Impaired fibrinolysis in multiple sclerosis: a role for tissue plasminogen activator inhibitors

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Abstract
Tissue plasminogen activator (tPA), a neuronal as well as the key fibrinolytic enzyme, is found concentrated on demyelinated axons in multiple sclerosis lesions together with fibrin(ogen) deposits. The decreased tPA activity in normal‐appearing white and grey matter and lesions of multiple sclerosis is reflected in diminished fibrinolysis as measured by a clot lysis assay. Nonetheless, peptide products of fibrin, including D‐dimer, accumulate on demyelinated axons—the result of fibrinogen entry through a compromised blood‐brain barrier (BBB). Analysis of tissue samples on reducing and non‐reducing polyacrylamide gels demonstrates complexes of tPA with plasminogen activator inhibitor‐1 (PAI‐1) but not with neuroserpin, a tPA‐specific inhibitor concentrated in grey matter. As total tPA protein remains unchanged in acute lesions and the concentration of PAI‐1 rises several fold, complex formation is a probable cause of the impaired fibrinolysis. Although the tPA‐plasmin cascade promotes neurodegeneration in excitotoxin‐induced neuronal death, in inflammatory conditions with BBB disruption it has been demonstrated to have a protective role in removing fibrin, which exacerbates axonal injury. The impaired fibrinolytic capacity resulting from increased PAI‐1 synthesis and complex formation with tPA, which is detectable prior to lesion formation, therefore has the potential to contribute to axonal damage in multiple sclerosis.