Alterations in serum thrombospondin in patients with amyotrophic lateral sclerosis

Thrombospondin (TSP), an endogenous extracellular matrix (ECM) glycoprotein, is secreted from platelet alpha-granules after thrombin stimulation. Alterations in blood TSP levels occur in different pathologic conditions, suggesting it is a marker for certain disorders. We previously found a marked increase in TSP deposition in the muscle ECM of patients with amyotrophic lateral sclerosis (ALS) in comparison with controls. Because the mechanism for this increase is unknown, we compared serum TSP levels in 11 patients to 15 controls using three different site-specific monoclonal antibodies (MA-I, MA-II and A6.1). We found mean serum TSP concentrations by indirect ELISA to be significantly decreased in the ALS patients. Using laser densitometry we calculated the ratio of fragmented to native TSP from Western immunoblots probed with A6.1, where a higher ratio corresponds to increased fragments. Mean values for this ratio were 6.3 +/- 4.9 and 18.3 +/- 8.2 for controls and patients, respectively. Thus significant decrease in native TSP and increase in its proteolytic fragments in ALS is consistent with increased proteolytic enzyme activity. Dysregulation of the protease: inhibitor balance in this degenerative condition may be reflected in the quantitative and qualitative changes in TSP.