Development of a Live Cell Phage Display Screening Protocol
Protein-protein interactions (PPIs) are essential for all biological functions. Developing peptides that disrupt these PPIs is an avid research effort, as peptides possess several advantages over small molecules and monoclonal antibodies. Peptide phage display is a useful tool in identifying peptides for targeting PPIs. This technology displays up to 10^10 unique polypeptides on the surface of bacteriophage, which after several rounds of panning enriches high affinity peptide sequences towards a target protein. Phage display is classically done on immobilized discrete protein; however, we propose to use this technology to identify peptides ligands for overexpressed oncogenic proteins on live cells in-vitro. This is a more accurate representation of the therapeutic target landscape and resembles how the peptide will interact with the receptor in-vivo. Several groups have explored live cell panning, such as Ruoslahti et al. and Cieslewicz et al., and while they demonstrate the capabilities of in-vitro style phage display, there are areas for improvement. We intend to improve on this previous work by 1. Identifying a peptide ligand against specific receptor/protein, and 2. By incorporating the use of covalent phage libraries to elucidate a high affinity binder. This work will be accomplished using the mammalian epidermal oncogenic cell line, A431, that is known to overexpress epidermal growth factor receptor (EGFR). Epidermal growth factor receptor (EGFR) is responsible for cellular proliferation, survival, differentiation and metastasis, which makes it an attractive target to inhibit oncogenic proliferation. Despite successfully marketed monoclonal antibodies and tyrosine kinase inhibitors, EGFR can mutate and develop resistance as diseases progress; this phenomenon, in addition to the benefits of peptides as therapeutics, are driving factors for pursuing this project. Despite our best efforts using non-covalent phage libraries to identify a viable ligand, screening against EGFR extracellular domain (ECD) has proven to be more difficult than anticipated. We hypothesize that non-covalent phage libraries do not possess any sequences with a high enough binding affinity for this protein, and that the use of covalent libraries will be needed to pull out a positive hit. Due to these findings, we have successfully constructed two phage libraries, a ACX7C and a ACX7C-TEV, where the latter introduced a TEV protease cleavage site on the C’-terminal side of the randomized amino acids suitable for covalent warhead modification and screening. Further, we have begun work on constructing an EGF-displaying phage construct to aid in optimizing a live cell panning protocol. In the future, we plan to evaluate ligand affinity and protein density, as well as determine the optimal covalent warhead/peptide combination for live cell screenings. With this information, we intend to apply this to other oncogenic cell lines, such as MCF-10CA1a, to identify potent peptide ligands for overexpressed oncogenic proteins.