SARS-CoV-2 nucleocapsid protein drives pulmonary injury by enhancing GRP75-dependent ER-mitochondria tethering and reprogramming of alveolar macrophages.

Publication date: Aug 08, 2026

To determine whether the SARS-CoV-2 nucleocapsid (N) protein promotes lung injury by augmenting GRP75-dependent endoplasmic reticulum (ER)-mitochondria tethering and reprogramming alveolar macrophages (AMϕs). MAMs, ER, and mitochondria were isolated from mouse lungs. Bone marrow-derived and RAW264. 7 macrophages were stimulated with recombinant N protein +/- lentiviral GRP75 knockdown. ER-mitochondria interactions, tethering complex formation, mitochondrial Ca dynamics, and function were assessed by confocal imaging, transmission electron microscopy, proximity ligation assay, co-immunoprecipitation, and biochemical approaches. In vivo, GRP75 was knocked down in C57BL/6 mouse lungs prior to intratracheal N protein challenge. Lung injury, AMϕ polarization, mitochondrial function, and single-cell transcriptomic changes were assessed. N protein promoted redistribution of GRP75 to MAMs without altering total lung GRP75 levels. It enhanced ER-mitochondria contacts and IP3R1-GRP75-VDAC1 complex assembly, resulting in excessive mitochondrial Ca accumulation, membrane depolarization, increased mitochondrial reactive oxygen species, ATP depletion, and proinflammatory cytokine production. GRP75 knockdown reversed these defects, restoring mitochondrial function and suppressing inflammation. In vivo, pulmonary GRP75 knockdown alleviated N protein-induced lung injury, inflammatory cell infiltration, cytokine release, M1 polarization, and mitochondrial dysfunction in AMϕs. Single-cell RNA sequencing further demonstrated that GRP75 knockdown reversed N protein-induced AMϕ transcriptional reprogramming by reducing the proinflammatory Car4 subset while expanding an anti-inflammatory CD74 subset. These findings establish GRP75-dependent ER-mitochondria tethering as a critical mechanism by which N protein disrupts AMϕ homeostasis and promotes COVID-19-associated lung injury, identifying GRP75-mediated MAM signaling as a potential therapeutic target.

Semantics

Original Article