Individual direct biomarkers of cartilage metabolism:
Biomarkers of anabolic processes
The carboxypropeptide of Type-II collagen (CPII) is a useful measure of Type-II collagen synthesis (Figure 4). Although CPII
concentrations were not notably higher in synovial fluid samples of joints with osteochondral fragmentation, their concentrations
were significantly higher in the serum of horses with osteochondral fragmentation in the first biomarker study done at CSU.
Figure 4: Diagram illustrating production of collagen propeptide molecule against which the CPII antibody reacts.
In the same study, the concentration of epitope 846 (CS-846), another anabolic marker (of chondroitin sulfate) was significantly
higher in both synovial fluids of joints with osteochondral fragmentation and the serum levels were also significantly higher.
The net result of this study was that using the combination of serum (CS-846) and CPII concentrations allowed 79 percent of
horses to be correctly classified as having osteochondral damage.
It has been shown in a study in Canada that repeated use of intra-articular methylprednisolone acetate leads to a potentially
harmful inhibition of CPII synthesis and an increased release of degradation products of aggrecan from the articular cartilage.
Biomarkers of catabolic processes
Measuring the degradation of Type-II collagen with biomarkers has proven to be a benefit in monitoring OA and OCD in the horse.
Antibodies have been developed to identify Type-II collagen fragments that have been cleaved and/or denatured, exposing previously
inaccessible regions (neoepitopes) of the molecule (Figure 5).
Figure 5: Diagram illustrating exposure of a collagen degradation epitope as exposed with breakdown of the triple helix.
Using these antibodies, notable increases in concentrations of degraded Type-II collagen have been demonstrated in synovial
fluid and serum samples from horses, dogs and rabbits with experimental OA.
Initially, the Col-2–3/4short immunoassay for detecting collagenase-cleaved collagen fragments (detect both Type-I and -II collagen degradation) was developed.
This assay had been used in the author's laboratory for monitoring collagenase-induced collagen degradation and to measure
the inhibitory effect of a synthetic MMP inhibitor on IL-1-induced degradation of equine articular cartilage explants.
More recently, a collagen degradation immunoassay that is specific for Type-II collagen degradation and is equine specific
was developed (called 234CEQ).
In a study of skeletal markers in osteochondrosis (OC) in foals done at CSU in collaboration with Utrecht University in Holland,
a combination of notably higher serum concentrations of CPII, higher concentrations of Col-2–3/4short and lower concentrations of 234CEQ correlated with high OC scores (radiographically).
This study suggests that there is increased collagen turnover in OC, and by measuring the serum amounts of specific biomarkers
of collagen metabolism, it was possible to identify horses with OC.
An earlier study in Canada in cases of OC found that there were notably higher concentrations of CPII and lower concentrations
of CS-846 and keratan sulfate (KS) epitopes in synovial fluids of affected (compared to non-affected) typical joints.
Other biomarkers that have been measured in the horse include KS and cartilage oligomeric matrix protein (COMP), but up until
now, these have proven less useful.
On the other hand, development of monoclonal antibodies that distinguish the two different sites of aggrecan degradation can
help identify which mediator is the most responsible for aggrecan degradation in the horse. Aggrecanase appears more important
than stromelysin in this degradation process and serves as an example how biomarkers can be used to define pathogenesis and