This is a summary of: Yoon, J.-H. et al. Nasopharyngeal lymphatic plexus is a hub for cerebrospinal fluid drainage. Nature https://doi.org/10.1038/s41586-023-06899-4 (2024).
The mission
Cerebrospinal fluid (CSF) bathes the central nervous system and is essential for brain health. The regulation of CSF production, circulation and drainage has received tremendous attention because of evidence that reduced CSF secretion and clearance during ageing might contribute to Alzheimer’s disease and other neurodegenerative conditions1. CSF is drained from the subarachnoid space, which exists between the innermost and middle (the arachnoid) layers of the meninges, the three membranes that line the brain and spinal cord. Among the known drainage routes are lymphatics in the cribriform plate — the bone that forms the roof of the nasal cavity — and the connective tissue that surrounds cranial nerves2,3. Lymphatic vessels drain extracellular fluid, along with immune cells, from tissues throughout the body. Lymphatics in the dura mater, the outermost layer of the meninges, also serve as a CSF-drainage route3,4. However, despite solid documentation of the contribution of lymphatics to CSF clearance3, the anatomical connections between the subarachnoid space and extracranial lymphatics have been challenging to elucidate.
The discovery
To identify these connections, we used fluorescence microscopy in anaesthetized transgenic mice that produce a protein marker of lymphatic vessels5. The marker can be tagged with a fluorescent reporter, facilitating the identification of these structures. A fluorescent tracer was infused into the subarachnoid space to track CSF outflow, and the nasopharyngeal lymphatics in the upper throat (behind the nose) and the cervical lymphatics in the neck were surgically exposed for imaging. In addition, we used immunofluorescence staining of the nasopharyngeal lymphatic plexus (network) and deep cervical lymphatics.
Our study identified the nasopharyngeal lymphatic plexus as a hub for CSF outflow (Fig. 1a). These lymphatics have abnormally shaped valves, structures that maintain one-way flow inside the vessels. They also have short lymphangions (a structural unit that lies between two valves) and no smooth-muscle coverage. CSF from the anterior and middle cranial regions of the subarachnoid space drained through the nasopharyngeal lymphatic plexus to deep cervical lymph nodes through medial cervical lymphatics (Fig. 1b). Unlike lymphatics in the plexus, medial cervical lymphatics have half-moon-shaped valves and smooth-muscle coverage, typical of lymphangions that pump lymphatic fluid toward the lymph nodes (Fig. 1b). CSF outflow through the plexus and medial cervical lymphatic route was, on average, 180% greater than through lateral dural lymphatics.
Figure 1 | CSF outflow through the nasopharyngeal lymphatic plexus and morphological features of medial cervical lymphatics. a, Fluorescence image showing the distribution of the CSF tracker in the nasopharyngeal lymphatic plexus (orange arrowheads) in a transgenic mouse. Green arrowheads indicate lymphatic valves. Scale bar, 250 micrometres. b, Immunofluorescence image showing the distribution of semi-lunar-shaped lymphatic valves (green arrowheads) and circular smooth-muscle cells (orange arrowheads) in medial and lateral deep cervical lymphatic vessels (dcLVs). dcLN, deep cervical lymph node. Scale bar, 1 millimetre. Yoon, J.-H. et al./Nature (CC BY 4.0).
To test whether CSF outflow could be regulated by pharmacological modulation of the contraction of medial cervical lymphatics, phenylephrine (which activates α1-adrenergic receptors, causing smooth-muscle contraction) or sodium nitroprusside (which releases nitric oxide, inducing muscle relaxation and vessel dilation), were applied locally to exposed medial cervical lymphatics of transgenic mice. CSF outflow through the medial cervical lymphatics was indeed modulated by α-adrenergic activation and nitric oxide signalling in lymphatic smooth muscle. Importantly, this feature was preserved during ageing, even when the nasopharyngeal lymphatic plexus had shrunk and was functionally impaired.
Future directions
The study implies that increasing CSF outflow during ageing or under pathological conditions (including Alzheimer’s disease, stroke and traumatic head injury) could be achieved by pharmacological activation of the medial cervical lymphatics. Of note, deep anaesthesia and removal of neck musculature were required to expose the nasopharynx and medial cervical lymphatics in the mice. Both procedures might alter the physiological dynamics of CSF drainage, because cerebral blood flow and blood pulsing through the vasculature contribute to CSF circulation, which in turn influences CSF outflow. Although the imaging techniques used were informative, advanced methods for imaging of live animals (such as synchrotron X-ray imaging) might reveal more features of the dynamics of CSF drainage under physiological conditions.
We plan to verify all the findings from the mice in primates, including monkeys and humans. We aim to investigate in a reliable animal model whether activating the cervical lymphatic vessels through pharmacological or mechanical means can prevent the exacerbation of Alzheimer’s disease progression by improving CSF clearance. — Hokyung Jin and Jin-Hui Yoon are at the Center for Vascular Research, Institute for Basic Science, Daejeon, South Korea.
Behind the paper
Accumulating evidence suggests that CSF can be drained through nasopharyngeal lymphatics. However, the connections and functional properties of these vessels in relation to CSF outflow, both under normal conditions and during ageing, have proven difficult to characterize. The main reason is the complicated structure of the lymphatics in the skull base and neck regions. However, we solved this issue using transgenic mice that produce a lymphatic reporter and our experience with microsurgical techniques.
We first identified the medial deep cervical lymphatics, which connect the nasopharyngeal lymphatics to the deep cervical lymph nodes, located in the neck. The lymphatics are a potential target for facilitating CSF outflow in people in whom it is compromised. Waste clearance from the brain could be enhanced in people with neurodegenerative conditions by stimulating the deep cervical lymphatics, which are located in a readily accessible region around the neck. —H.J. and J.-H.Y.
