The P11, tPA/plasminogen system and brain-derived neurotrophic factor: Implications for the pathogenesis of major depression and the therapeutic mechanism of antidepressants.

Major depressive disorder (MDD) is a common disabling psychiatric illness with an unknown etiology. Evidence from animal and human studies suggests that a disturbance in serotonergic (5-HT) activity and/or brain-derived neurotrophic factor (BDNF) signaling may be implicated in the pathogenesis of MDD. Recently, a protein, p11, has been found to increase the number of 5-HT(1B) receptors on the surface of cells and enhance 5-HT(1B) receptor function. Furthermore, mice over-expressing p11 acted as if they were undergoing treatment with antidepressants and p11 knockout mice exhibit a depression-like phenotype and reduced behavioural reactions to an antidepressant. As tissue-type plasminogen activator (tPA)/plasminogen proteolytic cascade is implicated in the cleavage of proBDNF to BDNF, and p11, a component of the Annexin II, which can greatly enhance the activation of plasmin by tPA, it is proposed that p11 may act through the tPA/plasminogen/BDNF pathway to achieve its antidepressant effect. Attempts to confirm this hypothesis may lead to new directions in the study of the pathogenesis of MDD and the development of a novel intervention for this disorder. In addition, BDNF is also implicated in several psychiatric diseases such as schizophrenia, bipolar disorder, attention-deficit hyperactivity disorder and Alzheimer's disease; whether p11 and other components related to the tPA/plasminogen pathway may be related to the pathogenesis of these diseases needs further exploration.