Lymphatic drainage, Immune cell recirculation and brain repair after stroke – BrainWash

Stroke is a leading cause of death, killing approximately 30.000 French people each year. Stroke occurs when blood flow to an area of the brain is cut off, causing death of oxygen- and nutriment-deprived brain cells downstream of obstructed blood vessels. Stroke also causes inflammatory cell accumulation and cerebral edema, which is accumulation of excessive fluid collecting in brain tissue. Because edema compresses brain tissue and aggravates neurological damage, it is a major risk factor for morbidity and mortality in stroke patients. In addition, inflammatory cell accumulation causes secondary brain damage and neurotoxicity. Better treatment options against cerebral edema and inflammation require insight into mechanisms promoting drainage of cerebrospinal (CSF) and interstitial fluid (ISF).

The drainage of cerebral fluids is performed by a circuitry including the cerebral glymphatic system and a meningeal lymphatic vasculature that connects with deep cervical lymph nodes. Lymphatic vasculature ensures both the clearance of toxic molecules produced by brain cell activity and the immune surveillance of brain tissues, allowing immune cell recirculation. Abnormal brain lymphangiogenesis is likely to affect homeostasis of cerebral fluids and brain immunity, with health-impairing consequences on the response to injuries and neurological diseases in humans.

VegfC is the major regulator of lymphangiogenesis; it signals via its high-affinity receptor, Vegfr3 that is expressed on the surface of lymphatic endothelial cells (LECs) and stimulates LEC migration, proliferation and survival. We have shown that, in addition to lymphangiogenesis, VegfC/Vegfr3 signaling is a potent and direct regulator of brain neurogenesis. VegfC is expressed in the adult brain, where it stimulates neurogenesis by activating neural stem cells (NSCs) both in mice and humans. The VegfC/Vegfr3 system is thus uniquely endowed with the dual ability to promote growth of lymphatic vessels and activate NSCs. We hypothesize that VegfC/Vegfr3 signaling regulates meningeal lymphangiogenesis and that promoting this signaling will allow to alleviate edema and immune cell accumulation in the brain, and simultaneously activate NSC, thereby enhancing brain repair.

In line with this hypothesis, preliminary results obtained by the consortium demonstrated that, in the mouse, the adult meningeal lymphatic vasculature is plastic and VegfC-dependent, allowing us to completely delete or expand the lymphatic vessel coverage of the skull though AAV-mediated expression of VegfC trap or VegfC, respectively. We have also established innovative tools and techniques to carry out live-imaging of meningeal vessels by multiphoton confocal microscopy, to perform 3D-imaging of the head volume including meningeal vessels, and to induce the controlled expression of VegfC in vivo.

The ultimate goal of this proposal is therefore to take advantage of our conceptual and technological advances on VegfC and meningeal lymphatic biology to develop novel strategies to improve brain repair which would simultaneously promote cerebral fluid drainage, immune cell recirculation and NSC activation. To this aim, we propose to characterize the role of meningeal lymphatic vasculature in stroke (WP2), the role of inflammation in mice with reduced/enhanced meningeal lymphatic vessels during stroke (WP3), and the potential of VegfC/Vegf3 signaling in NSCs to improve brain repair after stroke (WP4).

Hence, the major conceptual innovation of the present project is to integrate the lymphangiogenic and neurogenic functions of VegfC for improving brain tissue drainage and repair after an ischemic injury. Gain of knowledge in the function of recently discovered meningeal lymphatics could moreoever lead to the identification of innovative therapeutic targets to prevent or treat neurodegenerative diseases associated with defects in ISF drainage such as Alzheimer’s disease or other proteinopathies.