Endocytic recycling is central to circadian collagen fibrillogenesis and disrupted in fibrosis

Collagen-I fibrillogenesis is crucial to health and development, where dysregulation is a hallmark of fibroproliferative diseases. Here, we show that collagen-I fibril assembly required a functional endocytic system that recycles collagen-I to assemble new fibrils. Endogenous collagen production was not required for fibrillogenesis if exogenous collagen was available, but the circadian-regulated vacuolar protein sorting (VPS) 33b and collagen-binding integrin-alpha 11 subunit were crucial to fibrillogenesis. Cells lacking VPS33b secrete soluble collagen-I protomers but were deficient in fibril formation, thus secretion and assembly are separately controlled. Overexpression of VPS33b led to loss of fibril rhythmicity and over-abundance of fibrils, which was mediated through integrin alpha11beta1. Endocytic recycling of collagen-I was enhanced in human fibroblasts isolated from idiopathic pulmonary fibrosis, where VPS33b and integrin-alpha11 subunit were overexpressed at the fibrogenic front; this correlation between VPS33b, integrin-alpha11 subunit, and abnormal collagen deposition was also observed in samples from patients with chronic skin wounds. In conclusion, our study showed that circadian-regulated endocytic recycling is central to homeostatic assembly of collagen fibrils and is disrupted in diseases. ### Competing Interest Statement The authors have declared no competing interest.