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Joshi K, Diaz A, O'Keeffe K, Schaffer JD, Chiarot PR, Huang P. Flow in temporally and spatially varying porous media: a model for transport of interstitial fluid in the brain. J Math Biol 2024; 88:69. [PMID: 38664246 DOI: 10.1007/s00285-024-02092-x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/11/2023] [Revised: 02/02/2024] [Accepted: 04/02/2024] [Indexed: 05/12/2024]
Abstract
Flow in a porous medium can be driven by the deformations of the boundaries of the porous domain. Such boundary deformations locally change the volume fraction accessible by the fluid, creating non-uniform porosity and permeability throughout the medium. In this work, we construct a deformation-driven porous medium transport model with spatially and temporally varying porosity and permeability that are dependent on the boundary deformations imposed on the medium. We use this model to study the transport of interstitial fluid along the basement membranes in the arterial walls of the brain. The basement membrane is modeled as a deforming annular porous channel with the compressible pore space filled with an incompressible, Newtonian fluid. The role of a forward propagating peristaltic heart pulse wave and a reverse smooth muscle contraction wave on the flow within the basement membranes is investigated. Our results identify combinations of wave amplitudes that can induce either forward or reverse transport along these transport pathways in the brain. The magnitude and direction of fluid transport predicted by our model can help in understanding the clearance of fluids and solutes along the Intramural Periarterial Drainage route and the pathology of cerebral amyloid angiopathy.
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Affiliation(s)
- Ketaki Joshi
- Department of Mechanical Engineering, Watson College of Engineering and Applied Science, State University of New York at Binghamton, Binghamton, NY, 13902, USA
| | - Adrian Diaz
- Department of Mechanical Engineering, Watson College of Engineering and Applied Science, State University of New York at Binghamton, Binghamton, NY, 13902, USA
| | - Katherine O'Keeffe
- Department of Mechanical Engineering, Watson College of Engineering and Applied Science, State University of New York at Binghamton, Binghamton, NY, 13902, USA
| | - J David Schaffer
- Institute for Justice and Well-Being, State University of New York at Binghamton, Binghamton, NY, 13902, USA
| | - Paul R Chiarot
- Department of Mechanical Engineering, Watson College of Engineering and Applied Science, State University of New York at Binghamton, Binghamton, NY, 13902, USA
| | - Peter Huang
- Department of Mechanical Engineering, Watson College of Engineering and Applied Science, State University of New York at Binghamton, Binghamton, NY, 13902, USA.
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Sharp MK. Pulsatile cerebral paraarterial flow by peristalsis, pressure and directional resistance. Fluids Barriers CNS 2023; 20:41. [PMID: 37291600 DOI: 10.1186/s12987-023-00445-0] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/17/2022] [Accepted: 05/21/2023] [Indexed: 06/10/2023] Open
Abstract
BACKGROUND A glymphatic system has been proposed that comprises flow that enters along cerebral paraarterial channels between the artery wall and the surrounding glial layer, continues through the parenchyma, and then exits along similar paravenous channels. The mechanism driving flow through this system is unclear. The pulsatile (oscillatory plus mean) flow measured in the space surrounding the middle cerebral artery (MCA) suggests that peristalsis created by intravascular blood pressure pulses is a candidate for the paraarterial flow in the subarachnoid spaces. However, peristalsis is ineffective in driving significant mean flow when the amplitude of channel wall motion is small, as has been observed in the MCA artery wall. In this paper, peristalsis in combination with two additional mechanisms, a longitudinal pressure gradient and directional flow resistance, is evaluated to match the measured MCA paraarterial oscillatory and mean flows. METHODS Two analytical models are used that simplify the paraarterial branched network to a long continuous channel with a traveling wave in order to maximize the potential effect of peristalsis on the mean flow. The two models have parallel-plate and annulus geometries, respectively, with and without an added longitudinal pressure gradient. The effect of directional flow resistors was also evaluated for the parallel-plate geometry. RESULTS For these models, the measured amplitude of arterial wall motion is too large to cause the small measured amplitude of oscillatory velocity, indicating that the outer wall must also move. At a combined motion matching the measured oscillatory velocity, peristalsis is incapable of driving enough mean flow. Directional flow resistance elements augment the mean flow, but not enough to provide a match. With a steady longitudinal pressure gradient, both oscillatory and mean flows can be matched to the measurements. CONCLUSIONS These results suggest that peristalsis drives the oscillatory flow in the subarachnoid paraarterial space, but is incapable of driving the mean flow. The effect of directional flow resistors is insufficient to produce a match, but a small longitudinal pressure gradient is capable of creating the mean flow. Additional experiments are needed to confirm whether the outer wall also moves, as well as to validate the pressure gradient.
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Affiliation(s)
- M Keith Sharp
- Department of Mechanical Engineering, University of Louisville, 200 Sackett Hall, Louisville, KY, 40292, USA.
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Abstract
We review theoretical and numerical models of the glymphatic system, which circulates cerebrospinal fluid and interstitial fluid around the brain, facilitating solute transport. Models enable hypothesis development and predictions of transport, with clinical applications including drug delivery, stroke, cardiac arrest, and neurodegenerative disorders like Alzheimer’s disease. We sort existing models into broad categories by anatomical function: Perivascular flow, transport in brain parenchyma, interfaces to perivascular spaces, efflux routes, and links to neuronal activity. Needs and opportunities for future work are highlighted wherever possible; new models, expanded models, and novel experiments to inform models could all have tremendous value for advancing the field.
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Kedarasetti RT, Drew PJ, Costanzo F. Arterial vasodilation drives convective fluid flow in the brain: a poroelastic model. Fluids Barriers CNS 2022; 19:34. [PMID: 35570287 PMCID: PMC9107702 DOI: 10.1186/s12987-022-00326-y] [Citation(s) in RCA: 13] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2021] [Accepted: 03/29/2022] [Indexed: 01/26/2023] Open
Abstract
The movement of fluid into, through, and out of the brain plays an important role in clearing metabolic waste. However, there is controversy regarding the mechanisms driving fluid movement in the fluid-filled paravascular spaces (PVS), and whether the movement of metabolic waste in the brain extracellular space (ECS) is primarily driven by diffusion or convection. The dilation of penetrating arterioles in the brain in response to increases in neural activity (neurovascular coupling) is an attractive candidate for driving fluid circulation, as it drives deformation of the brain tissue and of the PVS around arteries, resulting in fluid movement. We simulated the effects of vasodilation on fluid movement into and out of the brain ECS using a novel poroelastic model of brain tissue. We found that arteriolar dilations could drive convective flow through the ECS radially outward from the arteriole, and that this flow is sensitive to the dynamics of the dilation. Simulations of sleep-like conditions, with larger vasodilations and increased extracellular volume in the brain showed enhanced movement of fluid from the PVS into the ECS. Our simulations suggest that both sensory-evoked and sleep-related arteriolar dilations can drive convective flow of cerebrospinal fluid not just in the PVS, but also into the ECS through the PVS around arterioles.
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Affiliation(s)
- Ravi Teja Kedarasetti
- grid.29857.310000 0001 2097 4281Department of Engineering Science and Mechanics, Pennsylvania State University, University Park, PA USA ,grid.29857.310000 0001 2097 4281Center for Neural Engineering, Pennsylvania State University, University Park, PA USA
| | - Patrick J. Drew
- grid.29857.310000 0001 2097 4281Department of Engineering Science and Mechanics, Pennsylvania State University, University Park, PA USA ,grid.29857.310000 0001 2097 4281Center for Neural Engineering, Pennsylvania State University, University Park, PA USA ,grid.29857.310000 0001 2097 4281Department of Biomedical Engineering, Pennsylvania State University, University Park, PA USA ,grid.29857.310000 0001 2097 4281Department of Neurosurgery, Pennsylvania State University, University Park, PA USA
| | - Francesco Costanzo
- grid.29857.310000 0001 2097 4281Department of Engineering Science and Mechanics, Pennsylvania State University, University Park, PA USA ,grid.29857.310000 0001 2097 4281Center for Neural Engineering, Pennsylvania State University, University Park, PA USA ,grid.29857.310000 0001 2097 4281Department of Biomedical Engineering, Pennsylvania State University, University Park, PA USA ,grid.29857.310000 0001 2097 4281Department of Mathematics, Pennsylvania State University, University Park, PA USA
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5
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Abstract
The brain harbors a unique ability to, figuratively speaking, shift its gears. During wakefulness, the brain is geared fully toward processing information and behaving, while homeostatic functions predominate during sleep. The blood-brain barrier establishes a stable environment that is optimal for neuronal function, yet the barrier imposes a physiological problem; transcapillary filtration that forms extracellular fluid in other organs is reduced to a minimum in brain. Consequently, the brain depends on a special fluid [the cerebrospinal fluid (CSF)] that is flushed into brain along the unique perivascular spaces created by astrocytic vascular endfeet. We describe this pathway, coined the term glymphatic system, based on its dependency on astrocytic vascular endfeet and their adluminal expression of aquaporin-4 water channels facing toward CSF-filled perivascular spaces. Glymphatic clearance of potentially harmful metabolic or protein waste products, such as amyloid-β, is primarily active during sleep, when its physiological drivers, the cardiac cycle, respiration, and slow vasomotion, together efficiently propel CSF inflow along periarterial spaces. The brain's extracellular space contains an abundance of proteoglycans and hyaluronan, which provide a low-resistance hydraulic conduit that rapidly can expand and shrink during the sleep-wake cycle. We describe this unique fluid system of the brain, which meets the brain's requisites to maintain homeostasis similar to peripheral organs, considering the blood-brain-barrier and the paths for formation and egress of the CSF.
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Affiliation(s)
- Martin Kaag Rasmussen
- Center for Translational Neuromedicine, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Humberto Mestre
- Center for Translational Neuromedicine, University of Rochester Medical Center, Rochester, New York
| | - Maiken Nedergaard
- Center for Translational Neuromedicine, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
- Center for Translational Neuromedicine, University of Rochester Medical Center, Rochester, New York
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Li Y, Rusinek H, Butler T, Glodzik L, Pirraglia E, Babich J, Mozley PD, Nehmeh S, Pahlajani S, Wang X, Tanzi EB, Zhou L, Strauss S, Carare RO, Theise N, Okamura N, de Leon MJ. Decreased CSF clearance and increased brain amyloid in Alzheimer's disease. Fluids Barriers CNS 2022; 19:21. [PMID: 35287702 PMCID: PMC8919541 DOI: 10.1186/s12987-022-00318-y] [Citation(s) in RCA: 33] [Impact Index Per Article: 16.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/13/2021] [Accepted: 02/21/2022] [Indexed: 01/05/2023] Open
Abstract
BACKGROUND In sporadic Alzheimer's disease (AD), brain amyloid-beta (Aβ) deposition is believed to be a consequence of impaired Aβ clearance, but this relationship is not well established in living humans. CSF clearance, a major feature of brain glymphatic clearance (BGC), has been shown to be abnormal in AD murine models. MRI phase contrast and intrathecally delivered contrast studies have reported reduced CSF flow in AD. Using PET and tau tracer 18F-THK5117, we previously reported that the ventricular CSF clearance of the PET tracer was reduced in AD and associated with elevated brain Aβ levels. METHODS In the present study, we use two PET tracers, 18F-THK5351 and 11C-PiB to estimate CSF clearance calculated from early dynamic PET frames in 9 normal controls and 15 AD participants. RESULTS we observed that the ventricular CSF clearance measures were correlated (r = 0.66, p < 0.01), with reductions in AD of 18 and 27%, respectively. We also replicated a significant relationship between ventricular CSF clearance (18F-THK5351) and brain Aβ load (r = - 0.64, n = 24, p < 0.01). With a larger sample size, we extended our observations to show that reduced CSF clearance is associated with reductions in cortical thickness and cognitive performance. CONCLUSIONS Overall, the findings support the hypothesis that failed CSF clearance is a feature of AD that is related to Aβ deposition and to the pathology of AD. Longitudinal studies are needed to determine whether failed CSF clearance is a predictor of progressive amyloidosis or its consequence.
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Affiliation(s)
- Yi Li
- Department of Radiology, Weill Cornell Medicine, Cornell University, Brain Health Imaging Institute, 407 East 61 Street, New York, NY, 10021, USA.
| | - Henry Rusinek
- Department of Radiology, New York University School of Medicine, New York, NY, USA
| | - Tracy Butler
- Department of Radiology, Weill Cornell Medicine, Cornell University, Brain Health Imaging Institute, 407 East 61 Street, New York, NY, 10021, USA
| | - Lidia Glodzik
- Department of Radiology, Weill Cornell Medicine, Cornell University, Brain Health Imaging Institute, 407 East 61 Street, New York, NY, 10021, USA
| | - Elizabeth Pirraglia
- Department of Psychiatry, New York University School of Medicine, New York, NY, USA
| | - John Babich
- Department of Radiology, Weill Cornell Medicine, Cornell University, Brain Health Imaging Institute, 407 East 61 Street, New York, NY, 10021, USA
| | - P David Mozley
- Department of Radiology, Weill Cornell Medicine, Cornell University, Brain Health Imaging Institute, 407 East 61 Street, New York, NY, 10021, USA
| | - Sadek Nehmeh
- Department of Radiology, Weill Cornell Medicine, Cornell University, Brain Health Imaging Institute, 407 East 61 Street, New York, NY, 10021, USA
| | - Silky Pahlajani
- Department of Radiology, Weill Cornell Medicine, Cornell University, Brain Health Imaging Institute, 407 East 61 Street, New York, NY, 10021, USA
| | - Xiuyuan Wang
- Department of Radiology, Weill Cornell Medicine, Cornell University, Brain Health Imaging Institute, 407 East 61 Street, New York, NY, 10021, USA
| | - Emily B Tanzi
- Department of Radiology, Weill Cornell Medicine, Cornell University, Brain Health Imaging Institute, 407 East 61 Street, New York, NY, 10021, USA
| | - Liangdong Zhou
- Department of Radiology, Weill Cornell Medicine, Cornell University, Brain Health Imaging Institute, 407 East 61 Street, New York, NY, 10021, USA
| | - Sara Strauss
- Department of Radiology, Weill Cornell Medicine, Cornell University, Brain Health Imaging Institute, 407 East 61 Street, New York, NY, 10021, USA
| | - Roxana O Carare
- Department of Clinical Neuroanatomy, University of Southampton, Southampton, UK
| | - Neil Theise
- Department of Pathology, New York University School of Medicine, New York, NY, USA
| | - Nobuyuki Okamura
- Division of Pharmacology, Faculty of Medicine, Tohoku Medical and Pharmaceutical University, Sendai, Japan
| | - Mony J de Leon
- Department of Radiology, Weill Cornell Medicine, Cornell University, Brain Health Imaging Institute, 407 East 61 Street, New York, NY, 10021, USA.
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Matsuzono K, Suzuki M, Miura K, Ozawa T, Mashiko T, Koide R, Tanaka R, Fujimoto S. Internal Jugular Vein Velocity and Spontaneous Echo Contrast Correlate with Alzheimer's Disease and Cognitive Function. J Alzheimers Dis 2021; 84:787-796. [PMID: 34602471 DOI: 10.3233/jad-210490] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
BACKGROUND Many issues persist in the today's Alzheimer's disease (AD) screening and the breakthrough method is desired. OBJECTIVE We aim to validate the association between venous reflux and AD, and to develop a new method for AD screening. METHODS We examined spontaneous echo contrast, area, diameter, retrograde velocity, and anterograde velocity of the bilateral cervical internal jugular vein (IJV) using carotid ultrasonography. RESULTS A total of 112 patients participated in this study, with 26 diagnosed as AD. The proportion of both or either IJV spontaneous echo contrast (+) occupied 25 of total 26 AD patients, which showed 96.2%of sensitivity and 98.5%negative predictive value. The IJV velocities also showed significant correlation with AD diagnosis, although the IJV area or diameter did not. CONCLUSION Our results indicate that the validation of the spontaneous echo contrast or velocities of the IJV are convenient AD diagnosis screening methods and that the venous reflux disturbance correlates with AD development.
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Affiliation(s)
- Kosuke Matsuzono
- Division of Neurology, Department of Medicine, Jichi Medical University, Tochigi, Japan
| | - Masayuki Suzuki
- Division of Neurology, Department of Medicine, Jichi Medical University, Tochigi, Japan
| | - Kumiko Miura
- Division of Neurology, Department of Medicine, Jichi Medical University, Tochigi, Japan
| | - Tadashi Ozawa
- Division of Neurology, Department of Medicine, Jichi Medical University, Tochigi, Japan
| | - Takafumi Mashiko
- Division of Neurology, Department of Medicine, Jichi Medical University, Tochigi, Japan
| | - Reiji Koide
- Division of Neurology, Department of Medicine, Jichi Medical University, Tochigi, Japan
| | - Ryota Tanaka
- Division of Neurology, Department of Medicine, Jichi Medical University, Tochigi, Japan
| | - Shigeru Fujimoto
- Division of Neurology, Department of Medicine, Jichi Medical University, Tochigi, Japan
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8
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Yankova G, Bogomyakova O, Tulupov A. The glymphatic system and meningeal lymphatics of the brain: new understanding of brain clearance. Rev Neurosci 2021; 32:693-705. [PMID: 33618444 DOI: 10.1515/revneuro-2020-0106] [Citation(s) in RCA: 14] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2020] [Accepted: 01/31/2021] [Indexed: 12/25/2022]
Abstract
The glymphatic system and meningeal lymphatics have recently been characterized. Glymphatic system is a glia-dependent system of perivascular channels, and it plays an important role in the removal of interstitial metabolic waste products. The meningeal lymphatics may be a key drainage route for cerebrospinal fluid into the peripheral blood, may contribute to inflammatory reaction and central nervous system (CNS) immune surveillance. Breakdowns and dysfunction of the glymphatic system and meningeal lymphatics play a crucial role in age-related brain changes, the pathogenesis of neurovascular and neurodegenerative diseases, as well as in brain injuries and tumors. This review discusses the relationship recently characterized meningeal lymphatic vessels with the glymphatic system, which provides perfusion of the CNS with cerebrospinal and interstitial fluids. The review also presents the results of human studies concerning both the presence of meningeal lymphatics and the glymphatic system. A new understanding of how aging, medications, sleep and wake cycles, genetic predisposition, and even body posture affect the brain drainage system has not only changed the idea of brain fluid circulation but has also contributed to an understanding of the pathology and mechanisms of neurodegenerative diseases.
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Affiliation(s)
- Galina Yankova
- Lavrentyev Institute of Hydrodynamics, Siberian Branch, Russian Academy of Sciences, Novosibirsk630090, Russia.,Novosibirsk State University, Novosibirsk630090,Russia
| | - Olga Bogomyakova
- International Tomography Center, Siberian Branch, Russian Academy of Sciences, Novosibirsk630090, Russia
| | - Andrey Tulupov
- Novosibirsk State University, Novosibirsk630090,Russia.,International Tomography Center, Siberian Branch, Russian Academy of Sciences, Novosibirsk630090, Russia.,Meshalkin National Medical Research Center, Ministry of Health of Russian Federation, Novosibirsk 630055, Russia
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9
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Faghih MM, Keith Sharp M. Mechanisms of tracer transport in cerebral perivascular spaces. J Biomech 2021; 118:110278. [PMID: 33548658 DOI: 10.1016/j.jbiomech.2021.110278] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/02/2021] [Accepted: 01/16/2021] [Indexed: 02/09/2023]
Abstract
Tracers infused into the brain appear to be transported along channels surrounding cerebral blood vessels. Bulk fluid flow has been hypothesized in paravascular "glymphatic" channels (outer space between the pial membrane and astrocyte endfeet), as well as in the periarterial space (inner space between smooth muscle cells). The plausibility of net flow in these channels due to steady and oscillatory pressures is reviewed, as is that of transport by oscillatory shear-enhanced dispersion in the absence of net flow. Models including 1D branching networks of annular channels and an expanded compartmental model for humans both predict that flow driven by physiologic steady pressure differences is unlikely in both periarterial and paraarterial spaces, whether the spaces are open or filled with porous media. One exception is that a small additional steady pressure difference could drive paraarterial flow if the space is open. The potential that the tracer injection itself could present such a pressure difference is outlined. Oscillatory (peristaltic) wall motion alone has been found to be insufficient to drive significant forward flow. However, a number of hypothesized mechanisms that have yet to be experimentally verified in the brain may create directional flow in combination with wall motion. Shear-augmented dispersion due to oscillatory pressure in channels with a range of porosity has been modeled analytically. Enhancement of axial dispersion is small for periarterial channels. In open paraarterial channels, dispersion enhancement with optimal lateral mixing is large enough that it may explain observed tracer transport without net forward fluid flow.
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Affiliation(s)
- Mohammad M Faghih
- Department of Mechanical Engineering, University of Louisville Louisville, KY 40292, United States
| | - M Keith Sharp
- Department of Mechanical Engineering, University of Louisville Louisville, KY 40292, United States.
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10
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Howe MD, McCullough LD, Urayama A. The Role of Basement Membranes in Cerebral Amyloid Angiopathy. Front Physiol 2020; 11:601320. [PMID: 33329053 PMCID: PMC7732667 DOI: 10.3389/fphys.2020.601320] [Citation(s) in RCA: 14] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2020] [Accepted: 10/28/2020] [Indexed: 12/25/2022] Open
Abstract
Dementia is a neuropsychiatric syndrome characterized by cognitive decline in multiple domains, often leading to functional impairment in activities of daily living, disability, and death. The most common causes of age-related progressive dementia include Alzheimer's disease (AD) and vascular cognitive impairment (VCI), however, mixed disease pathologies commonly occur, as epitomized by a type of small vessel pathology called cerebral amyloid angiopathy (CAA). In CAA patients, the small vessels of the brain become hardened and vulnerable to rupture, leading to impaired neurovascular coupling, multiple microhemorrhage, microinfarction, neurological emergencies, and cognitive decline across multiple functional domains. While the pathogenesis of CAA is not well understood, it has long been thought to be initiated in thickened basement membrane (BM) segments, which contain abnormal protein deposits and amyloid-β (Aβ). Recent advances in our understanding of CAA pathogenesis link BM remodeling to functional impairment of perivascular transport pathways that are key to removing Aβ from the brain. Dysregulation of this process may drive CAA pathogenesis and provides an important link between vascular risk factors and disease phenotype. The present review summarizes how the structure and composition of the BM allows for perivascular transport pathways to operate in the healthy brain, and then outlines multiple mechanisms by which specific dementia risk factors may promote dysfunction of perivascular transport pathways and increase Aβ deposition during CAA pathogenesis. A better understanding of how BM remodeling alters perivascular transport could lead to novel diagnostic and therapeutic strategies for CAA patients.
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Affiliation(s)
| | | | - Akihiko Urayama
- Department of Neurology, McGovern Medical School, The University of Texas Health Science Center at Houston, Houston, TX, United States
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11
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Abstract
Despite its small size, the brain consumes 25% of the body’s energy, generating its own weight in potentially toxic proteins and biological debris each year. The brain is also the only organ lacking lymph vessels to assist in removal of interstitial waste. Over the past 50 years, a picture has been developing of the brain’s unique waste removal system. Experimental observations show cerebrospinal fluid, which surrounds the brain, enters the brain along discrete pathways, crosses a barrier into the spaces between brain cells, and flushes the tissue, carrying wastes to routes exiting the brain. Dysfunction of this cerebral waste clearance system has been demonstrated in Alzheimer’s disease, traumatic brain injury, diabetes, and stroke. The activity of the system is observed to increase during sleep. In addition to waste clearance, this circuit of flow may also deliver nutrients and neurotransmitters. Here, we review the relevant literature with a focus on transport processes, especially the potential role of diffusion and advective flows.
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12
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Martinac AD, Bilston LE. Computational modelling of fluid and solute transport in the brain. Biomech Model Mechanobiol 2019; 19:781-800. [DOI: 10.1007/s10237-019-01253-y] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/20/2019] [Accepted: 11/05/2019] [Indexed: 01/10/2023]
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13
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Abstract
The flow of cerebrospinal fluid along perivascular spaces (PVSs) is an important part of the brain’s system for delivering nutrients and eliminating metabolic waste products (such as amyloid-β); it also offers a pathway for the delivery of therapeutic drugs to the brain parenchyma. Recent experimental results have resolved several important questions about this flow, setting the stage for advances in our understanding of its fluid dynamics. This review summarizes the new experimental evidence and provides a critical evaluation of previous fluid-dynamic models of flows in PVSs. The review also discusses some basic fluid-dynamic concepts relevant to these flows, including the combined effects of diffusion and advection in clearing solutes from the brain.
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Affiliation(s)
- John H Thomas
- Department of Mechanical Engineering, University of Rochester, Rochester, NY 14627, USA
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14
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Keith Sharp M, Carare RO, Martin BA. Dispersion in porous media in oscillatory flow between flat plates: applications to intrathecal, periarterial and paraarterial solute transport in the central nervous system. Fluids Barriers CNS 2019; 16:13. [PMID: 31056079 PMCID: PMC6512764 DOI: 10.1186/s12987-019-0132-y] [Citation(s) in RCA: 39] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/14/2018] [Accepted: 04/16/2019] [Indexed: 01/22/2023] Open
Abstract
Background As an alternative to advection, solute transport by shear-augmented dispersion within oscillatory cerebrospinal fluid flow was investigated in small channels representing the basement membranes located between cerebral arterial smooth muscle cells, the paraarterial space surrounding the vessel wall and in large channels modeling the spinal subarachnoid space (SSS). Methods Geometries were modeled as two-dimensional. Fully developed flows in the channels were modeled by the Darcy–Brinkman momentum equation and dispersion by the passive transport equation. Scaling of the enhancement of axial dispersion relative to molecular diffusion was developed for regimes of flow including quasi-steady, porous and unsteady, and for regimes of dispersion including diffusive and unsteady. Results Maximum enhancement occurs when the characteristic time for lateral dispersion is matched to the cycle period. The Darcy–Brinkman model represents the porous media as a continuous flow resistance, and also imposes no-slip boundary conditions at the walls of the channel. Consequently, predicted dispersion is always reduced relative to that of a channel without porous media, except when the flow and dispersion are both unsteady. Discussion/conclusions In the basement membranes, flow and dispersion are both quasi-steady and enhancement of dispersion is small even if lateral dispersion is reduced by the porous media to achieve maximum enhancement. In the paraarterial space, maximum enhancement Rmax = 73,200 has the potential to be significant. In the SSS, the dispersion is unsteady and the flow is in the transition zone between porous and unsteady. Enhancement is 5.8 times that of molecular diffusion, and grows to a maximum of 1.6E+6 when lateral dispersion is increased. The maximum enhancement produces rostral transport time in agreement with experiments. Electronic supplementary material The online version of this article (10.1186/s12987-019-0132-y) contains supplementary material, which is available to authorized users.
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Affiliation(s)
- M Keith Sharp
- Biofluid Mechanics Laboratory, Department of Mechanical Engineering, University of Louisville, Louisville, KY, 40292, USA.
| | - Roxana O Carare
- Faculty of Medicine, Southampton General Hospital, University of Southampton, Southampton, SO16 6YD, UK
| | - Bryn A Martin
- Department of Biological Engineering, University of Idaho, Moscow, ID, 83844, USA
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Coloma M, Schaffer JD, Huang P, Chiarot PR. Boundary waves in a microfluidic device as a model for intramural periarterial drainage. BIOMICROFLUIDICS 2019; 13:024103. [PMID: 30867887 PMCID: PMC6408319 DOI: 10.1063/1.5080446] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/08/2018] [Accepted: 02/28/2019] [Indexed: 06/09/2023]
Abstract
The failure to clear amyloid-Beta from an aging brain leads to its accumulation within the walls of arteries and potentially to Alzheimer's disease. However, the clearance mechanism through the intramural periarterial pathway is not well understood. We previously proposed a hydrodynamic reverse transport model for the cerebral arterial basement membrane pathway. In our model, solute transport results from fluidic forcing driven by the superposition of forward and reverse propagating boundary waves. The aim of this study is to experimentally validate this hydrodynamic reverse transport mechanism in a microfluidic device where reverse transport in a rectangular conduit is driven by applying waveforms along its boundaries. Our results support our theory that while the superimposed boundary waves propagate in the forward direction, a reverse flow in the rectangular conduit can be induced by boundary wave reflections. We quantified the fluid transport velocity and direction under various boundary conditions and analyzed numerical simulations that support our experimental findings. We identified a set of boundary wave parameters that achieved reverse transport, which could be responsible for intramural periarterial drainage of cerebral metabolic waste.
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Affiliation(s)
- Mikhail Coloma
- Department of Mechanical Engineering, State University of New York at Binghamton, Binghamton, New York 13902, USA
| | - J. David Schaffer
- Institute for Justice and Well-Being, State University of New York at Binghamton, Binghamton, New York 13902, USA
| | - Peter Huang
- Department of Mechanical Engineering, State University of New York at Binghamton, Binghamton, New York 13902, USA
| | - Paul R. Chiarot
- Department of Mechanical Engineering, State University of New York at Binghamton, Binghamton, New York 13902, USA
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16
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Aldea R, Weller RO, Wilcock DM, Carare RO, Richardson G. Cerebrovascular Smooth Muscle Cells as the Drivers of Intramural Periarterial Drainage of the Brain. Front Aging Neurosci 2019; 11:1. [PMID: 30740048 PMCID: PMC6357927 DOI: 10.3389/fnagi.2019.00001] [Citation(s) in RCA: 128] [Impact Index Per Article: 25.6] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2018] [Accepted: 01/07/2019] [Indexed: 12/25/2022] Open
Abstract
The human brain is the organ with the highest metabolic activity but it lacks a traditional lymphatic system responsible for clearing waste products. We have demonstrated that the basement membranes of cerebral capillaries and arteries represent the lymphatic pathways of the brain along which intramural periarterial drainage (IPAD) of soluble metabolites occurs. Failure of IPAD could explain the vascular deposition of the amyloid-beta protein as cerebral amyloid angiopathy (CAA), which is a key pathological feature of Alzheimer's disease. The underlying mechanisms of IPAD, including its motive force, have not been clarified, delaying successful therapies for CAA. Although arterial pulsations from the heart were initially considered to be the motive force for IPAD, they are not strong enough for efficient IPAD. This study aims to unravel the driving force for IPAD, by shifting the perspective of a heart-driven clearance of soluble metabolites from the brain to an intrinsic mechanism of cerebral arteries (e.g., vasomotion-driven IPAD). We test the hypothesis that the cerebrovascular smooth muscle cells, whose cycles of contraction and relaxation generate vasomotion, are the drivers of IPAD. A novel multiscale model of arteries, in which we treat the basement membrane as a fluid-filled poroelastic medium deformed by the contractile cerebrovascular smooth muscle cells, is used to test the hypothesis. The vasomotion-induced intramural flow rates suggest that vasomotion-driven IPAD is the only mechanism postulated to date capable of explaining the available experimental observations. The cerebrovascular smooth muscle cells could represent valuable drug targets for prevention and early interventions in CAA.
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Affiliation(s)
- Roxana Aldea
- Mathematical Sciences, University of Southampton, Southampton, United Kingdom
| | - Roy O Weller
- Clinical Neurosciences, Faculty of Medicine, University of Southampton, Southampton General Hospital, Southampton, United Kingdom
| | - Donna M Wilcock
- Department of Physiology, Sanders-Brown Center on Aging, University of Kentucky, Lexington, KY, United States
| | - Roxana O Carare
- Clinical Neurosciences, Faculty of Medicine, University of Southampton, Southampton General Hospital, Southampton, United Kingdom
| | - Giles Richardson
- Mathematical Sciences, University of Southampton, Southampton, United Kingdom
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17
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Rey J, Sarntinoranont M. Pulsatile flow drivers in brain parenchyma and perivascular spaces: a resistance network model study. Fluids Barriers CNS 2018; 15:20. [PMID: 30012159 PMCID: PMC6048913 DOI: 10.1186/s12987-018-0105-6] [Citation(s) in RCA: 41] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/13/2018] [Accepted: 07/03/2018] [Indexed: 11/10/2022] Open
Abstract
Background In animal models, dissolved compounds in the subarachnoid space and parenchyma have been found to preferentially transport through the cortex perivascular spaces (PVS) but the transport phenomena involved are unclear. Methods In this study two hydraulic network models were used to predict fluid motion produced by blood vessel pulsations and estimate the contribution made to solute transport in PVS and parenchyma. The effect of varying pulse amplitude and timing, PVS dimensions, and tissue hydraulic conductivity on fluid motion was investigated. Results Periodic vessel pulses resulted in oscillatory fluid motion in PVS and parenchyma but no net flow over time. For baseline parameters, PVS and parenchyma peak fluid velocity was on the order of 10 μm/s and 1 nm/s, with corresponding Peclet numbers below 103 and 10−1 respectively. Peak fluid velocity in the PVS and parenchyma tended to increase with increasing pulse amplitude and vessel size, and exhibited asymptotic relationships with hydraulic conductivity. Conclusions Solute transport in parenchyma was predicted to be diffusion dominated, with a negligible contribution from convection. In the PVS, dispersion due to oscillating flow likely plays a significant role in PVS rapid transport observed in previous in vivo experiments. This dispersive effect could be more significant than convective solute transport from net flow that may exist in PVS and should be studied further.
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Affiliation(s)
- Julian Rey
- Department of Mechanical and Aerospace Engineering, University of Florida, PO Box 116250, Gainesville, FL, 32611, USA
| | - Malisa Sarntinoranont
- Department of Mechanical and Aerospace Engineering, University of Florida, PO Box 116250, Gainesville, FL, 32611, USA.
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18
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Faghih MM, Sharp MK. Is bulk flow plausible in perivascular, paravascular and paravenous channels? Fluids Barriers CNS 2018; 15:17. [PMID: 29903035 PMCID: PMC6003203 DOI: 10.1186/s12987-018-0103-8] [Citation(s) in RCA: 60] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/02/2018] [Accepted: 05/15/2018] [Indexed: 01/12/2023] Open
Abstract
BACKGROUND Transport of solutes has been observed in the spaces surrounding cerebral arteries and veins. Indeed, transport has been found in opposite directions in two different spaces around arteries. These findings have motivated hypotheses of bulk flow within these spaces. The glymphatic circulation hypothesis involves flow of cerebrospinal fluid from the cortical subarachnoid space to the parenchyma along the paraarterial (extramural, Virchow-Robin) space around arteries, and return flow to the cerebrospinal fluid (CSF) space via paravenous channels. The second hypothesis involves flow of interstitial fluid from the parenchyma to lymphatic vessels along basement membranes between arterial smooth muscle cells. METHODS This article evaluates the plausibility of steady, pressure-driven flow in these channels with one-dimensional branching models. RESULTS According to the models, the hydraulic resistance of arterial basement membranes is too large to accommodate estimated interstitial perfusion of the brain, unless the flow empties to lymphatic ducts after only several generations (still within the parenchyma). The estimated pressure drops required to drive paraarterial and paravenous flows of the same magnitude are not large, but paravenous flow back to the CSF space means that the total pressure difference driving both flows is limited to local pressure differences among the different CSF compartments, which are estimated to be small. CONCLUSIONS Periarterial flow and glymphatic circulation driven by steady pressure are both found to be implausible, given current estimates of anatomical and fluid dynamic parameters.
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Affiliation(s)
- Mohammad M Faghih
- Biofluid Mechanics Laboratory, Department of Mechanical Engineering, University of Louisville, Louisville, KY, 40292, USA
| | - M Keith Sharp
- Biofluid Mechanics Laboratory, Department of Mechanical Engineering, University of Louisville, Louisville, KY, 40292, USA.
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19
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Pizzo ME, Wolak DJ, Kumar NN, Brunette E, Brunnquell CL, Hannocks M, Abbott NJ, Meyerand ME, Sorokin L, Stanimirovic DB, Thorne RG. Intrathecal antibody distribution in the rat brain: surface diffusion, perivascular transport and osmotic enhancement of delivery. J Physiol 2018; 596:445-475. [PMID: 29023798 PMCID: PMC5792566 DOI: 10.1113/jp275105] [Citation(s) in RCA: 170] [Impact Index Per Article: 28.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/14/2017] [Accepted: 09/29/2017] [Indexed: 12/11/2022] Open
Abstract
KEY POINTS It is unclear precisely how macromolecules (e.g. endogenous proteins and exogenous immunotherapeutics) access brain tissue from the cerebrospinal fluid (CSF). We show that transport at the brain-CSF interface involves a balance between Fickian diffusion in the extracellular spaces at the brain surface and convective transport in perivascular spaces of cerebral blood vessels. Intrathecally-infused antibodies exhibited size-dependent access to the perivascular spaces and tunica media basement membranes of leptomeningeal arteries. Perivascular access and distribution of full-length IgG could be enhanced by intrathecal co-infusion of hyperosmolar mannitol. Pores or stomata present on CSF-facing leptomeningeal cells ensheathing blood vessels in the subarachnoid space may provide unique entry sites into the perivascular spaces from the CSF. These results illuminate new mechanisms likely to govern antibody trafficking at the brain-CSF interface with relevance for immune surveillance in the healthy brain and insights into the distribution of therapeutic antibodies. ABSTRACT The precise mechanisms governing the central distribution of macromolecules from the cerebrospinal fluid (CSF) to the brain and spinal cord remain poorly understood, despite their importance for physiological processes such as antibody trafficking for central immune surveillance, as well as several ongoing intrathecal clinical trials. In the present study, we clarify how IgG and smaller single-domain antibodies (sdAb) distribute throughout the whole brain in a size-dependent manner after intrathecal infusion in rats using ex vivo fluorescence and in vivo three-dimensional magnetic resonance imaging. Antibody distribution was characterized by diffusion at the brain surface and widespread distribution to deep brain regions along the perivascular spaces of all vessel types, with sdAb accessing a four- to seven-fold greater brain area than IgG. Perivascular transport involved blood vessels of all caliber and putative smooth muscle and astroglial basement membrane compartments. Perivascular access to smooth muscle basement membrane compartments also exhibited size-dependence. Electron microscopy was used to show stomata on leptomeningeal coverings of blood vessels in the subarachnoid space as potential access points allowing substances in the CSF to enter the perivascular space. Osmolyte co-infusion significantly enhanced perivascular access of the larger antibody from the CSF, with intrathecal 0.75 m mannitol increasing the number of perivascular profiles per slice area accessed by IgG by ∼50%. The results of the present study reveal potential distribution mechanisms for endogenous IgG, which is one of the most abundant proteins in the CSF, as well as provide new insights with respect to understanding and improving the drug delivery of macromolecules to the central nervous system via the intrathecal route.
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Affiliation(s)
- Michelle E. Pizzo
- School of PharmacyDivision of Pharmaceutical Sciences, University of Wisconsin‐MadisonMadisonWIUSA
- Clinical Neuroengineering Training ProgramUniversity of Wisconsin‐MadisonMadisonWIUSA
| | - Daniel J. Wolak
- School of PharmacyDivision of Pharmaceutical Sciences, University of Wisconsin‐MadisonMadisonWIUSA
- Clinical Neuroengineering Training ProgramUniversity of Wisconsin‐MadisonMadisonWIUSA
| | - Niyanta N. Kumar
- School of PharmacyDivision of Pharmaceutical Sciences, University of Wisconsin‐MadisonMadisonWIUSA
| | - Eric Brunette
- Human Health Therapeutics Research CentreNational Research Council of CanadaOttawaCanada
| | | | - Melanie‐Jane Hannocks
- Institute of Physiological Chemistry and PathobiochemistryMuenster UniversityMuensterGermany
- Cells‐in‐Motion Cluster of ExcellenceMuenster UniversityMuensterGermany
| | - N. Joan Abbott
- Institute of Pharmaceutical ScienceKing's College LondonLondonUK
| | - M. Elizabeth Meyerand
- Clinical Neuroengineering Training ProgramUniversity of Wisconsin‐MadisonMadisonWIUSA
- Department of Medical PhysicsUniversity of Wisconsin‐MadisonMadisonWIUSA
- Department of Biomedical EngineeringUniversity of Wisconsin‐MadisonMadisonWIUSA
| | - Lydia Sorokin
- Institute of Physiological Chemistry and PathobiochemistryMuenster UniversityMuensterGermany
- Cells‐in‐Motion Cluster of ExcellenceMuenster UniversityMuensterGermany
| | - Danica B. Stanimirovic
- Human Health Therapeutics Research CentreNational Research Council of CanadaOttawaCanada
| | - Robert G. Thorne
- School of PharmacyDivision of Pharmaceutical Sciences, University of Wisconsin‐MadisonMadisonWIUSA
- Clinical Neuroengineering Training ProgramUniversity of Wisconsin‐MadisonMadisonWIUSA
- Neuroscience Training ProgramUniversity of Wisconsin‐MadisonMadisonWIUSA
- Cellular and Molecular Pathology Graduate ProgramUniversity of Wisconsin‐MadisonMadisonWIUSA
- Institute for Clinical and Translational ResearchUniversity of Wisconsin‐MadisonWIUSA
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20
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Louveau A, Plog BA, Antila S, Alitalo K, Nedergaard M, Kipnis J. Understanding the functions and relationships of the glymphatic system and meningeal lymphatics. J Clin Invest 2017; 127:3210-3219. [PMID: 28862640 PMCID: PMC5669566 DOI: 10.1172/jci90603] [Citation(s) in RCA: 377] [Impact Index Per Article: 53.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Recent discoveries of the glymphatic system and of meningeal lymphatic vessels have generated a lot of excitement, along with some degree of skepticism. Here, we summarize the state of the field and point out the gaps of knowledge that should be filled through further research. We discuss the glymphatic system as a system that allows CNS perfusion by the cerebrospinal fluid (CSF) and interstitial fluid (ISF). We also describe the recently characterized meningeal lymphatic vessels and their role in drainage of the brain ISF, CSF, CNS-derived molecules, and immune cells from the CNS and meninges to the peripheral (CNS-draining) lymph nodes. We speculate on the relationship between the two systems and their malfunction that may underlie some neurological diseases. Although much remains to be investigated, these new discoveries have changed our understanding of mechanisms underlying CNS immune privilege and CNS drainage. Future studies should explore the communications between the glymphatic system and meningeal lymphatics in CNS disorders and develop new therapeutic modalities targeting these systems.
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Affiliation(s)
- Antoine Louveau
- Center for Brain Immunology and Glia, Department of Neuroscience, School of Medicine, University of Virginia, Charlottesville, Virginia, USA
| | - Benjamin A. Plog
- Center for Translational Neuromedicine, University of Rochester Medical Center, Rochester, New York, USA
| | - Salli Antila
- Wihuri Research Institute and Translational Cancer Biology Program, Biomedicum Helsinki, University of Helsinki, Helsinki, Finland
| | - Kari Alitalo
- Wihuri Research Institute and Translational Cancer Biology Program, Biomedicum Helsinki, University of Helsinki, Helsinki, Finland
| | - Maiken Nedergaard
- Center for Translational Neuromedicine, University of Rochester Medical Center, Rochester, New York, USA
- Center of Neuroscience, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Jonathan Kipnis
- Center for Brain Immunology and Glia, Department of Neuroscience, School of Medicine, University of Virginia, Charlottesville, Virginia, USA
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21
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Coles JA, Myburgh E, Brewer JM, McMenamin PG. Where are we? The anatomy of the murine cortical meninges revisited for intravital imaging, immunology, and clearance of waste from the brain. Prog Neurobiol 2017; 156:107-148. [PMID: 28552391 DOI: 10.1016/j.pneurobio.2017.05.002] [Citation(s) in RCA: 78] [Impact Index Per Article: 11.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2016] [Revised: 04/25/2017] [Accepted: 05/08/2017] [Indexed: 12/15/2022]
Abstract
Rapid progress is being made in understanding the roles of the cerebral meninges in the maintenance of normal brain function, in immune surveillance, and as a site of disease. Most basic research on the meninges and the neural brain is now done on mice, major attractions being the availability of reporter mice with fluorescent cells, and of a huge range of antibodies useful for immunocytochemistry and the characterization of isolated cells. In addition, two-photon microscopy through the unperforated calvaria allows intravital imaging of the undisturbed meninges with sub-micron resolution. The anatomy of the dorsal meninges of the mouse (and, indeed, of all mammals) differs considerably from that shown in many published diagrams: over cortical convexities, the outer layer, the dura, is usually thicker than the inner layer, the leptomeninx, and both layers are richly vascularized and innervated, and communicate with the lymphatic system. A membrane barrier separates them and, in disease, inflammation can be localized to one layer or the other, so experimentalists must be able to identify the compartment they are studying. Here, we present current knowledge of the functional anatomy of the meninges, particularly as it appears in intravital imaging, and review their role as a gateway between the brain, blood, and lymphatics, drawing on information that is scattered among works on different pathologies.
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Affiliation(s)
- Jonathan A Coles
- Centre for Immunobiology, Institute of Infection, Immunity and Inflammation, College of Medical, Veterinary and Life Sciences, Sir Graeme Davis Building, University of Glasgow, Glasgow, G12 8TA, United Kingdom.
| | - Elmarie Myburgh
- Centre for Immunology and Infection Department of Biology, University of York, Wentworth Way, Heslington, York YO10 5DD, United Kingdom
| | - James M Brewer
- Centre for Immunobiology, Institute of Infection, Immunity and Inflammation, College of Medical, Veterinary and Life Sciences, Sir Graeme Davis Building, University of Glasgow, Glasgow, G12 8TA, United Kingdom
| | - Paul G McMenamin
- Department of Anatomy & Developmental Biology, School of Biomedical and Psychological Sciences and Monash Biomedical Discovery Institute, Faculty of Medicine, Nursing and Health Sciences, Monash University, 10 Chancellor's Walk, Clayton, Victoria, 3800, Australia
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22
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Partyka PP, Godsey GA, Galie JR, Kosciuk MC, Acharya NK, Nagele RG, Galie PA. Mechanical stress regulates transport in a compliant 3D model of the blood-brain barrier. Biomaterials 2016; 115:30-39. [PMID: 27886553 DOI: 10.1016/j.biomaterials.2016.11.012] [Citation(s) in RCA: 112] [Impact Index Per Article: 14.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2016] [Revised: 10/19/2016] [Accepted: 11/09/2016] [Indexed: 12/20/2022]
Abstract
Transport of fluid and solutes is tightly controlled within the brain, where vasculature exhibits a blood-brain barrier and there is no organized lymphatic network facilitating waste transport from the interstitial space. Here, using a compliant, three-dimensional co-culture model of the blood-brain barrier, we show that mechanical stimuli exerted by blood flow mediate both the permeability of the endothelial barrier and waste transport along the basement membrane. Application of both shear stress and cyclic strain facilitates tight junction formation in the endothelial monolayer, with and without the presence of astrocyte endfeet in the surrounding matrix. We use both dextran perfusion and TEER measurements to assess the initiation and maintenance of the endothelial barrier, and microparticle image velocimetry to characterize the fluid dynamics within the in vitro vessels. Application of pulsatile flow to the in vitro vessels induces pulsatile strain to the vascular wall, providing an opportunity to investigate stretch-induced transport along the basement membrane. We find that a pulsatile wave speed of approximately 1 mm/s with Womersley number of 0.004 facilitates retrograde transport of high molecular weight dextran along the basement membrane between the basal endothelium and surrounding astrocytes. Together, these findings indicate that the mechanical stress exerted by blood flow is an important regulator of transport both across and along the walls of cerebral microvasculature.
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Affiliation(s)
- Paul P Partyka
- Department of Biomedical Engineering, Rowan University, United States
| | - George A Godsey
- School of Biomedical Sciences, Rowan University, United States
| | - John R Galie
- Department of Physics, Camden County College, United States
| | - Mary C Kosciuk
- School of Osteopathic Medicine, Rowan University, United States
| | | | - Robert G Nagele
- School of Biomedical Sciences, Rowan University, United States; School of Osteopathic Medicine, Rowan University, United States
| | - Peter A Galie
- Department of Biomedical Engineering, Rowan University, United States.
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23
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Jin BJ, Smith AJ, Verkman AS. Spatial model of convective solute transport in brain extracellular space does not support a "glymphatic" mechanism. J Gen Physiol 2016; 148:489-501. [PMID: 27836940 PMCID: PMC5129742 DOI: 10.1085/jgp.201611684] [Citation(s) in RCA: 130] [Impact Index Per Article: 16.3] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2016] [Accepted: 10/21/2016] [Indexed: 01/12/2023] Open
Abstract
A “glymphatic mechanism” has been proposed to mediate convective fluid transport from para-arterial to paravenous extracellular space in the brain. Jin et al. model such a system and find that diffusion, rather than convection, can account for the transport of solutes. A “glymphatic system,” which involves convective fluid transport from para-arterial to paravenous cerebrospinal fluid through brain extracellular space (ECS), has been proposed to account for solute clearance in brain, and aquaporin-4 water channels in astrocyte endfeet may have a role in this process. Here, we investigate the major predictions of the glymphatic mechanism by modeling diffusive and convective transport in brain ECS and by solving the Navier–Stokes and convection–diffusion equations, using realistic ECS geometry for short-range transport between para-arterial and paravenous spaces. Major model parameters include para-arterial and paravenous pressures, ECS volume fraction, solute diffusion coefficient, and astrocyte foot-process water permeability. The model predicts solute accumulation and clearance from the ECS after a step change in solute concentration in para-arterial fluid. The principal and robust conclusions of the model are as follows: (a) significant convective transport requires a sustained pressure difference of several mmHg between the para-arterial and paravenous fluid and is not affected by pulsatile pressure fluctuations; (b) astrocyte endfoot water permeability does not substantially alter the rate of convective transport in ECS as the resistance to flow across endfeet is far greater than in the gaps surrounding them; and (c) diffusion (without convection) in the ECS is adequate to account for experimental transport studies in brain parenchyma. Therefore, our modeling results do not support a physiologically important role for local parenchymal convective flow in solute transport through brain ECS.
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Affiliation(s)
- Byung-Ju Jin
- Department of Medicine, University of California, San Francisco, San Francisco, CA 94143.,Department of Physiology, University of California, San Francisco, San Francisco, CA 94143
| | - Alex J Smith
- Department of Medicine, University of California, San Francisco, San Francisco, CA 94143.,Department of Physiology, University of California, San Francisco, San Francisco, CA 94143
| | - Alan S Verkman
- Department of Medicine, University of California, San Francisco, San Francisco, CA 94143 .,Department of Physiology, University of California, San Francisco, San Francisco, CA 94143
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