CCL2 and Cognitive impairment: The sera, MGCs, Mφs, and midbrain regions of PARK2-associated human PD also displayed decreased expression of parkin and its link to increased generation of pro-inflammatory cytokines (e.g., IFNβ1, TNFα, IL-1β, IL-6, IL-12, IL-13, CCL2, CCL4, and CXCL1), loss of DA neurons, and cognitive defects in PD (Table 5B).