Macroscopic distribution pathways of apoE in CNS

Deane, Rashid

Abstract

Apolipoprotein E (apoE) is essential for the transport and metabolism of lipids in the CNS. Of the three apoE isoforms (E2, E3 and E4), E4 is a major risk factor for Alzheimer's disease (AD). Under normal conditions, it is recognized that the highly specialized mature neurons outsource apoE production to astrocytes, but the mechanism for apoE delivery to neurons remains unclear. Recently, a brain-wide pathway that facilitates convective flow of interstitial fluid (ISF) was shown to play a major role in clearance of by-products of neural activity, including amyloid-beta. This perivascular system of cerebrospinal fluid (CSF) channels-termed the glymphatic system- allows solute exchange with parenchymal ISF by three main pathways: (1) influx of CSF via the peri-arterial space, (2) astrocytic aquaporin 4 (AQP4)-dependent convective flow of ISF through the brain parenchyma, and (3) efflux via para-venous clearance. We propose to test the idea that the glymphatic system also serves as a brain-wide distribution path for the delivery of essential substances to neurons. We choose to test this concept for apoE, since brain apoE is independent of peripheral apoE, CSF could be a source of apoE and because lipid dysregulation is a major risk factor for AD. Our pilot data show, (1) that lipidated apoE (lip-apoE) was circulated by the glymphatic system and entered brain ISF via peri-arterial influx;(2) unexpectedly, fluorescently-tagged lip-apoE was distributed in an isoform specific pattern of zones around penetrating arteries (E4 Aim 1 will characterize the macroscopic delivery and distribution of apoE isoforms.
Aim 2 will assess the effect of aging and AQP4 on the macroscopic distribution of apoE isoforms in conditional inducible Aqp4-/- and control mice.
Aim 3 will determine the role of apoE receptors in the macroscopic apoE delivery using specific receptor blocking antibodies. We hypothesize that apoE receptors trap apoE with different efficacy dependent on the apoE isoforms, resulting in the differential distribution profiles, and that reduced peri-arterial convective CSF influx in aging and Aqp4-/- animals will reduce apoE distribution. The implication of the proposed concept is that failure of brain-wide glymphatic distribution of apoE contributes to apoE isoform specific related disorders, including cerebral amyloid angiopathy (CAA) and AD. Our hope is that these studies will generate entirely novel targets for slowing or even preventing AD related neurocognitive decline by improving the glymphatic distribution of apoE, a target that has been ignored so far.

Public Health Relevance

Lipids, essential for normal neuronal function, are not synthesized by mature neurons, but instead this function is outsourced to astrocytes, important supporting cells in the central nervous system. We propose that the recently described glymphatic system, fundamental for the convective flow of brain interstitial fluid (ISF), also serves as a brain-wide distribution pathway for delivering of lipids, in the form of apoE, to neurons. We choose to test this concept for apoE, since it is brain derived, its CSF levels are greater than ISF exchangeable levels, dysregulation of brain lipid homeostasis is a major risk factor for Alzheimer's disease (AD), and because manipulating brain apoE levels and its lipidation has been proposed as therapeutic targets for the treatment of AD.