**Background:** SARS-CoV-2 infection is known to cause neurological symptoms and may trigger or accelerate neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD). The frontal cortex is critically involved in cognitive function and is affected in both AD and PD, as well as in COVID-19 patients. This study aimed to identify shared molecular mechanisms among COVID-19, AD, and PD using bioinformatic analysis of frontal cortex transcriptomic data.
**Methods:** Three training datasets were obtained from the GEO database: GSE188847 (COVID-19, 12 cases vs. 12 controls), and GSE150696 (AD, 9 cases vs. 9 controls; PD, 12 cases vs. 9 controls). Differential expression analysis was performed using the limma package in R (cutoff: p < 0.05, |logFC| > 1), controlling for age and sex. Common DEGs among the three diseases were identified via Venn diagram analysis. Functional annotation included Gene Ontology (GO), KEGG, Reactome pathway enrichment, and Gene Set Enrichment Analysis (GSEA). A protein-protein interaction (PPI) network was constructed using STRING and visualized in Cytoscape. Hub genes were identified using three algorithms (MCC, Degree, Betweenness Centrality) via CytoHubba. Candidate drugs were predicted using DrugBank, and transcription factors were identified using RcisTarget. Validation was performed using independent datasets: GSE164332 (COVID-19), GSE104704 (AD), and merged GSE20168/GSE8397 (PD).
**Key Results:** A total of 1344 DEGs were identified for COVID-19 (927 up, 417 down), 2655 for AD (651 up, 2004 down), and 2589 for PD (882 up, 1707 down). Cross-comparison yielded 52 common DEGs (9 up-regulated, 43 down-regulated). GO analysis showed enrichment in synaptic signaling terms including neurotransmitter transport (p = 1.64 × 10⁻⁵), modulation of chemical synaptic transmission (p = 0.0001), and regulation of trans-synaptic signaling (p = 0.0001). Cellular component terms were dominated by exocytic vesicle (p = 3.31 × 10⁻¹⁰) and synaptic vesicle membrane (p = 4.42 × 10⁻¹⁰). KEGG analysis identified the synaptic vesicle cycle as the top pathway (p = 2.06 × 10⁻⁶). Reactome analysis highlighted transmission across chemical synapses (p = 8.21 × 10⁻⁷) and neuronal system (p = 1.84 × 10⁻⁶). GSEA confirmed that synapse-related terms were consistently down-regulated across all three diseases, while cytokine-cytokine receptor interaction and humoral immune response were up-regulated. The PPI network contained 52 nodes and 320 edges. MCODE analysis extracted a key module (score 18.222, 19 nodes) also linked to the synaptic vesicle cycle. Five hub genes were identified: TAGLN3, GAD2, SST, SYP, and KCNJ4, all of which were validated in independent datasets. Six drug targets and 18 related drugs were identified from DrugBank. Five drugs were considered potentially therapeutic: Ibutilide (target: CACNB1, p = 0.0058), Azelnidipine (target: CACNB1, p = 0.0058), Dotarizine (target: CACNA1A, p = 0.0068), Copper (target: SFPQ, p = 0.0233), and Artenimol (target: SFPQ, p = 0.0233). Ten transcription factors (SP2, SIN3A, REST, ATF3, MYF6, TBX5, RFX1, RPL6, NR3C1, HDAC2) were predicted to regulate 42 of the common DEGs.
**Clinical Implications:** This study provides evidence that synaptic vesicle cycle dysfunction is a shared molecular mechanism linking COVID-19 with AD and PD, suggesting that SARS-CoV-2 infection may cause synaptic down-regulation in the frontal cortex that could trigger or accelerate neurodegenerative processes. The five identified hub genes (particularly GAD2, involved in GABA synthesis, and SYP, a key synaptic protein) represent potential therapeutic targets. The candidate drugs identified, especially azelnidipine (a calcium channel blocker with known neuroprotective effects) and artenimol (an artemisinin derivative with anti-SARS-CoV-2 potential), warrant further investigation for treating COVID-19-related neurological complications. However, the authors caution that these findings are based on bioinformatic analyses with limited sample sizes and require validation through cellular and animal experiments.