Convective influx/glymphatic system: tracers injected into the CSF enter and leave the brain along separate periarterial basement membrane pathways

Abstract

Tracers injected into CSF pass into the brain alongside arteries and out again. This has been recently termed the “glymphatic system” that proposes tracers enter the brain along periarterial “spaces” and leave the brain along the walls of veins. The object of the present study is to test the hypothesis that: (1) tracers from the CSF enter the cerebral cortex along pial-glial basement membranes as there are no perivascular “spaces” around cortical arteries, (2) tracers leave the brain along smooth muscle cell basement membranes that form the Intramural Peri-Arterial Drainage (IPAD) pathways for the elimination of interstitial fluid and solutes from the brain. 2 μL of 100 μM soluble, fluorescent fixable amyloid β (Aβ) were injected into the CSF of the cisterna magna of 6–10 and 24–30 month-old male mice and their brains were examined 5 and 30 min later. At 5 min, immunocytochemistry and confocal microscopy revealed Aβ on the outer aspects of cortical arteries colocalized with α-2 laminin in the pial-glial basement membranes. At 30 min, Aβ was colocalised with collagen IV in smooth muscle cell basement membranes in the walls of cortical arteries corresponding to the IPAD pathways. No evidence for drainage along the walls of veins was found. Measurements of the depth of penetration of tracer were taken from 11 regions of the brain. Maximum depths of penetration of tracer into the brain were achieved in the pons and caudoputamen. Conclusions drawn from the present study are that tracers injected into the CSF enter and leave the brain along separate periarterial basement membrane pathways. The exit route is along IPAD pathways in which Aβ accumulates in cerebral amyloid angiopathy (CAA) in Alzheimer’s disease. Results from this study suggest that CSF may be a suitable route for delivery of therapies for neurological diseases, including CAA.Electronic supplementary materialThe online version of this article (10.1007/s00401-018-1862-7) contains supplementary material, which is available to authorized users.

Emerging links between cerebrovascular and neurodegenerative diseases—a special role for pericytes

“…It has also been shown that the majority of Ab removal (85%) occurs via the BBB and only a minority via ISF bulk flow in mice [72,73], and similar findings are reported from humans [74]. An intriguing notion is that the perivascular fibroblasts reside between the two basement membranes-the astrocyte-derived pial basement membrane and the basement membrane embedding the arteriolar and arterial VSMC-implicated in inward (arachnoid-to-brain) and outward (brain-to-arachnoid) tracer transport, respectively, indicating a possible role for the perivascular fibroblasts in the separation and compartmentalization of these tightly apposed transport routes [70]. Do pericytes play a role in the glymphatic system?…”

“…In the glymphatic system, astrocytes are important for ion buffering and fluid exchange between the CSF and ISF, and it has been proposed that glymphatic clearance is higher in the sleep state [61]. Tracer flow does not connect to perivenous drainage, and studies of the routing of tracers show that tracers injected into the CSF enter along pial-glial membranes but exit the brain along VSMC basement membranes, going against the direction of blood flow [70]. The exchange through the glymphatic system has been suggested to be dependent on the water channel aquaporin-4 (Aqp4) present in the astrocytic endfeet ( Fig 3A).…”

Rapid lymphatic efflux limits cerebrospinal fluid flow to the brain

“…Thus, quantifications of tracer that are performed within the PVS or parenchyma of the brain ex vivo are likely overestimates of the levels that are present in vivo. This phenomenon may also account for some of the previous findings based on ex vivo assessments that have concluded that CSF influx was occurring into the brain within minutes of tracer administration [2, 8, 45, 52]. …”

“…After a small skin incision over the occipital bone/cervical spinal cord was made, the three covering muscle layers were carefully dissected under a stereomicroscope using fine forceps and scissors. A beveled glass capillary micropipette (Sutter instruments, Novato, CA, USA) with a diameter of < 60 μm was made using a Sutter P97 Pipette puller (Sutter instruments) and was positioned perpendicular to the ear bars and advanced to penetrate the dura until resistance was overcome, indicating entry into the cisterna magna as previously described [2]. Overall, 5 µL of a Gadospin D solution at 25 mM gadolinium (nanoPET Pharma GmbH) was infused at the speed of 1 µL/min with a NanoJet syringe pump (Chemyx).…”

Brain perivascular macrophages: Recent advances and implications in health and diseases

“…Therefore, PVMs may help control the velocity of IPAD through regulating VSMC constriction and relaxation. Of note, the velocity of IPAD is much slower in aged mice …”