Test of the 'glymphatic' hypothesis demonstrates diffusive and aquaporin-4-independent solute transport in rodent brain parenchyma
Open accessAbstract
Transport of solutes through brain involves diffusion and convection. The importance of convective flow in the subarachnoid and paravascular spaces has long been recognized; a recently proposed ‘glymphatic’ clearance mechanism additionally suggests that aquaporin-4 (AQP4) water channels facilitate convective transport through brain parenchyma. Here, the major experimental underpinnings of the glymphatic mechanism were re-examined by measurements of solute movement in mouse brain following intracisternal or intraparenchymal solute injection. We found that: (i) transport of fluorescent dextrans in brain parenchyma depended on dextran size in a manner consistent with diffusive rather than convective transport; (ii) transport of dextrans in the parenchymal extracellular space, measured by 2-photon fluorescence recovery after photobleaching, was not affected just after cardiorespiratory arrest; and (iii) Aqp4 gene deletion did not impair transport of fluorescent solutes from sub-arachnoid space to brain in mice or rats. Our results do not support the proposed glymphatic mechanism of convective solute transport in brain parenchyma.
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A recent report in AQP4 knockout rodents has shown that loss of AQP4 does not affect transport of fluorescent solutes from the subarachnoid space to the brain in rats and mice, suggesting that water production by astrocyte end feet does not control transport of solutes across brain extracellular spaces.
“A recent report in AQP4 knockout rodents has shown that loss of AQP4 does not affect transport of fluorescent solutes from subarachnoid space to brain in rats and mice, suggesting that water production by astrocyte end feet does not control transport of solutes across brain ECS (547).”
From Blood-Brain Barrier: From Physiology to Disease and Back (2019) · cites this paper as [547]
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Experimental — accuracy not yet measuredA later animal study found that interstitial flow could be explained by diffusion alone, independent of AQP4 status.
“Moreover, a later animal study utilized a similar set of experiments as those bringing evidence for a glymphatic system and found, contrary to the previous observations, that interstitial flow could be explained by diffusion alone, independent of AQP4 status (9).”
From Brain-wide glymphatic enhancement and clearance in humans assessed with MRI. (2018) · cites this paper as [9]
Animal studies have shown communication between the subarachnoid CSF space and perivascular compartments of the brain and spinal cord.
“Previous animal studies have shown communication between the subarachnoid cerebrospinal fluid (CSF) space and perivascular compartments of the brain (6-10) and spinal cord (11).”
From Brain-wide glymphatic enhancement and clearance in humans assessed with MRI. (2018) · cites this paper as [9]
Other recent studies did not support the proposed glymphatic mechanism, nor the convective, pressure-driven fluid flow of cerebrospinal fluid from para-arterial to paravenous spaces throughout the parenchymal extracellular spaces.
“Other recent studies (252, 255, 547, 555), however, did not support the proposed glymphatic mechanism, nor the convective, pressure-driven fluid flow of CSF from para-arterial to paravenous spaces throughout the parenchymal ECS (31, 30, 255, 296, 547, 255).”
From Blood-Brain Barrier: From Physiology to Disease and Back (2019) · cites this paper as [547]
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- Authors
- Smith, A. J., Yao, X., Dix, J. A., Jin, B.-J., & Verkman, A. S.
- Affiliations
- University of California, San Francisco (US)
- Corresponding author
- Smith, Alex J
- Verkman, Alan S
- Venue
- eLife
- Publisher
- eLife Sciences Publications, Ltd
- Published
- 2017-08-21
- Volume
- 6
- ISSN
- 2050-084X (ISSN-L)
- PMC
- PMC5578736
- OpenAlex
- W2746870493
- Cited by
- 362
- References
- 47
- Citations by year
- 2026: 32 · 2025: 36 · 2024: 35 · 2023: 32 · 2022: 43 · 2021: 43 · 2020: 47 · 2019: 47 · 2018: 38 · 2017: 9
- FWCI
- 18.86
- Type
- journal article
- Language
- en
- Open access
- gold
- doi.org
- Licence
- Locations
- eLife (publishedVersion, cc-by)
- PubMed Central (publishedVersion, cc-by)
- PubMed (publishedVersion)
- DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
- eScholarship (California Digital Library) (submittedVersion)
- eScholarship (California Digital Library) (submittedVersion, cc-by)
- Funders
- National Institutes of Health (EB00415, EY13574, DK72517, DK35124, DK101273)
- Guthy-Jackson Charitable Foundation
- Topics
- Life Sciences › Neuroscience › Cellular and Molecular Neuroscience › Cerebrospinal fluid and hydrocephalus
- Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Ion Transport and Channel Regulation
- Life Sciences › Neuroscience › Neurology › Barrier Structure and Function Studies
- Keywords
- glymphatic system, aquaporin-4, solute transport, brain parenchyma, diffusion, convection, extracellular space, dextran, FRAP, cardiac arrest, subarachnoid space, perivascular spaces, mouse brain
- MeSH terms
- Parenchymal Tissue, Animals, Biological Transport, Carbocyanines, Dextrans, Diffusion, Fluoresceins, Fluorescent Dyes, Heart Arrest, Injections, Intraventricular, Models, Biological, Subarachnoid Space, Gene Expression, Mice, Knockout, Fluorescence Recovery After Photobleaching, Mice, Rats, Aquaporin 4
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