The paper describes a cheap, easy lab method using engineered HEK cells to test lots of tiny proteins (peptides) for their ability to turn on a mast‑cell receptor called MRGPRX2, which is involved in immune responses. It doesn’t give any direct advice on using LL‑37 or other peptides in humans, just a way to screen them in the lab.

Identifying ligands specific to therapeutically significant cell receptors is crucial for many applications, including the design and development of new therapeutics. Mas related G-protein receptor-X2 (MRGPRX2) is an important receptor that regulates mast cell activation and, thus, directs the general immune response. Numerous ligands for MRGPRX2 have been identified and include endogenous peptides like PAMPs, defensins, LL-37 and other protein fragments (i.e., degraded albumin). Further identification of MRGPRX2 specific ligands requires the screening of a large number of peptides (i.e., peptide library); however, mast cells are difficult and expensive to maintain in vitro and, therefore, not economical to use for screening large numbers of molecules. The present paper demonstrates a method to design, develop, and screen a library of small peptide molecules using MRGPRX2 expressing HEK cells. This cell line is relatively easy and inexpensive to maintain and can be used for in vitro high-throughput analysis. A calcium sensitive Fura-2 fluorescent dye to mark intracellular calcium flux upon activation was used to monitor the activation. The ratio of fluorescence intensity of Fura-2 at 510 nm against excitation wavelengths of 340 and 380 nm was used to calculate calcium concentration. The peptide library used to verify this system was based on the endogenous proadrenomedullin N-terminal 12 (PAMP-12) secretagogue, which is known to bind MRGPRX2 with high specificity and affinity. Subsequent peptides were generated through amino acid truncation and alanine scanning techniques applied to PAMP-12. The method described here is simple and inexpensive yet robust for screening a large library of compounds to identify binding domains and other important parameters that play an important role in receptor activation.