Mechanisms of blood-brain barrier penetration: A molecular dynamics study on R9 and MPG peptide translocation

One of the major obstacles in treating diseases that affect the central nervous system is delivering drugs across the blood-brain-barrier (BBB). Cell-penetrating peptides (CPPs) can be used as delivery vectors, but their translocation mechanism is still poorly understood, in part due to the simplistic membrane models applied to their interpretation. Here we investigate the translocation mechanism of two CPPs, R9 and MPG, using molecular dynamics and enhanced sampling techniques on a realistic membrane model of human brain microvascular endothelial cells. The results suggest that R9 induces greater membrane disruption compared to MPG, yet that both face a significant free energy barrier to translocation. In both peptides the first interactions were initiated by the N-terminus and prominently involved arginine residues even for MPG. The crucial role of the plasticity of both partners (BBB bending, partial CPP unfolding) on the translocation energetics was also explored by sampling ad hoc collective variables, revealing the important role of long polyunsaturated acyl chain lipids. Together, these findings provide mechanistic insight into CPP-mediated transport and offer guidelines for rational design.