Matrix remodelling in the superficial digital flexor tendon of the horse
Authors: Vaughan‐Thomas A. , Anderson V.M. , Kipar A. , Innes J.F. , Clegg P.D. 
Published: September 2009

Introduction  Injuries to the superficial digital flexor tendon (SDFT) are very common in the horse. The macroscopic degeneration of the tendon that is often apparent is associated with an altered matrix composition indicating substantial tissue‐remodelling activity.

The matrix metalloproteinases (MMPs) are responsible for degrading matrix components during tissue remodelling or turnover. There is evidence that mediators of angiogenesis, particularly vascular endothelial growth factor (VEGF), regulate the activities of gelatinases, and it is likely that tendon overuse or injury leads to disruption of tissue homeostasis, leading to hypoxia and VEGF up‐regulation. In this study, we investigated the presence and levels of MMP‐2 and MMP‐9 and VEGF within both grossly normal and pathological tendon tissue. Furthermore, we investigated the presence of cartilage oligomeric matrix protein (COMP) within tendon.

Materials and methods  SDFT samples were collected post euthanasia. From tendons showing no gross pathology, samples were taken along the length of the SDFT and pulverized in a liquid nitrogen‐cooled mill. Tendon tissue in which a central degenerate core lesion was observed was sampled and processed identically. Extracts of soluble protein in SDS‐PAGE sample buffer were analysed by gelatin zymography for the presence of MMP‐2 and MMP‐9 and also by Western blot analysis for the presence of VEGF isoforms and COMP. SDS‐PAGE followed by coomassie‐blue staining was used to obtain a profile of the proteins extracted, and casein substrate gel zymography was used for anaylsis of serum‐associated proteolytic activities.

Results  In the nonpathological tendon samples, no significant differences were observed along the length of the tendon. Basically, protein profiles, MMP‐2 activity, activities co‐migrating with serum‐derived plasminogen and VEGF isoforms were constant. Pro‐MMP‐2 was the predominant gelatinase activity but active MMP‐2 was also present. The main immunoreactive band detected using a polyclonal antiserum to VEGF had an apparent Mr of 16.5 kDa. Analysis of the pathological tendon samples revealed less consistent expression of these molecules and activities. The protein profile showed an apparent absence of some serum‐derived soluble proteins in some areas, with raised levels of both pro and active MMP‐2. An activity co‐migrating with an active MMP‐9 standard was also observed. Intact subunits of COMP were observed in all samples of pathological tissue. The presence of VEGF within the pathological tissue is being investigated.

Discussion  These results indicate that mediators of tissue remodelling are present both within normal and pathological SDFT. However, it is apparent that there may be imbalance between some protein components and enzyme activities within the pathological tendon. We will undertake further studies to determine whether our findings are common to further samples. This study should aid our understanding of the mechanisms that underlie SDFT pathology in the horse.