SPP1 and Parkinson disease: In addition, elevated plasma OPN levels have been also shown to play an important role in inflammatory and degenerative processes of the central nervous system: OPN plasma levels were elevated in secondary progressive MS compared to relapsing-remitting MS patients in remission and healthy controls, supporting a role for OPN in the chronic disease activity [38]; OPN serum levels were elevated in Parkinson's disease and higher serum levels were associated with more severe motor symptoms [39].