Over 100 of the 700 GPCR proteins encoded in the genome have known or presumed therapeutic potential but have escaped targeting due mostly to their inaccessible location on the intracellular face of cell membranes, according to Dr. Kuliopulos. While diverse in their primary amino acid sequences and functions, all GPCRs share a highly conserved spatial configuration.
GPCR proteins consist of three different subunits (heterotrimers) consisting of a seven-transmembrane helical core domain held together by three intracellular loops, three extracellular loops, and N- and C-terminal domains that span the entire cell membrane.
Key to changing the receptor from inactive to active are ligand-induced conformational changes of transmembrane helices 3 (TM3) and 6 (TM6). These helical movements in turn change the conformation of the intracellular loops of the receptor to promote activation of associated heterotrimers.
“Most people are looking for molecules, particularly small molecules that will engage the binding site for GPCR that is on the outside of the cell. We focus on a site on the opposite side of the receptor, inside the cell membrane,” Anchor’s president and CEO, Frederick Jones, M.D., explained to GEN.
“We create peptide libraries based on the exposed areas of the intracellular GPCR loops to find peptides that act as mimics and compete with the loops that they are modeled on. These peptides can change the normal interactions among the loops. For example, preventing the shape change may make it more difficult for the ligand to signal, thereby downregulating signaling, while facilitating the shape change upregulates receptor activity.”
Dr. Kuliopulos’ team has shown that pepducins activate or inhibit a class of GPCRs, protease-activated receptors (PARs), that play key roles in thrombosis, inflammation, and vascular biology.