Collagen fibril formation at the plasma membrane occurs independently from collagen secretion.

Collagen fibrils are the primary supporting scaffold of vertebrate tissues but how they are assembled is unclear. Here, using CRISPR-tagging of type I collagen and SILAC labelling, we elucidate the cellular mechanism for the spatiotemporal assembly of collagen fibrils, in cultured fibroblasts. Our findings reveal multifaceted trafficking of collagen, including constitutive secretion, intracellular pooling, and plasma membrane-directed fibrillogenesis. Notably, we differentiate the processes of collagen secretion and fibril assembly and identify the crucial involvement of endocytosis in regulating fibril formation. By employing Col1a1 knockout fibroblasts we demonstrate the incorporation of exogenous collagen into nucleation sites at the plasma membrane through these recycling mechanisms. Our study sheds light on the assembly process and its regulation in health and disease. Mass spectrometry data are available via ProteomeXchange with identifier PXD036794.

Discoidin domain receptor 2 signaling through PIK3C2α in fibroblasts promotes lung fibrosis.

Pulmonary fibrosis, especially idiopathic pulmonary fibrosis (IPF), portends significant morbidity and mortality, and current therapeutic options are suboptimal. We have previously shown that type I collagen signaling through discoidin domain receptor 2 (DDR2), a receptor tyrosine kinase expressed by fibroblasts, is critical for the regulation of fibroblast apoptosis and progressive fibrosis. However, the downstream signaling pathways for DDR2 remain poorly defined and could also be attractive potential targets for therapy. A recent phosphoproteomic approach indicated that PIK3C2α, a poorly studied member of the PI3 kinase family, could be a downstream mediator of DDR2 signaling. We hypothesized that collagen I/DDR2 signaling through PIK3C2α regulates fibroblast activity during progressive fibrosis. To test this hypothesis, we found that primary murine fibroblasts and IPF-derived fibroblasts stimulated with endogenous or exogenous type I collagen led to the formation of a DDR2/PIK3C2α complex, resulting in phosphorylation of PIK3C2α. Fibroblasts treated with an inhibitor of PIK3C2α or with deletion of PIK3C2α had fewer markers of activation after stimulation with TGFß and more apoptosis after stimulation with a Fas-activating antibody. Finally, mice with fibroblast-specific deletion of PIK3C2α had less fibrosis after bleomycin treatment than did littermate control mice with intact expression of PIK3Cα. Collectively, these data support the notion that collagen/DDR2/PIK3C2α signaling is critical for fibroblast function during progressive fibrosis, making this pathway a potential target for antifibrotic therapy. © 2024 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

CD36/Lyn kinase interactions within macrophages promotes pulmonary fibrosis in response to oxidized phospholipid.

Recent data from human studies and animal models have established roles for type II alveolar epithelial cell (AEC2) injury/apoptosis and monocyte/macrophage accumulation and activation in progressive lung fibrosis. Although the link between these processes is not well defined, we have previously shown that CD36-mediated uptake of apoptotic AEC2s by lung macrophages is sufficient to drive fibrosis. Importantly, apoptotic AEC2s are rich in oxidized phospholipids (oxPL), and amongst its multiple functions, CD36 serves as a scavenger receptor for oxPL. Recent studies have established a role for oxPLs in alveolar scarring, and we hypothesized that uptake and accrual of oxPL by CD36 would cause a macrophage phenotypic change that promotes fibrosis. To test this hypothesis, we treated wild-type and CD36-null mice with the oxPL derivative oxidized phosphocholine (POVPC) and found that CD36-null mice were protected from oxPL-induced scarring. Compared to WT mice, fewer macrophages accumulated in the lungs of CD36-null animals, and the macrophages exhibited a decreased accumulation of intracellular oxidized lipid. Importantly, the attenuated accrual of oxPL in CD36-null macrophages was associated with diminished expression of the profibrotic mediator, TGFß. Finally, the pathway linking oxPL uptake and TGFß expression was found to require CD36-mediated activation of Lyn kinase. Together, these observations elucidate a causal pathway that connects AEC2 injury with lung macrophage activation via CD36-mediated uptake of oxPL and suggest several potential therapeutic targets.