Cargo receptor-assisted endoplasmic reticulum export of pathogenic α1-antitrypsin polymers

Circulating polymers of alpha1-antitrypsin (1AT) are chemo-attractant for neutrophils and contribute to inflammation in pulmonary, vascular and adipose tissues. Cellular factors affecting the intracellular itinerary of mutant polymerogenic 1AT remain obscure. Here, we report on an unbiased genome-wide CRISPR/Cas9 screen for regulators of trafficking of the polymerogenic 1ATH334D variant. Single guide RNAs targeting genes whose inactivation enhanced accumulation of polymeric 1AT were enriched by iterative construction of CRISPR libraries based on genomic DNA from fixed cells selected for high polymer content by fluorescence-activated cell sorting. This approach bypassed the limitation to conventional enrichment schemes imposed by cell fixation and identified 121 genes involved in polymer retention at false discovery rate < 0.1. From that set of genes, the pathway cargo loading into COPII-coated vesicles was overrepresented with 16 significant genes, including two transmembrane cargo receptors, LMAN1 (ERGIG-53) and SURF4. LMAN1 and SURF4-disrupted cells displayed a secretion defect extended beyond 1AT monomers to polymers, whose low-level secretion was especially dependent on SURF4 and correlated with SURF4-1ATH334D physical interaction and with enhanced co-localisation of polymeric 1ATH334D with the endoplasmic reticulum (ER). These findings suggest that ER cargo receptors co-ordinate intracellular progression of 1AT out of the ER and modulate the accumulation of polymeric 1AT not only by controlling the concentration of precursor monomers but also through a previously-unrecognised role in secretion of the polymers themselves.