Cargo Receptor-Assisted Endoplasmic Reticulum Export Of Pathogenic Alpha 1-Antitrypsin Polymers

Circulating polymers of alpha 1-antitrypsin (alpha 1AT) are neutrophil chemo-attractants and contribute to inflammation, yet cellular factors affecting their secretion remain obscure. We report on a genome-wide CRISPR-Cas9 screen for genes affecting trafficking of polymerogenic alpha 1AT(H334D). A CRISPR enrichment approach based on recovery of single guide RNA (sgRNA) sequences from phenotypically selected fixed cells reveals that cells with high-polymer content are enriched in sgRNAs targeting genes involved in "cargo loading into COPII-coated vesicles,'' where "COPII'' is coat protein II, including the cargo receptors lectin mannose binding1 (LMAN1) and surfeit protein locus 4 (SURF4). LMAN1- and SURF4-disrupted cells display a secretion defect extending beyond alpha 1AT monomers to polymers. Polymer secretion is especially dependent on SURF4 and correlates with a SURF4-alpha 1AT(H334D) physical interaction and with their co-localization at the endoplasmic reticulum (ER). These findings indicate that ER cargo receptors co-ordinate progression of alpha 1AT out of the ER and modulate the accumulation of polymeric alpha 1AT not only by controlling the concentration of precursor monomers but also by promoting secretion of polymers.