Caspase 1 Enhances Transport and Golgi Organization Protein 1 Expression to Promote Procollagen Export From the Endoplasmic Reticulum in Systemic Sclerosis Contributing to Fibrosis

OBJECTIVE

Transport and Golgi Organization protein 1 (TANGO1) is a protein that regulates the export of procollagen from the endoplasmic reticulum and has a role in the organization of exit sites for general protein export. What regulates the expression of TANGO1 and the role of TANGO1 in fibrosis is poorly understood and has never been studied in the setting of systemic sclerosis (SSc). We undertook this study to determine the role of TANGO1 in SSc fibrosis.

METHODS

SSc (n = 15) and healthy (n = 12) primary fibroblast lung cell lines were investigated for the expression of TANGO1. Histologic analyses for TANGO1 were performed on lung biopsy samples (n = 12 SSc patient samples and n = 8 healthy control samples).

RESULTS

SSc fibroblasts showed increased expression of TANGO1 protein in cultured fibroblasts. TANGO1 colocalizes with α-smooth muscle actin (α-SMA)-positive cells in SSc lung tissue and is highly up-regulated in the neointima of SSc vessels. TANGO1 expression was dependent on the inflammasome activation of caspase 1. It was also dependent on signaling from the interleukin-1 (IL-1) and transforming growth factor β (TGFβ) receptors. The decrease in TANGO1 down-regulated export of larger cargos including collagen and laminin. Reduced TANGO1 protein had no effect on smaller molecular weight cargoes; however, the secretion of elastin was significantly reduced.

CONCLUSION

TANGO1 is markedly increased in SSc fibroblasts and was found to be elevated in lung tissue in association with α-SMA-positive cells. TANGO1 expression is driven by inflammasome-dependent caspase 1 activation and is mediated by IL-1 and TGFβ downstream signaling. These observations suggest that during fibrosis, caspase 1 promotes the up-regulation of TANGO1 and the organization of endoplasmic reticulum exits sites, ultimately contributing to procollagen export and fibrosis.

A physical mechanism of TANGO1-mediated bulky cargo export

The endoplasmic reticulum (ER)-resident protein TANGO1 assembles into a ring around ER exit sites (ERES), and links procollagens in the ER lumen to COPII machinery, tethers, and ER-Golgi intermediate compartment (ERGIC) in the cytoplasm (Raote et al., 2018). Here, we present a theoretical approach to investigate the physical mechanisms of TANGO1 ring assembly and how COPII polymerization, membrane tension, and force facilitate the formation of a transport intermediate for procollagen export. Our results indicate that a TANGO1 ring, by acting as a linactant, stabilizes the open neck of a nascent COPII bud. Elongation of such a bud into a transport intermediate commensurate with bulky procollagens is then facilitated by two complementary mechanisms: (i) by relieving membrane tension, possibly by TANGO1-mediated fusion of retrograde ERGIC membranes and (ii) by force application. Altogether, our theoretical approach identifies key biophysical events in TANGO1-driven procollagen export.