Golgi casein kinase-mediated phosphorylation of osteopontin orchestrates macrophage- lung-resident mesenchymal stem cells crosstalk and fibrogenesis: a targetable macromolecular axis

Idiopathic pulmonary fibrosis (IPF) involves aberrant crosstalk between immune cells and mesenchymal compartments. While secreted phosphoproteins are crucial in this process, the upstream kinases regulating their post-translational modifications and biological functions remain poorly understood. Integrating single-cell RNA Sequencing and macromolecular interaction analysis, we identified a pro-fibrotic FPR3⁺ macrophage subset. We utilized co-immunoprecipitation and mass spectrometry to map the interaction between the Golgi kinase Fam20C and its substrate Osteopontin (also known as SPP1). To validate the functional requirement of this kinase in vivo, we employed a Fab'-functionalized macromolecular delivery system to specifically silence Fam20C in macrophages. We demonstrate that Fam20C phosphorylates SPP1, a critical modification that facilitates its secretion and subsequent binding to CD44 receptors on lung-resident mesenchymal stem cells (LR-MSCs). This ligand-receptor interaction inhibits the Hippo pathway, driving LR-MSC differentiation into myofibroblasts. Importantly, specific silencing of Fam20C using the targeted siRNA delivery strategy significantly attenuated fibrotic progression and blocked the macrophage-LR-MSC fibrogenic crosstalk in mouse models. This study reveals the Fam20C-SPP1 phosphorylation axis as a critical macromolecular switch in pulmonary fibrosis. Our findings provide mechanistic insights into immune-stromal communication and highlight Fam20C as a viable target for precision intervention.