Loss of aquaporin-4 results in glymphatic system dysfunction via brain-wide interstitial fluid stagnation

Abstract

The glymphatic system is a fluid transport network of cerebrospinal fluid (CSF) entering the brain along arterial perivascular spaces, exchanging with interstitial fluid (ISF), ultimately establishing directional clearance of interstitial solutes. CSF transport is facilitated by the expression of aquaporin-4 (AQP4) water channels on the perivascular endfeet of astrocytes. Mice with genetic deletion of AQP4 (AQP4 KO) exhibit abnormalities in the brain structure and molecular water transport. Yet, no studies have systematically examined how these abnormalities in structure and water transport correlate with glymphatic function. Here we used high-resolution 3D magnetic resonance (MR) non-contrast cisternography, diffusion-weighted MR imaging (MR-DWI) along with intravoxel-incoherent motion (IVIM) DWI, while evaluating glymphatic function using a standard dynamic contrast-enhanced MR imaging to better understand how water transport and glymphatic function is disrupted after genetic deletion of AQP4. AQP4 KO mice had larger interstitial spaces and total brain volumes resulting in higher water content and reduced CSF space volumes, despite similar CSF production rates and vascular density compared to wildtype mice. The larger interstitial fluid volume likely resulted in increased slow but not fast MR diffusion measures and coincided with reduced glymphatic influx. This markedly altered brain fluid transport in AQP4 KO mice may result from a reduction in glymphatic clearance, leading to enlargement and stagnation of fluid in the interstitial space. Overall, diffusion MR is a useful tool to evaluate glymphatic function and may serve as valuable translational biomarker to study glymphatics in human disease.

Glymphatic fluid transport is suppressed by the aquaporin‐4 inhibitor AER‐271

“…These brain regions exhibited the highest AQP4 expression in WT mice (Gomolka et al, 2023) which supports the notion that the drug is inhibiting AQP4, as areas with the most AQP4 were most affected by drug treatment. Our findings further validate that slow MR diffusion is a surrogate indicator of glymphatic function (Gomolka et al, 2023), and support the observation that AER-271 reduces glymphatic flow. Here we find the inhibitor reduced transmembrane water exchange leading to inhibition of ISF volume increases in WT mice, as measured using ADC.…”

“…The motion-correction was performed four times or until no further improvement in reference to the first image acquired using the first bvalue (b0), with visual inspection to confirm correctness. To assess the water molecules mobility within the brain parenchyma in the motioncorrected DWI images from each diffusion-encoding direction, we performed evaluation as presented previously (Gomolka et al, 2023).…”

(S)‐roscovitine, a CDK inhibitor, decreases cerebral edema and modulates AQP4 and α1‐syntrophin interaction on a pre‐clinical model of acute ischemic stroke

“…The polarized distribution of AQP4 in the brain is important for regulating water transport in both normal and pathological conditions (Kitchen et al, 2020; Manley et al, 2004; Nakada, 2014), with evidence indicating that vascular polarized AQP4 expression is more important for effective fluid transport, rather than total AQP4 levels in the tissue (Gomolka et al, 2023; Hablitz et al, 2020; Mestre et al, 2018; Sylvain et al, 2021). It is also noteworthy that the effect of (S)‐roscovitine took place in the cortex but did not impact the striatum where AQP4 is more diffusely expressed in astrocyte processes, suggesting that AQP4 expression is regulated in a region‐specific manner which may be related to the functional differences of astrocytes in different brain regions (Aoyama et al, 2012; Guo et al, 2021).…”

Multifaceted Roles of Aquaporins in the Pathogenesis of Alzheimer’s Disease

“…The requirement of AQP4 in the glymphatic system has been confirmed in multiple lines of AQP4 KO mice, as well as pharmacological blockade by the AQP4 inhibitor, TGN-020 [ 38 , 39 ]. It has recently been shown that AQP4 KO mice display an enlarged interstitial volume and decreased CSF volume without an altered CSF production rate [ 40 ]. The stagnation of ISF and enlargement of the ISF space might be a consequence of an overall reduction in glymphatic transport.…”