Mapping Solute Clearance From the Mouse Hippocampus Using a 3D Imaging Cryomicrotome.

Daphne M P Naessens,Judith De Vos,Ed Vanbavel,Johannes G G Dobbe,Erik N T P Bakker

doi:10.3389/fnins.2021.631325

Abstract

The hippocampus is susceptible to protein aggregation in neurodegenerative diseases such as Alzheimer’s disease. This protein accumulation is partially attributed to an impaired clearance; however, the removal pathways for fluids and waste products are not fully understood. The aim of this study was therefore to map the clearance pathways from the mouse brain. A mixture of two fluorescently labeled tracers with different molecular weights was infused into the hippocampus. A small subset of mice (n = 3) was sacrificed directly after an infusion period of 10 min to determine dispersion of the tracer due to the infusion, while another group was sacrificed after spreading of the tracers for an additional 80 min (n = 7). Upon sacrifice, mice were frozen and sectioned as a whole by the use of a custom-built automated imaging cryomicrotome. Detailed 3D reconstructions were created to map the tracer spreading. We observed that tracers distributed over the hippocampus and entered adjacent brain structures, such as the cortex and cerebroventricular system. An important clearance pathway was found along the ventral part of the hippocampus and its bordering interpeduncular cistern. From there, tracers left the brain via the subarachnoid spaces in the directions of both the nose and the spinal cord. Although both tracers followed the same route, the small tracer distributed further, implying a major role for diffusion in addition to convection. Taken together, these results reveal an important clearance pathway of solutes from the hippocampus.

Highlights

Several neurodegenerative diseases are characterized by the accumulation and aggregation of proteins
Solutes and cerebrospinal fluid (CSF) can exit the skull through arachnoid villi or along perineural routes through the foramina in the skull that drain into the extracranial lymphatic vessels, as shown in mice (Ma et al, 2017)
The two tracers distributed over the hippocampus, entered adjacent brain structures, and left the brain both in the direction of the nose and the spinal cord

Summary

Introduction

Several neurodegenerative diseases are characterized by the accumulation and aggregation of proteins These include aggregates of amyloid-β in Alzheimer’s disease, α-synuclein in Parkinson’s disease, and huntingtin in Huntington’s disease (Ross and Poirier, 2004). A study by Iliff and colleagues challenged this view and proposed an additional clearance route via the so-called glymphatic system In this pathway, subarachnoid CSF enters the brain parenchyma via perivascular spaces along penetrating arteries, where it mixes with the ISF. The fluid disperses toward the perivenous spaces and leaves the brain interstitium into the subarachnoid space, a process that depends on glial cells (Iliff et al, 2012) Once in this compartment, solutes and CSF can exit the skull through arachnoid villi or along perineural routes through the foramina in the skull that drain into the extracranial lymphatic vessels, as shown in mice (Ma et al, 2017)

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