Probing into receptor-mediated trafficking and internalization functions of biologically active peptides

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Background: The blood-brain barrier (BBB) is a formidable gatekeeper toward exogenous substances, playing an important role in brain homeostasis for complex neuronal activities. Accordingly, in order for the majority of bioactive proteins and peptides to cross the BBB, macromolecular receptors are required to circumvent the BBB restrictive permeability. Whether it is for brain imaging contrast formulations or in the development of brain penetrating agents, two molecular goals must therefore be achieved : Greater targeting selectivity and better delivery efficacy. A rational approach to attain these goals is to understand how biologically active ligands can selectively bind to and be transported through specific receptors at the BBB. The identity and interdependent functional relationship between specific macromolecular (co)receptors-mediated transport, that enable endocytosis to specific intracellular compartments, remain however poorly documented. An attractive strategy to increase the intracerebral delivery of a bioactive molecule is also to specifically deliver it within an intracellular targeted organelle. Thus, receptor-mediated ligand intracellular trafficking/compartmentation processes also remain to be better defined and understood at the BBB.Originality of our research program: Although peptide transcytosis into brain tissue occurs, in part, via the low-density lipoprotein receptor-related protein 1 (LRP1) at the BBB interface, we have discovered novel undocumented compensatory adaptive mechanisms, in LRP1/brain endothelial cells, which allowed us to postulate for the significant involvement of alternate transcytosis processes through new macromolecular receptors from heparin sulfate proteoglycans and Vps10p-domain receptors. Impact of our research program leading to technology advancements: By examining whether different, yet undocumented, mechanisms exist for differential internalization and trafficking of extracellular ligands in both cerebral endothelial and non-endothelial cell types, we will delineate at the molecular level the functional interdependence of cell surface macromolecular receptors. The value of identifying new macromolecular entities that mediate transcytosis across the BBB is then two-fold: as research tools to help understand fundamental in vitro molecular and cellular transport processes, and as pharmacological tools to address the significant mechanistic needs to efficiently target the BBB. We are confident that our research program is original as it will ultimately provide rationale for greater targeting selectivity towards the BBB phenotype and help in the design of better intracerebral delivery strategies. As such, an immediate impact derived from our research program should inspire the future generation of bioconjugates to selectively bind to those receptors we propose to investigate.