A lymphatic waste-disposal system implicated in Alzheimer’s disease

Abstract

The discovery that a set of lymphatic vessels interacts with blood vessels to remove toxic waste products from the brain has implications for cognition, aging and disorders such as Alzheimer’s disease.

A network of lymphatic vessels acts in tandem with the blood vasculature to regulate fluid balance in the body¹. The brain does not have its own lymphatic network, but the cellular membranes around the brain, known as the meninges, do have a network of lymphatic vessels. This meningeal lymphatic system was first found² in 1787 and has been ‘rediscovered’ this decade^3–5. Do the meningeal lymphatics have a role in brain diseases, as systemic lymphatic vessels do in systemic diseases such as cancer¹? In this issue of Nature, Da Masquita et al.⁶ show that meningeal lymphatic vessels help to maintain both the proper levels of proteins in brain fluids (a process called proteostasis) and cognitive function. The finding has implications for normal aging and disorders such as Alzheimer’s disease.

In the body, lymphatic vessels drain tissues of interstitial fluid (ISF), which contains waste products such as cellular debris and toxic molecules. The ISF forms a protein-rich fluid called lymph that circulates through the lymphatic system back to the circulating blood¹. On its way, lymph is filtered through the lymph nodes, which can initiate immune responses if foreign particles are detected.

The brain does not have its own lymphatic vessels. As such, proteins and waste from the main body of the brain (the parenchyma) are transported within the ISF along the walls of blood vessels to reach the cerebrospinal fluid (CSF), which circulates through the ventricles and brain meninges⁷. It is well established that proteins, metabolic waste products and other molecules in these fluids can be removed from the brain by being transported across the walls of blood vessels (primarily capillaries), thus crossing the blood–brain barrier^7,9 — a process called transvascular removal. But whether the meningeal lymphatic vessels are also involved in waste clearance has been unknown.

Da Masquita et al. destroyed the meningeal lymphatic vessels of mice by injecting a vessel-damaging drug into the cisterna magna — a large CSF-filled space in the meninges. They then administered a fluorescent tracer molecule into the cisterna magna. In mice lacking meningeal lymphatic vessels, the tracer did not reach the deep cervical lymph nodes, to which the meningeal lymphatics drain. Previous work has shown⁸ that injecting high concentrations of tracer into CSF can cause diffusion of tracer into the brain along blood vessels — but this transport was also reduced. Additionally, tracer injection into the brain parenchyma also showed reduced ISF drainage into deep cervical lymph nodes. The authors confirmed these results using several alternative approaches: using different tracers; surgically closing off drainage to the deep cervical lymph nodes; and examining mice genetically engineered such that lymphatic-vessel development was impaired.

Destruction of the meningeal lymphatics also led to deficits in spatial orientation and fear memory. The brain’s hippocampus has a key role in these behaviours, and the researchers found changes in gene expression in this region resembling those seen in neurodegenerative disorders. Collectively, these experiments suggest that drainage of brain ISF and CSF by the meningeal lymphatics is necessary for proper cognitive functions.

These findings also raise an interesting question: where did the injected tracers go? One study¹⁰ has shown that tracers injected into the cisterna magna are primarily transported into the blood, and only secondarily into the lymphatic system. Simultaneous measurements of tracer movements into the meningeal lymphatics, other lymphatic vessels (for instance in the neck) and the blood could reveal whether impairment of the meningeal lymphatics leads to a shift in the pathways used to control brain proteostasis, increasing transvascular removal of waste products across the blood–brain barrier (Fig. 1), or their drainage into the venous system within the meninges across the arachnoid granulations⁷.

Figure 1 |. Regulation of waste clearance in the brain.

a, The brain does not have its own lymphatic vessels. Proteins and waste are transported from brain interstitial fluid (ISF) along the walls of blood vessels to reach the cerebrospinal fluid (CSF) in the subarachnoid space within brain meninges. Tracers injected into the CSF can also diffuse back into the brain in the opposite direction (indicated by arrow). Da Masquita et al.⁶ report that lymphatic vessels, located in the brain meninges, drain CSF and ISF containing waste products. b, In a healthy mouse brain, lymphatic drainage of both fluids requires signalling between VEGF-C protein and its receptor VEGFR3, which is expressed in lymphatic endothelium. The protein amyloid-β (Aβ), which is associated with Alzheimer’s disease, is primarily removed by blood vessels. But during aging, both vessel systems can become dysfunctional. The diameter of the meningeal lymphatic vessels decreases, causing decreased waste clearance by this route. Decreased lymphatic drainage, along with impaired clearance by blood vessels, leads to amyloid-β accumulation in the brain.

Da Masquita et al. next showed an aging-induced decrease in the diameter and coverage of meningeal lymphatic vessels, and decreased drainage of tracers from the ISF and CSF into deep cervical lymph nodes. Lymphatic-vessel growth in mice is promoted by a signalling pathway involving vascular endothelial growth factor C (VEGF-C) and its receptor VEGFR3, whereas impairments in the pathway leads to a loss of meningeal lymphatic vessels^1,3. Furthermore, treatment with VEGF-C protein increases the diameter of meningeal lymphatic vessels, improving lymphatic drainage⁴. Consistent with these findings, the authors showed that local delivery of the Vegf-c gene into the cisterna magna of old mice using a virus restored both the drainage of CSF tracer into deep cervical lymph nodes and the rate of its influx into the brain parenchyma. These changes were accompanied by restoration of spatial orientation in old mice.

Age-related impairments in transvascular clearance of waste have been implicated in the accumulation of amyloid-β protein in the brain^7,11,12 — a hallmark of Alzheimer’s disease. Da Masquita and colleagues investigated the effects of ablating the meningeal lymphatics in two mouse models of Alzheimer’s disease producing amyloid-β protein in neurons. Ablation led to amyloid-β accumulation in the meninges, accelerated amyloid-β deposition in the brain parenchyma and cognitive deficits. The authors also showed that amyloid-β accumulated in the meninges of people who had Alzheimer’s disease, pointing to the potential relevance of these findings for humans.

Notably, the researchers found that the mouse models did not develop any apparent structural or functional changes in the meningeal lymphatics at the time when amyloid-β deposition in the brain parenchyma first became apparent. Viral delivery of Vegf-c at this timepoint could not prevent the cognitive impairments in either model, suggesting that the early amyloid-β deposition and cognitive impairments in these animals were caused by disruption in another clearance pathway — most likely transvascular clearance. As transvascular-clearance routes gradually deteriorate with age, an increasing burden is probably put on the meningeal lymphatic system. If the capacity of the lymphatic system is reached, this might lead to faulty lymphatic drainage of amyloid-β and other proteins from ISF and CSF (Fig. 1). Thus, a dynamic relationship between the meningeal lymphatics and blood vessels seems to regulate proteostasis in the brain.

Future work should aim to improve our understanding of waste-clearance pathways from the brain, how the ISF and CSF drain into the meningeal lymphatics, and how these lymphatic vessels interact with the blood vessels at the blood–brain barrier. Such analyses will open up new directions for research into cognition, neurodegeneration and Alzheimer’s disease. Da Masquita et al. showed that strategies that promote local growth of lymphatic vessels have the potential to improve clearance by meningeal lymphatics to rebuild brain proteostasis, and might lessen amyloid-β deposition. Whether treatments directed at the meningeal lymphatics can also improve the impaired function of blood vessels with age, and whether enhancing clearance at the blood–brain barrier can improve lymphatic drainage function, remains to be addressed.