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The Emerging Field of Perivascular Flow Dynamics: Biological Relevance and Clinical Applications

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The Emerging Field of Perivascular Flow Dynamics: Biological Relevance and Clinical Applications Technology and Innovation, Vol. 18, pp. 63-74, 2016 ISSN 1949-8241 • E-ISSN 1949-825X Printed in the USA. All rights reserved. http://dx.doi.org/10.21300/18.1.2016.63 Copyright © 2016 National Academy of Inventors. www.technologyandinnovation.org THE EMERGING FIELD OF PERIVASCULAR FLOW DYNAMICS: BIOLOGICAL RELEVANCE AND CLINICAL APPLICATIONS Jacob Huffman1, Sarah Phillips1, George T. Taylor,1,3, and Robert Paul1,2 1Behavioral Neuroscience Program, Department of Psychological Sciences, University of Missouri – St. Louis, MO, USA 2Missouri Institute of Mental Health (MIMH), St. Louis, MO, USA 3Interfakultäre Biomedizinische Forschungseinrichtung (IBF) der Universität Heidelberg, Heidelberg, Germany Brain-wide pathways of perivascular flow help clear the brain of proteins and metabolic waste linked to the onset and progression of neurodegenerative diseases. Recent studies on the glymphatic system and novel lymphatic vessels of the meninges have prompted new insight into the clinical significance of perivascular flow. Current techniques in both humans and animals are unable to fully reveal the complex functional and anatomical features of these clearance pathways. While much research has stemmed from fluorescence microscopy and MR imaging, clinical and experimental investigations are hindered by the lack of more advanced and precise technology. In this review, we discuss the biological relevance of the glymphatic and perivascular clearance systems, the innovative technology that has defined these pathways, and the potential for new studies to advance our understanding of degenerative brain diseases using similar technology. Key words: Perivascular space; Glymphatic; Imaging; Fluorescence; Contrast-enhance; MRI INTRODUCTION differences among neurodegenerative disorders, Maintaining homeostatic function of the central imaging modalities have been employed in both nervous system (CNS) relies not only on the proper animal and human research to uncover the etiolog- regulation and composition of cerebrospinal fluid ical mechanisms associated with neurodegenerative (CSF) but its distribution and drainage from the brain. diseases. The dynamic flow of CSF and interstitial fluid (ISF) along brain-wide networks of fluid motion permit CSF AND PERIVASCULAR FLOW the transport of molecules and substrates throughout Clinical evaluations of CSF production, flow, and multiple brain regions (68). Dysfunction of these reabsorption in humans rely primarily on phase delivery and clearance systems may render the brain contrast magnetic resonance imaging (PC-MRI) vulnerable to the accumulation of metabolic waste (7,27,40,69). However, CSF moves with relatively and aggregation of proteins, such as amyloid β (Aβ), low velocity and is confined to small anatomical areas; possibly leading to the onset and/or progression therefore, a number of limitations arise with image of diseases such as Alzheimer’s Disease (AD) and quality and accuracy. As a result, the vast majority Cerebral Amyloid Angiopathy (CAA) (36,53,72). of studies focused on mechanisms of CSF physiol- Due to the inherent physiological and behavioral ogy rely on animal models. A brief review of these _____________________ Accepted December 10, 2015. Address correspondence to Jacob Huffman, Behavioral Neuroscience/Psychology, University of Missouri – Saint Louis, One University Boulevard, St. Louis, MO 63121, USA, Tel: +1 (314) 486-3407; Fax: (314) 516-5392, E-mail: [email protected] 63 64 HUFFMAN ET. AL mechanisms is provided below, followed by dis- Earlier studies initially identified this perivascular cussion of advanced technological approaches to space where tracers move along periarterial spaces enhance these models. (33,63,64,71,83). More recently, Iliff and colleagues CSF is primarily produced in choroid plexuses of used in vivo two-photon microscopy to show that the lateral, third, and fourth ventricles and is modestly ISF moves by bulk flow toward perivenous path- regulated by the blood-CSF barrier (62). Functionally ways (36). This movement is made possible by the similar to the blood brain barrier, the blood-CSF influx of CSF and subsequent CSF-ISF exchange in barrier controls the production of CSF and its ionic the interstitium (Figure 1). The authors refer to this and molecular composition while also serving as a as the ‘glymphatic’ pathway due to the involvement pathway for solute and waste removal (18,39). CSF of glial processes in fluid transport and its similarity travels through the ventricular system from the lateral to peripheral lymphatic function (36). ventricles to the third and fourth ventricles, passing There is also evidence of a different mechanism through the foramen of Magendie and Luschka where for ISF clearance in which bulk flow transports sol- it mixes with existing fluid in the subarachnoid space utes from the interstitium into periarterial pathways, (SAS) (54). The movement of CSF is then derived where they are subsequently transported to cervical from multiple mechanisms, including hydrostatic lymphatics along arteries in the opposite direction pressure gradients between CSF compartments and of blood flow (1,4,10,11) (Figure 1). The concept sites of reabsorption (52), the pulsatility of the cardiac of periarterial ISF efflux from the parenchyma and cycle and vascular smooth muscle (2,23,37,43,74), the CSF influx via glymphatic flow can be viewed as two respiratory cycle (13,16,43), and body posture (47). mechanisms acting in opposition to one another. The gap between the arachnoid membrane and pia Described in more detail below, it is important to note mater gives way to the SAS where both arteries and that the precise anatomical differences between these veins along the pial surface of the brain are bathed pathways are not yet fully understood. For the pur- in CSF. Surface arteries penetrate the pia and project pose of this review, ‘perivascular space’ will broadly into the parenchyma, carrying the pial membrane denote a single space between vascular endothelium for a short distance. Astrocyte endfeet wrap around and astrocyte endfeet that permits the movement of these penetrating vessels, presumably covering the CSF to, and ISF from, the parenchyma. ‘Glymphatic pial membrane, to form a canal or perivascular space flow’ will represent the movement of subarachnoid (also known as the Virchow-Robin space). There is a CSF along periarterial pathways, into the intersti- distinct gap between the basement membrane of the tium to exchange with ISF, and then drained along vascular smooth muscle and the astrocyte endfeet perivenous pathways. (38). Studies utilizing fluorescent tracers injected into the cisterna magna have identified CSF traveling from CLEARING THE BRAIN INTERSTITIUM the SAS into deeper periarterial pathways toward brain capillary beds (36,37,45,79). As reviewed by CSF Clearance Jessen et al. (38), these periarterial spaces become Subarachnoid CSF can be transported to periph- tighter as smooth muscle dissipates and eventually eral blood and lymphatics along four main routes of joins with the basal lamina surrounding the capil- reabsorption: 1) arachnoid villi present in the dural lary endothelium. From there, CSF can either move sinuses, 2) drainage pathways at the cranial nerves, across the astrocyte endfeet and into the interstitial 3) nerve sheaths along spinal roots, and 4) the more space or continue into perivenous pathways of drain- recently discovered lymphatic vessels in the meninges ing venules and veins. Because the bulk flow of CSF (5,50,57). These drainage pathways are necessary through the perivascular space is driven in part by for the removal of large solutes and metabolic waste the pulsatile activity of the vascular smooth muscle dumped into ventricular and subarachnoid CSF. (23,37,74), the low resistance basal lamina facilitates Clearance of the interstitial space relies in part on the exchange of CSF and interstitial fluid (ISF) at either the bulk flow of ISF from the parenchyma to deeper capillary beds (38). CSF compartments or the exchange of CSF and ISF IMAGING THE PERIVASCULAR SPACE 65 of the glymphatic flow. Nevertheless, both systems flow, which has been implicated in the removal of require proficient passage along perivascular spaces. Aβ (4,10,11,28,29,53), a protein well-known for its role in AD (72). ISF bulk flow pathways are thought Perivascular and Glymphatic Clearance to originate in the walls of cerebral vessels, running As mentioned above, previous studies derived along the basement membrane of capillaries and mostly from a single group of researchers have arteries that drive ISF toward CSF compartments. shown that ISF can move along the perivascular While the removal of ISF along periarterial path- spaces of arteries in the opposite direction of blood ways provides one mechanism for solute and waste Figure 1. Perivascular and Glymphatic Flow Perivascular clearance comprises perivascular drainage and glymphatic pathways. The perivascular drainage pathway (white arrows) moves waste into the periarterial space (located along smooth muscle cells and the capillary basement membrane) and towards the subarachnoid space in the direction opposite to blood flow. The glymphatic pathway (black arrows) clears waste from the ISF through the brain paren- chyma, and comprises three functional components. (1) CSF influx, unidirectionally with blood flow, into the periarterial space (between the basement membrane of smooth muscle cells and pia mater), where the water component of CSF crosses astrocytic AQP4 channels to enter the
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