Periodic Reporting for period 2 - Ctrl-BBB (Blood-brain barrier: from molecular mechanisms to intervention strategies)
Période du rapport: 2022-07-01 au 2023-12-31
The delicate brain needs to be protected from blood-borne substances. The blood-brain barrier (BBB) is one of the structures that provide neuroprotection. At the anatomical level, the BBB is a manifestation of brain endothelial cells (ECs). The vertebrate EC-based BBB is both a physical and metabolic barrier. The apical tight junctional complexes seal the intercellular clefts between adjacent cells to severely restrict paracellular permeability. The down-regulation of transcytosis contributes to the barrier function by blocking molecular transit through the cells. The BBB also precludes the diffusion of lipophilic compounds into the brain parenchyma by expressing an array of apically-located efflux pumps. Various transporters compensate for barrier impermeability by providing essential nutrients to the brain. Finally, brain ECs are enriched with detoxifying enzymes that constitute a metabolic barrier. Taken together, these adaptations control brain homeostasis for reliable synaptic transmission and protect the brain from harmful components of the blood.
BBB dysfunction is a hallmark of brain tumors, stroke, brain trauma, congenital vascular malformations, and several neurodegenerative diseases. Leaky barriers compromise neuronal function and survival and often have disabling or fatal consequences. Furthermore, the BBB provides a stubborn obstacle to the treatment of many neurological diseases, impeding an overwhelming majority of neuroactive molecules from reaching effective concentrations in target brain regions. Hence, there is great interest in better understanding the molecular mechanisms that shape the anatomy and control the functionality of the BBB, not only to understand better how the brain develops and works but also to elaborate innovative therapeutic strategies to control BBB in health and disease. This is the overarching goal of this project.
BBB dysfunction is a hallmark of brain tumors, stroke, brain trauma, congenital vascular malformations, and several neurodegenerative diseases. Leaky barriers compromise neuronal function and survival and often have disabling or fatal consequences. Furthermore, the BBB provides a stubborn obstacle to the treatment of many neurological diseases, impeding an overwhelming majority of neuroactive molecules from reaching effective concentrations in target brain regions. Hence, there is great interest in better understanding the molecular mechanisms that shape the anatomy and control the functionality of the BBB, not only to understand better how the brain develops and works but also to elaborate innovative therapeutic strategies to control BBB in health and disease. This is the overarching goal of this project.
Among the many signals that control BBB functionality, neural progenitor-derived Wnt7a/b ligands play arguably important roles. During embryogenesis, they control CNS vascularization. In adults, they contribute to maintaining BBB function. In this project, we explored if these endogenous BBB-inducing signals can be repurposed as BBB repair agents in disease settings in which the BBB dysfunctions. To that end, we implemented a two-step strategy. In step one, we investigated the molecular mechanisms by which Wnt7a/b ligands trigger Wnt signaling in CNS endothelial cells. Prior evidence indicated that two membrane proteins, Reck and Gpr124 played critical roles. Our observations suggest that the two proteins share tasks in the process. The GPI-anchored Reck binds to Wnt7a/b but is unable to relay a signal within the endothelial cell. Signal transduction requires Gpr124. This member of the adhesion class of G protein-coupled receptors acts as a linker molecule between Reck and the signaling initiating Frizzled receptors. Our observations reveal three distinct linking mechanisms, of varying importance among vertebrate species. In step two, based on the molecular understanding of the Gpr124/Reck mechanism of action, we explored whether the endogenous Wnt7a/b ligands can be engineered into BBB-specific repair agents. By introducing subtle mutations in the sequence of these ligands, we could shift their signaling properties, making them strictly specific to the Gpr124/Reck receptor complex of the BBB. Thereby, the engineered ligands became safer and could be used to reduce glioblastoma progression and stroke infarct volumes, when delivered in the mouse brain.
Our results illustrate that Wnt7a/b ligands, important endogenous BBB-inducing signals, can be repurposed as BBB repair agents. This observation provides proof of concept that exploring the developmental signals orchestrating neurovascular development can lead to the identification of therapeutic intervention strategies to reduce BBB dysfunction in adults. Such BBB-focused strategies have large potential across a range of neurological conditions, including neurovascular, neurodegenerative, and neuroinflammatory disorders. This project will continue exploring this approach, by implementing various gene-discovery programs aiming at identifying important BBB regulatory molecules, exploring their mechanism of action, and eventually, their therapeutic potential.