Identification of Small Molecules Affecting the Secretion of Therapeutic Antibodies with the Retention Using Selective Hook (RUSH) System.

Unlocking cell secretion capacity is of paramount interest for the pharmaceutical industry focused on biologics. Here, we leveraged retention using a selective hook (RUSH) system for the identification of human osteosarcoma U2OS cell secretion modulators, through automated, high-throughput screening of small compound libraries. We created a U2OS cell line which co-expresses a variant of streptavidin addressed to the lumen-facing membrane of the endoplasmic reticulum (ER) and a recombinant anti-PD-L1 antibody. The heavy chain of the antibody was modified at its C-terminus, to which a furin cleavage site, a green fluorescent protein (GFP), and a streptavidin binding peptide (SBP) were added. We show that the U2OS cell line stably expresses the streptavidin hook and the recombinant antibody bait, which is retained in the ER through the streptavidin-SBP interaction. We further document that the addition of biotin to the culture medium triggers the antibody release from the ER, its trafficking through the Golgi where the GFP-SBP moiety is clipped off, and eventually its release in the extra cellular space, with specific antigen-binding properties. The use of this clone in screening campaigns led to the identification of lycorine as a secretion enhancer, and nigericin and tyrphostin AG-879 as secretion inhibitors. Altogether, our data support the utility of this approach for the identification of agents that could be used to improve recombinant production yields and also for a better understanding of the regulatory mechanism at work in the conventional secretion pathway.

Methods for measuring HMGB1 release during immunogenic cell death.

The exodus of the alarmin high mobility group box 1 (HMGB1) from the nucleus constitutes a crucial cellular danger signal and manifests as a sequential process in which HMGB1 first exits the nucleus into the cytoplasm and then is secreted or passively released through the permeabilized plasma membrane. Extracellular HMGB1 can interact with pattern recognition receptors to stimulate innate immune responses. Here, we describe a discovery pipeline for the identification of pharmacological agents endowed with HMGB1 releasing properties. The "retention using selective hooks" (RUSH) system in which a streptavidin-NLS3 fusion protein serves as a nuclear hook to sequester streptavidin-binding peptide (SBP) fused with HMGB1 and green fluorescent protein (GFP) allowed for synchronizing HMGB1 increase. Thus, exclusively in the presence of biotin, which liberates HMGB1-SBP-GFP from its nuclear hook, immunogenic cell death (ICD) inducers such as anthracyclines are able to cause the nucleo-cytoplasmic translocation of HMGB1-SBP-GFP. This system facilitates the identification of HMGB1 releasing agents in medium- to high-throughput screening assays.