Translational implications of the discovery of brain-draining lymphatics

Project Information

BrainDrain

Grant agreement ID: 743155

DOI

10.3030/743155

Project closed

EC signature date 14 July 2017

Start date 1 August 2017

End date 30 June 2023

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 420 429,00

EU contribution

€ 2 420 429,00

Coordinated by

HELSINGIN YLIOPISTO
Finland

Periodic Reporting for period 4 - BrainDrain (Translational implications of the discovery of brain-draining lymphatics)

Reporting period: 2022-02-01 to 2023-06-30

Summary of the context and overall objectives of the project

In 2010, 800 billion Euros was spent on brain diseases in Europe and the cost is expected to increase due to the aging population. In this project we exploited our new discovery for research aimed at alleviating this disease burden. In work selected by Nature Medicine among the top 10 ”Notable Advances” and by Science as one of the 10 ”Breakthroughs of the year” 2015, we discovered a meningeal lymphatic vascular system that serves brain homeostasis. We have reassessed current concepts about cerebrovascular dynamics, fluid drainage and cellular trafficking in physiological conditions and in Alzheimer’s disease (AD) and neuroinflammation models in mice and in postmortem tissues in humans. First, we studied the development and properties of meningeal lymphatics and how they are formed and maintained. Then, analysed the clearance of macromolecules and protein aggregates in mouse models of Alzheimer’s disease, and in mice that lack the newly discovered meningeal lymphatic drainage system. We examined if growth factor-mediated expansion of lymphatic vessels alleviates the parenchymal accumulation of neurotoxic molecules and brain disease pathogenesis and brain damage after traumatic brain injury. Our major conclusion from studies of neuroinflammation and Alzheimer’s disease mouse models is that the meningeal lymphatic vessels are not directly involved in the disease progression, which challenges current views based on models where laser damage was used to destroy these vessels. However, they are pivotal for a proper and specific neuro-immune response after traumatic brain injury and the lymphangiogenic factors modulate the pathogenesis of ischemic and hemorrhagic stroke.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

We demonstrated that essentially meningeal lymphatic vessels around the brain and spinal cord develop during the first postnatal month and that VEGFR3/VEGFC signaling is indispensable for their development and maintenance. We showed that tuning the VEGFR3/VEGFC pathway by genetic and biochemical methods allows regression or expansion of meningeal lymphatic vessels in adulthood, resulting in a corresponding decrease or increase of cerebrospinal fluid clearance via the lymphatic vessels. We found that the lymphatic network is extensive, it functions in transport of cerebrospinal fluid and it is plastic, reacting to injury or inflammation. Furthermore, we have developed multiple new and improved methods for imaging and analysis of the meningeal lymphatic vessels and their function, including imaging of the cleared transparent skull and analysis of tracer drainage from CSF to lymph nodes and blood. Based on this, we have reported on the effects of VEGF-C and VEGFR-3 on cerebrospinal fluid flow and protein clearance.

We demonstrated a significant role of the main lymphangiogenic factor VEGF-C in regulation of cerebrospinal fluid (CSF) flow and drainage. Administration of VEGF-C protein immediately after ischemic stroke ensure faster recovery and improved behavioral outcome in mice. Analysis of transgenic K14-sVEGFR3-Ig mice, in which lymphatic vessel development to the dura fails, revealed a number of interesting changes in immune cells in CNS tissues. We have also described and characterized meningeal lymphatics in primates (marmosets).

We have characterized the role of meningeal lymphatic vessels in Alzheimer’s disease in two widely used mouse models. The basic characterization of tissues related to mLVs, namely dura mater, brain and cervical lymph nodes were examined at different time-points of disease course by IHC and functional drainage analysis. The exceptional dependence of meningeal lymphatic vessels the lymphangiogenic growth factor VEGF-C allowed us to selectively modulate the amount and functionality of mLVs and study their role in Alzheimer’s disease.

In addition to the VEGFC-VEGFR3 signaling pathway, angiopoietin (Angpt)-Tie signaling pathway plays important roles in regulating vascular permeability and leukocyte trafficking. Experimental autoimmune encephalomyelitis (EAE) is a well-characterized experimental model of multiple sclerosis (MS), which is the most common human demyelinating CNS autoimmune disease. To our surprise, the lack of mLVs did not alter the clinical severity of EAE or the percentage of body weight loss. During our study, we found that neuroinflammation in this model increases the expression of the Angpt2 ligand that is required for lymphangiogenic signaling. Transgenic mice expressing Angpt2 specifically in ECs developed a more severe EAE than control mice whereas both prophylactic and preemptive treatment with an Angpt2-blocking antibody ameliorated EAE. Thus, Angpt2 targeting may serve as an alternative therapeutic option for the treatment of CNS autoimmune disease.

Our current results (in collaboration with Dr. Francesco Noe) showed that mLVs are pivotal for a proper and specific neuro-immune response after traumatic brain injury TBI, which is principally mediated by the resident memory cytotoxic CD8+ T cells.

Further studies should determine how the obtained knowledge could be translated to the treatment of CNS autoimmune disease, TBI and stroke.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Our top-ranking discovery that the brain is served by an elaborate system of meningeal lymphatic vessels opens up a possibility to address a totally new area of biomedical research.

With our discovery, the concept of how the brain drainage and immune systems should be targeted for disease benefit is likely to be dramatically changed, thus greatly influencing the development of novel therapeutic targets. Our results could spin off useful biomarkers and indicators for clinical pathology, patient stratification and personalized medicine. For example, in collaboration with Dr. Christer Betsholtz, we have contributed significantly in the characterization of cell types in the human meninges (Del Glaudio et al., Distinct fibroblast subtypes populate specific brain barriers, accepted for publication in Neuron). Our studies could also expose genetic and epigenetic risk factors that may alter normal brain function and predispose to neurologic disease through dysfunctional lymphatic circulation. - Our research group has previously invented and patented vascular technology that has so far led to a startup company and altogether three clinical trials, thus further possibilities in translational biomedical research should be explored based on our findings.

The ambitious objective of our plan was to elucidate the development and physiological function of meningeal lymphatic vasculature, to characterize its role in the pathogenesis of neurodegenerative and neuroinflammatory diseases, and to explore the new translational diagnostic and therapeutic potential of lymphangiogenic growth factors in these diseases. This research should have a global impact by incorporating new concepts into the state-of-the-art in human physiology and into neurodegenerative and neuroinflammatory pathogenesis research to bring it to a new level.

Barriers at the surface of the human brain

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