Apoptosis, necrosis, chondrocytes aging and loss of proteoglycans were universally acknowledged as the primary features of cartilage injure(17–21), As direct consequences of these, the density of the living cells in the cartilage decreases. The results of H&E staining verified that such a phenomenon really was obvious in the cartilage of OA and KBD.
Proteoglycans are complex macromolecules distributing in the ECM of articular cartilage, having a protective effect on joints. Dominant proteoglycans in cartilage are aggrecan (22). Increasing number of research has demonstrated that there is a close association between loss of aggrecan and cartilage diseases(23–25).It could be observed in the TB staining that the staining intensity of proteoglycans in KBD was quite low compared to OA group and control group, which showed that the depletion of aggrecan is more serious in cartilage of KBD.
It is generally accepted that two families of metalloproteases are the major contributors in the progression of degradation of ECM: matrix metalloproteinase (MMP) family and ADAMTS family(26). However, in comparison to the MMPs, aggrecanases seemingly have more association with the depletion of aggrecan, because it was the greatest in areas of cartilage adjacent to sites of cartilage erosion(25).
When referring to the roles of ADAMTS family played in the degradation of aggrecan, researchers often pay more attention to the ADAMTS4 and ADAMTS5, since other members of family related to degradation of aggrecan, such as ADAMTS1, ADAMTS8, ADAMTS9, has been shown to have a rather low aggrecan-degrading activity(27, 28). ADAMTS4 and ADAMTS5 leave the aggrecan core protein at the aggrecanase-specific Glu373- Ala374 bond in the interglobular domain (IGD) region(29, 30), therefore, they have been regarded as the key aggrecanase. ADAMTS4 and ADAMTS5 have similar structure, containing a disintegrin domain, a thrombospondin domain, a cysteine-rich domain, and a spacer domain, but ADAMTS5 has an additional TS domain after the spacer domain. A study reported that in the IGD region the aggrecanolytic activity of ADAMTS5 are fourfold compared to ADAMTS4, in the CS-2 region of aggrecan 2.5 fold, and under physiological conditions the aggrecanase activity of ADAMTS5 was at least 1000-fold greater than that of ADAMTS4(31, 32).
Based on the research of activity and molecular basis of ADAMTS4 and ADAMTS5, advanced works from lots of laboratories have further illustrated the impact of aggrecanase on increase of aggrecan loss related to cartilage diseases in different ways(33–36). Although there are some reviews comprehensively summarizing the regulation of ADAMTS4 and ADAMTS5 in OA(26, 37–39), it is still controversial that which of them is the main aggrecanaes in the destruction of cartilage. Some reports argue that ADAMTS4 is primarily expressed in an active form in osteoarthritic cartilage, while ADAMTS5 would be constitutively expressed in both normal and OA cartilage(40).
Others suggested that owing to the high affinity of its non-catalytic domains for glycosaminoglycan chains, ADAMTS5 is the most active aggrecanase to increase the loss of aggrecan(32). It has also been proposed that ADAMTS4 is the most inducible aggrecanase upon cytokine stimulation, whereas ADAMTS5 is the most abundant aggrecanase(41), which manifests that ADAMTS5 is constitutive. In the murine models of OA, ADAMTS5 deficiency could protect mice from OA, but deletion of ADAMTS4 didn’t have any influence on the normal growth and physiology of mice(42, 43). As discussed above, ADAMTS5 appears to play the key role in the cartilage depletion, though these conclusions are limited to animal model, and the discrepancy between human and murine should not been ignored. Therefore, aims of this study not only investigated the main aggrecanases involving the cartilage degradation, but also explored whether ADAMST4 and ADAMTS5 play the equivalent roles in the progression of KBD.
In our study, the expression of ADAMTS4 is not in accordance with the points of some previous studies: the percentage of positive staining of the control was more intensive than OA and KBD in the middle and the deep zones. Generally speaking, ADAMTS4 should be overexpressed in the damaged cartilage. However, the mRNA expression of ADAMTS4 was significantly repressed at the early stage of OA(44). Besides, in the superficial zone, the expressions of ADAMTS4 of OA and KBD had a similar proportion with the control. These results indicated that ADAMTS4-induced cartilage degradation maybe not happened at the early stage of cartilage diseases, and the role of ADAMTS4 is less significant, especially in the superficial zone.
Nevertheless, the expression of ADAMTS5 in OA and KBD were greatly increased compared to the control, particularly in the cartilages from KBD patients. In addition, there was gradual upregulation of ADAMTS5 in the OA group from superficial to deep zone, while the expression in KBD group down-regulated in the deep zone. It confirmed that ADAMTS5 was the crucial aggrecanase, which led to the cartilage damage. This adverse effect is evidence in the deep zone of OA cartilage. What’s more, the expression of ADAMTS5 in KBD group was higher than the one in OA group. Based on the results of TB staining of the aggrecan positive cell rates among three groups, it could be inferred that the increased expression of ADAMTS5 is more likely to cause the decrease of aggrecan in OA and KBD.
However, mainly owing to the ethical issue, it was hard to collect enough cartilage from KBD patients. It couldn’t manifest adequately the casual relationships between cartilage degradation and ADAMTS5. In order to investigate the role of ADAMTS5 in the cartilage disease, increasing amount of research is required, which could further explain the complex pathogenesis of KBD and provide a potential therapeutic target for KBD patients.
In conclusion, we found that the function of ADAMTS4 in degradation aggrecan is seemingly slight. Hence, we prefer to favor of the opinion that ADAMTS5 is the key enzyme in the cartilage destruction, in particularly in KBD cartilage. Most of previous studies have focused on the roles of ADAMTS4 and ADAMTS5 in OA, while it is rare to investigate the importance of ADAMTS4 and ADAMTS5 in KBD. The test of the expression of ADAMTS4 and ADAMTS5 in OA and KBD cartilage further affirmed our standpoint that ADAMTS5 was more significant in the loss of aggrecan and cartilage damage.