis exhibits similarities with the lumbar vertebrae of ob/ob
mice, manifesting increased trabecular bone volume and decreased subchondral cortical bone thickness relative to wild-type controls (7
). The extent to which this phenotype is influenced by altered joint loading patterns is unknown. However, our observation that the relative trabecular bone volume is also increased in the tibial epiphysis of ob/+
mice, similar to that observed in vertebral bodies of ob/+
), supports a direct role for the involvement of leptin signaling in mediating trabecular bone morphology in the knee. Thus, leptin signaling appears to regulate both cortical and trabecular bone mass in ways that may be relevant to osteoarthritis pathogenesis.
Leptin may further regulate tissue mineralization is by targeting chondrocytes. In the growth plate, leptin is localized in prehypertrophic chondrocytes and the leptin receptor is localized in hypertrophic chondrocytes (44
). Leptin deficiency, as observed in ob/ob
mice, increases hypertrophic chondrocyte apoptosis and impairs endochondral ossification (44
). In the current study, we observed that ob/ob
mice had significantly fewer hypertrophic chondrocytes in the calcified cartilage of the tibia. Dumond et al. (13
) has previously shown that rat articular chondrocytes express leptin receptors (Ob-Rb) and that injections of leptin into the knee joint increases the expression transforming growth factor β1, insulin-like growth factor 1, and leptin messenger RNA. These localized pro-anabolic effects of leptin, when considered in conjunction with the proposed inflammatory and pro-catabolic effects of leptin, are consistent with an overall increase in anabolic and catabolic activities of chondrocytes in osteoarthritis.
An additional way that leptin may mediate the etiology of osteoarthritis is via its actions on the reproductive system. Administration of leptin rescues infertility in ob/ob
). Articular chondrocytes express functional estrogen receptors (47
), and the concurrence of a spike in osteoarthritis onset with menopause has implicated an osteoarthritis-protective effect for estrogen in women. Many ovariectomy and estrogen treatment animal models also show a protective effect of estrogen on osteoarthritis pathogenesis (48
). Although little is known about the interaction between leptin and estrogen in osteoarthritis, both ovariectomy and menopause precede increases in adiposity (49
) indicating that osteoarthritis associated with these low-estrogen transitions is also associated with increasing levels of leptin. Interestingly, ob/ob
mice have low estrogen levels, impaired leptin signaling, and unaltered incidence of knee osteoarthritis.
There are several additional factors to consider in the interpretation of our findings. First, given the significant variation in susceptibility to obesity and osteoarthritis among different mouse strains, additional studies with leptin-impaired mice created on different backgrounds strains are needed to generalize the findings from this study. Furthermore, a high-fat diet pair-feeding experimental design for comparing wild-type and leptin deficient mice would provide additional weight and dietary controls for evaluating the protective effects of impaired leptin signaling on the development of osteoarthritis.
In conclusion, extremely obese leptin-impaired mice do not develop increased incidence of knee osteoarthritis. This finding is consistent with recent studies that implicate leptin as a pro-inflammatory and pro-catabolic mediator of osteoarthritis associated with obesity (9
). Leptin, however, has many pleiotropic effects on the body, including significant roles in the musculoskeletal, immune, and reproductive systems. Our findings indicate that impaired leptin signaling significantly alters subchondral bone morphology without altering knee osteoarthritis, suggesting that obesity, other obesity-dependent factors, or the absence of leptin signaling independently moderate subchondral bone morphology. Furthermore, adiposity alone, in the absence of leptin signaling, is insufficient to induce systemic inflammation. Additional insight into the potential chondroprotective effects of disrupting leptin signaling may be obtained by examining leptin-impaired mice in acute models of osteoarthritis, such as instability or injury models.