GRM5 and Parkinson disease: Conversely, topological analysis of the microglia cell-specific PPI network, which was constructed using the downregulated DEGs identified when comparing PD to MDD, revealed high centrality genes such as CAMK2A, GRIN2B, GRM5, SYP, GRIN2A, and NRXN. These genes are closely associated with synaptic transmission, specifically glutamate receptor signaling.