Rescue of ΔF508-CFTR Trafficking via a GRASP-Dependent Unconventional Secretion Pathway

Abstract

The most prevalent disease-causing mutation of CFTR is the deletion of Phe508 (ΔF508), which leads to defects in conventional Golgi-mediated exocytosis and cell surface expression. We report that ΔF508-CFTR surface expression can be rescued in vitro and in vivo by directing it to an unconventional GRASP-dependent secretion pathway. An integrated molecular and physiological analysis indicates that mechanisms associated with ER stress induce cell surface trafficking of the ER core-glycosylated wild-type and ΔF508-CFTR via the GRASP-dependent pathway. Phosphorylation of a specific site of GRASP and the PDZ-based interaction between GRASP and CFTR are critical for this unconventional surface trafficking. Remarkably, transgenic expression of GRASP in ΔF508-CFTR mice restores CFTR function and rescues mouse survival without apparent toxicity. These findings provide insight into how unconventional protein secretion is activated, and offer a potential therapeutic strategy for the treatment of cystic fibrosis and perhaps diseases stemming from other misfolded proteins.

Golgi Feels Its Own Wound

“…These findings are relevant for a recent study which shows the importance of conventional Golgi assembly and secretion in a genetic disorder and offers a potential therapeutic strategy for the treatment of disease related to Golgi secretion. 17 The pericentrosomal positioning of the Golgi apparatus could be a byproduct of a strategy using microtubules plus end-directed motility to orient delivery of post-Golgi membrane carriers over large distances and to specific cellular sites. 16 The remodeling and reorientation of Golgi was shown in another study to be regulated by phosphorylation of Golgi reassembly stacking protein 65 (GRASP65) by the extracellular signal-regulated kinase, causing loss of GRASP65 oligomerization and cisternal unstacking.…”

Autophagy at the crossroads of catabolism and anabolism

“…Studies have primarily focused on autophagy in unconventional secretion, a collection of processes through which certain proteins are secreted from cells either via direct trafficking from the endoplasmic reticulum (ER) to the plasma membrane in a Golgi-independent manner, or via the transport of cytoplasmic proteins lacking an amino-terminal ER signal sequence to the cell surface, completely bypassing the ER-Golgi route 126 . Autophagy-related proteins (ATGs) have been genetically implicated in the unconventional secretion of the acyl-CoA-binding protein Acb1 in yeast (AcbA in Dictyostelium discoideum), and inflammatory mediators such as interleukin-1β (IL-1β) and IL-18, the high mobility group protein B1 (HMGB1) and the integral membrane protein ΔF508 CFTR (cystic fibrosis transmembrane conductance regulator) in mammalian cells [127][128][129][130][131] . The unconventional secretion of these proteins is also dependent on Golgi membrane-binding proteins of the GRASP family in both yeast and mammals 129,130,132 .…”

Reduced PDZ Interactions of Rescued ΔF508CFTR Increases Its Cell Surface Mobility

“…In the Golgi, modulation of the function or abundance of PDZ-domain containing proteins has been shown to correct ⌬F508CFTR trafficking (10,22,23). Similarly, EBP50 has been implicated in r⌬F508CFTR biosynthetic processing.…”