Long COVID elevated MMP-9 and release from microglia by SARS-CoV-2 Spike protein

Objective: Long COVID is a major health concern because many patients develop chronic neuropsychiatric symptoms, but the precise pathogenesis is unknown. Matrix metalloproteinase-9 (MMP-9) can disrupt neuronal connectivity and be elevated in patients with long COVID. Methods: In this study, MMP-9 was measured in the serum of long COVID patients and healthy controls, as well as in the supernatant fluid of cultured human microglia cell line stimulated by recombinant severe acute respiratory syndrome coronavirus 2 Spike protein, as well as lipopolysaccharide (LPS) and neurotensin (NT) used as positive controls. MMP-9 was measured by commercial enzyme-linked immunosorbent assay. Results: MMP-9 was significantly elevated in the serum of long COVID patients compared to healthy controls. Moreover, there was significant release of MMP-9 from a cultured human microglia cell line stimulated by LPS, NT, or Spike protein. We further show that pretreatment with the flavonoids luteolin and tetramethoxyluteolin (methlut) significantly inhibited the release of MMP-9 stimulated by the Spike protein. Conclusion: MMP-9 from Spike protein-stimulated microglia could contribute to the development of long COVID and may serve as a target for treatment including the use of luteolin.