The Yin-Yang of osteopontin in nervous system diseases: Damage versus repair

Osteopontin is a broadly expressed pleiotropic protein, and is attracting increased attention because of its role in the pathophysiology of several inflammatory, degenerative, autoimmune, and oncologic diseases. In fact, in the last decade, several studies have shown that osteopontin contributes to tissue damage not only by recruiting harmful inflammatory cells to the site of lesion, but also increasing their survival. The detrimental role of osteopontin has been indeed well documented in the context of different neurological conditions (i.e., multiple sclerosis, Parkinson's, and Alzheimer's diseases). Intriguingly, recent findings show that osteopontin is involved not only in promoting tissue damage (the Yin), but also in repair/regenerative mechanisms (the Yang), mostly triggered by the inflammatory response. These two apparently discordant roles are partly related to the presence of different functional domains in the osteopontin molecule, which are exposed after thrombin or metalloproteases cleavages. Such functional domains may in turn activate intracellular signaling pathways and mediate cell-cell and cell-matrix interactions. This review describes the current knowledge on the Yin and Yang features of osteopontin in nervous system diseases. Understanding the mechanisms behind the Yin/Yang would be relevant to develop highly specific tools targeting this multifunctional protein.