Twists and turns of ocular glymphatic clearance – new study reveals surprising findings in glaucoma

Editor,

The central nervous system (CNS) is largely devoid of functional lymphatic vessels, apart from within the meningeal lining (Louveau et al. 2015). The brain can eliminate cellular waste and excess interstitial fluid from deep in the parenchyma using paravascular spaces facilitated by the water channel aquaporin-4 (AQP4) (Iliff et al. 2012). The hallmarks of the brain glymphatic system are inflow of fluid and solutes along arteries driven by hydrostatic pressure, convective interstitial fluid movement facilitated by AQP4 on astroglial endfeet, and perivenous drainage, especially at the skull base. Despite the retina having higher metabolic and fluidic demands per gram than any other tissue in the body, all known pathways for fluid clearance exist in the anterior globe. Several groups have hypothesised the existence of an analogous glymphatic system in the eye (Denniston et al. 2015; Wostyn et al. 2016). The presence of a glymphatic pathway in the optic nerve and the theory that a dysfunctional glymphatic system may be involved in the pathogenesis of glaucoma were first proposed by Wostyn et al. (Wostyn et al. 2015). In 2017, Mathieu et al. demonstrated the movement of small cerebrospinal fluid (CSF) tracers from the brain into the optic nerve, consistent with glymphatic transport (Mathieu et al. 2017).

Our group recently discovered an “antegrade” ocular glymphatic clearance system from the retina and optic nerve to the CSF and meningeal lymphatics in rodents (Wang et al. 2020). Following tracer infusion into both CSF and the vitreous, whole mouse tissue clearing showed evidence of bidirectional glymphatic transport (Figure 1). Tracer movement was facilitated by glial AQP4 in both the retina and optic nerve. Small tracer molecules like amyloid-beta and radio-labelled potassium entered retinal ganglion cell (RGC) axons and the perivenous spaces of the retina and optic nerve head (ONH) before being cleared by the antegrade glymphatic pathway. Larger dextrans were blocked from posterior outflow both by the intact glial lamina in mice and the more developed lamina cribrosa in rats. Directional water and small solute movement inside axons is distinct from and sometimes has a different direction from the ATP-driven axonal transport along microtubules (Beaulieu 2002). We found that RGC axons appeared to use the hydrostatic pressure gradient to promote fluid and solute delivery across the ONH, where the axons take a sharp turn before exiting the eye. Raised intracranial pressure (ICP) abrogated the intra-axonal tracer movement along the optic nerve, whilst lowering ICP or stimulating pupil movement increased it. The lamina cribrosa appears to have not only an anatomical function of supporting axon bundles, but also a vital physiological role as a hydrostatic barrier redirecting fluid and solute movement into axons and the perivenous spaces at the ONH and retrolaminar nerve (Wang et al. 2020). One of our most surprising findings was that two distinct animal models of glaucoma were both characterised by excessive and misdirected glymphatic clearance, and not by impaired glymphatic outflow as previously suggested (Mathieu et al. 2018). In both models, the excessive outflow actually was also observed after normalisation of intraocular pressure, thus being a result of barrier failure rather than an increased pressure gradient in the long term.

Figure 1.

Macroscopic overview of the brain and ocular glymphatic systems, emphasising the role played by pressure gradients, hydrostatic barriers and lymphatic drainage, shown in the context of known pathways for aqueous humour and cerebrospinal fluid (CSF) efflux. ICP, intracranial pressure; IOP, intraocular pressure.

We found enhanced glymphatic clearance by stimulation of the pupillary light response. This enhancement was blocked by atropine, suggesting causality linked to the movement of the pupil. A proportion of suprachoroidal tracer also exited via the glymphatic pathway. These results hint at an interplay between uveoscleral and glymphatic outflow, and a potential functional coupling with accommodation. The amount of fluid that exits the eye via this posterior clearance route in normal and glaucomatous eyes is unknown. Glymphatic outflow is likely to be orders of magnitude lower than clearance via anteriorly located routes. Large fluid movements across the ONH cannot be accommodated due to space constraints, as is seen in papilloedema, and may promote wash out of important metabolites within axons. In fact, excessive extracellular tracer movement across the ONH in glaucomatous mice masked an impairment of intra-axonal tracer clearance. The lamina no longer re-directed fluid into axons, but allowed passage via large defects in this barrier. We speculate that slowing of intra-axonal tracer clearance in our glaucoma models reflects build-up of metabolic waste like amyloid beta within RGC, leading to dysfunction and cell death.

As the eye is considered an extension of the CNS, glaucoma is sometimes described as ocular dementia. A key finding in our study is that glaucomatous damage to the lamina barrier impairs the clearance of small and potentially neurotoxic solutes such as amyloid-beta via a novel glymphatic pathway. Several important questions remain. What other small solutes and ions are dependent on the ocular glymphatic clearance pathway for efficient transport? Can we develop diagnostic tracers to study this pathway in a clinical setting and modulate transport to delay or prevent optic nerve diseases like glaucoma? Further characterisation of the ocular glymphatic system could be vital in the development of new diagnostics and treatments for several potentially blinding conditions.