Glymphatic distribution of CSF-derived apoE into brain is isoform specific and suppressed during sleep deprivation

Abstract

BackgroundApolipoprotein E (apoE) is a major carrier of cholesterol and essential for synaptic plasticity. In brain, it’s expressed by many cells but highly expressed by the choroid plexus and the predominant apolipoprotein in cerebrospinal fluid (CSF). The role of apoE in the CSF is unclear. Recently, the glymphatic system was described as a clearance system whereby CSF and ISF (interstitial fluid) is exchanged via the peri-arterial space and convective flow of ISF clearance is mediated by aquaporin 4 (AQP4), a water channel. We reasoned that this system also serves to distribute essential molecules in CSF into brain. The aim was to establish whether apoE in CSF, secreted by the choroid plexus, is distributed into brain, and whether this distribution pattern was altered by sleep deprivation.MethodsWe used fluorescently labeled lipidated apoE isoforms, lenti-apoE3 delivered to the choroid plexus, immunohistochemistry to map apoE brain distribution, immunolabeled cells and proteins in brain, Western blot analysis and ELISA to determine apoE levels and radiolabeled molecules to quantify CSF inflow into brain and brain clearance in mice. Data were statistically analyzed using ANOVA or Student’s t- test.ResultsWe show that the glymphatic fluid transporting system contributes to the delivery of choroid plexus/CSF-derived human apoE to neurons. CSF-delivered human apoE entered brain via the perivascular space of penetrating arteries and flows radially around arteries, but not veins, in an isoform specific manner (apoE2 > apoE3 > apoE4). Flow of apoE around arteries was facilitated by AQP4, a characteristic feature of the glymphatic system. ApoE3, delivered by lentivirus to the choroid plexus and ependymal layer but not to the parenchymal cells, was present in the CSF, penetrating arteries and neurons. The inflow of CSF, which contains apoE, into brain and its clearance from the interstitium were severely suppressed by sleep deprivation compared to the sleep state.ConclusionsThus, choroid plexus/CSF provides an additional source of apoE and the glymphatic fluid transporting system delivers it to brain via the periarterial space. By implication, failure in this essential physiological role of the glymphatic fluid flow and ISF clearance may also contribute to apoE isoform-specific disorders in the long term.Electronic supplementary materialThe online version of this article (doi:10.1186/s13024-016-0138-8) contains supplementary material, which is available to authorized users.

Understanding the functions and relationships of the glymphatic system and meningeal lymphatics

“…The glymphatic pathway is important for the brain-wide delivery of nutrients, specifically glucose (17), the circulation and distribution of apolipoprotein E isoforms produced by the choroid plexus (18), and even astrocytic paracrine signaling with lipid molecules (19); however, its most fundamental role is the "lymphatic" function it serves in clearing extracellular metabolites and waste products from the parenchyma into the CSF. Reductions in brain efflux of radiolabeled mannitol and Aβ were observed in AQP4-deficient mice with slowed glymphatic flow (15).…”

Perivascular spaces, glymphatic dysfunction, and small vessel disease

“…This system has been previously reviewed in detail(11). Bulk flow entry of CSF into the periarterial spaces (glymphatic influx) has been shown to be important for the brain-wide delivery of glucose(12) in energy metabolism, transport of lipids and signaling molecules(13), and apolipoprotein E (apoE)-derived from the choroid plexus(14). While glymphatic efflux of ISF has shown to play a vital role in the clearance of several metabolic waste products and other solutes such as Aβ and tau(15).…”

The serine protease HtrA1 contributes to the formation of an extracellular 25-kDa apolipoprotein E fragment that stimulates neuritogenesis

“…These APOE genotypes produce the three major apoE isoforms, apoE2, apoE3 and apoE4, which differ in their Cys/Arg composition at positions 112 (3). ApoE is present in the central nervous system (CNS) as both spherical and discoidal lipoprotein complexes (4) and is strongly expressed in the choroid plexus (5,6). Astrocytes are thought to be the primary source of apoE in the brain, although microglia and neurons also contribute to the apoE CNS pool under certain circumstances (4,(7)(8)(9).…”