The Glymphatic System and Brain Interstitial Fluid Homeostasis

Introduction

The maintenance of the interstitial compartment is a basic element of an organ’s function that is of utmost importance in the brain, given neural cells’ exquisite sensitivity to changes in their extracellular environment. In peripheral tissues, interstitial fluid (ISF) is formed from the filtration of plasma across the permeable capillary endothelium. Although a portion of the ISF is reabsorbed into postcapillary venules, much of the ISF and virtually all extracellular proteins are collected into primary lymphatic vessels, which return lymph fluid and proteins to the blood circulation. In the central nervous system (CNS), the presence of the blood–brain barrier (BBB), which restricts the filtration of water and proteins from the plasma, and the apparent absence of lymphatic vessels from brain tissues require that the “lymphatic” function of interstitial homeostasis be subserved by an alternative mechanism. In the CNS, the cerebrospinal fluid (CSF) circulation supports this function, serving as a sink and a crossroad for extracellular proteins and metabolites that cannot readily cross the BBB .

The exchange of ISF and CSF is a physiological process in the CNS that likely affects many aspects of brain function, including waste clearance, lipid metabolism, growth factor and neurohormone distribution, and immune surveillance. Dysfunction of these processes appears to contribute to the edema formation after cerebral ischemia and traumatic brain injury (TBI), to the accumulation of aggregated proteins in neurodegenerative conditions such as Alzheimer disease (AD), and to neuroinflammatory diseases such as multiple sclerosis .

Interstitial Solute Clearance

Under physiological conditions, the brain extracellular space comprises approximately 14–23% of the overall brain volume. ISF is formed from a combination of water crossing the BBB, water produced through cellular metabolism, and fluid from the CSF compartments. The relative contributions of these processes to ISF production is not clearly known, and may vary by brain region and physiological state. Interstitial solutes move through brain tissue both by the process of diffusion and bulk flow, with the relative contribution of each process being determined by the physical and chemical properties of the solute and the properties of the extracellular environment .

Diffusion and the Blood–Brain Barrier

Many interstitial solutes, including nonpolar molecules and those with specific BBB efflux transporters are readily cleared to the blood stream across the BBB. The process of diffusion, which is the movement of molecules down their concentration gradients driven by thermal motion, is very efficient on the microscopic scales of the brain microcirculation. Hence, the clearance of nonpolar molecules such as CO ₂ and BBB efflux transporter substrates such as the drug verapamil (a substrate for P-glycoprotein) is diffusion limited . Diffusion-limited clearance kinetics will be strongly influenced by molecular weight and the dimensions and nature of the extracellular space, with larger solutes moving more slowly than smaller, and with more rapid diffusion as the extracellular space becomes larger and less tortuous . Beyond clearance of nonpolar molecules and those with specific transporters at the BBB, diffusion also dominates the exchange of ISF and CSF in tissue closest to internal and external CSF compartments, including periventricular and subpial brain tissue ( Fig. 3.1 ).

Perivascular Bulk Flow and the Glymphatic System

Perivascular spaces surrounding cerebral blood vessels have long been known to facilitate the exchange of CSF and ISF. Although this process was generally held to occur slowly along the vasculature, a series of studies carried out in dogs and cats in the mid- to late 1980s suggested that the interaction of CSF and ISF along perivascular spaces was both rapid and polarized along the arterial and venous sides of the circulation, with CSF entering the brain along perivascular spaces surrounding penetrating cerebral arteries and ISF being cleared from the brain along perivascular spaces surrounding cerebral veins . More recent studies have substantiated these findings employing dynamic imaging approaches in living animals rather than analysis of fixed or frozen tissues. In mice, in vivo two-photon microscopy demonstrated that fluorescent tracers injected into the subarachnoid CSF at the cisterna magna moved rapidly into and through the brain parenchyma along perivascular spaces surrounding cerebral arteries ( Fig. 3.2 ). Fluorescent tracers injected into the brain interstitium were in turn cleared along specific anatomical pathways including perivascular spaces surrounding large-caliber draining veins that drain to extraparenchymal venous sinuses, such as the internal cerebral veins that form the origin of the straight sinus. These findings have been confirmed both in rats and with brain-wide CSF tracer imaging using dynamic contrast-enhanced MRI. As detailed later, because perivascular bulk flow along this pathway depends on glial water transport, and because it assumes the lymphatic function of interstitial solute clearance, this brain-wide perivascular network has been termed the “glymphatic” system .