Impaired Glymphatic Function and Pulsation Alterations in a Mouse Model of Vascular Cognitive Impairment

ACKNOWLEDGMENTS Schematic diagrams in Figures 2, 8 are created withBiorender.com. FUNDING We gratefully acknowledge the grant support from the Alzheimer’s Society (152 (PG-157); 290 (AS-PG-15b-018); 228 (AS-DTC-2014-017), 314 (AS –PhD-16-006), and Alzheimer’s Research United Kingdom (ART-PG2010-3; ARUK-PG2013- 22; ARUK-PG2016B-6), and The University of Edinburgh Centre for Cognitive Ageing and Cognitive Epidemiology, part of the cross council Lifelong Health and Wellbeing Initiative (G0700704/84698). ML and JB are funded by an Alzheimer’s Society Scotland Doctoral Training Programme and RS Macdonald Trust. ML was also funded by a China Scholarship Council (CSC)/University of Edinburgh scholarship.Peer reviewedPublisher PD

Emerging roles for dynamic aquaporin-4 subcellular relocalization in CNS water homeostasis

Aquaporin channels facilitate bidirectional water flow in all cells and tissues. AQP4 is highly expressed in astrocytes. In the CNS, it is enriched in astrocyte endfeet, at synapses, and at the glia limitans, where it mediates water exchange across the blood-spinal cord and blood-brain barriers (BSCB/BBB), and controls cell volume, extracellular space volume, and astrocyte migration. Perivascular enrichment of AQP4 at the BSCB/BBB suggests a role in glymphatic function. Recently, we have demonstrated that AQP4 localization is also dynamically regulated at the subcellular level, affecting membrane water permeability. Ageing, cerebrovascular disease, traumatic CNS injury, and sleep disruption are established and emerging risk factors in developing neurodegeneration, and in animal models of each, impairment of glymphatic function is associated with changes in perivascular AQP4 localization. CNS oedema is caused by passive water influx through AQP4 in response to osmotic imbalances. We have demonstrated that reducing dynamic relocalization of AQP4 to the BSCB/BBB reduces CNS oedema, and accelerates functional recovery in rodent models. Given the difficulties in developing pore-blocking AQP4 inhibitors, targeting AQP4 subcellular localization opens up new treatment avenues for CNS oedema, neurovascular and neurodegenerative diseases, and provides a framework to address fundamental questions about water homeostasis in health and disease

Mapping the lymphatic system across body scales and expertise domains: A report from the 2021 National Heart, Lung, and Blood Institute workshop at the Boston Lymphatic Symposium

Enhancing our understanding of lymphatic anatomy from the microscopic to the anatomical scale is essential to discern how the structure and function of the lymphatic system interacts with different tissues and organs within the body and contributes to health and disease. The knowledge of molecular aspects of the lymphatic network is fundamental to understand the mechanisms of disease progression and prevention. Recent advances in mapping components of the lymphatic system using state of the art single cell technologies, the identification of novel biomarkers, new clinical imaging efforts, and computational tools which attempt to identify connections between these diverse technologies hold the potential to catalyze new strategies to address lymphatic diseases such as lymphedema and lipedema. This manuscript summarizes current knowledge of the lymphatic system and identifies prevailing challenges and opportunities to advance the field of lymphatic research as discussed by the experts in the workshop

Loss of cholinergic innervation differentially affects eNOS-mediated blood flow, drainage of Aβ and cerebral amyloid angiopathy in the cortex and hippocampus of adult mice

Vascular dysregulation and cholinergic basal forebrain degeneration are both early pathological events in the development of Alzheimer’s disease (AD). Acetylcholine contributes to localised arterial dilatation and increased cerebral blood flow (CBF) during neurovascular coupling via activation of endothelial nitric oxide synthase (eNOS). Decreased vascular reactivity is suggested to contribute to impaired clearance of β-amyloid (Aβ) along intramural periarterial drainage (IPAD) pathways of the brain, leading to the development of cerebral amyloid angiopathy (CAA). However, the possible relationship between loss of cholinergic innervation, impaired vasoreactivity and reduced clearance of Aβ from the brain has not been previously investigated. In the present study, intracerebroventricular administration of mu-saporin resulted in significant death of cholinergic neurons and fibres in the medial septum, cortex and hippocampus of C57BL/6 mice. Arterial spin labelling MRI revealed a loss of CBF response to stimulation of eNOS by the Rho-kinase inhibitor fasudil hydrochloride in the cortex of denervated mice. By contrast, the hippocampus remained responsive to drug treatment, in association with altered eNOS expression. Fasudil hydrochloride significantly increased IPAD in the hippocampus of both control and saporin-treated mice, while increased clearance from the cortex was only observed in control animals. Administration of mu-saporin in the TetOAPPSweInd mouse model of AD was associated with a significant and selective increase in Aβ40-positive CAA. These findings support the importance of the interrelationship between cholinergic innervation and vascular function in the aetiology and/or progression of CAA and suggest that combined eNOS/cholinergic therapies may improve the efficiency of Aβ removal from the brain and reduce its deposition as CAA

Analysis of convective and diffusive transport in the brain interstitium

Abstract Background Despite advances in in vivo imaging and experimental techniques, the nature of transport mechanisms in the brain remain elusive. Mathematical modelling verified using available experimental data offers a powerful tool for investigating hypotheses regarding extracellular transport of molecules in brain tissue. Here we describe a tool developed to aid in investigation of interstitial transport mechanisms, especially the potential for convection (or bulk flow) and its relevance to interstitial solute transport, for which there is conflicting evidence. Methods In this work, we compare a large body of published experimental data for transport in the brain to simulations of purely diffusive transport and simulations of combined convective and diffusive transport in the brain interstitium, incorporating current theories of perivascular influx and efflux. Results The simulations show (1) convective flow in the interstitium potentially of a similar magnitude to diffusive transport for molecules of interest and (2) exchange between the interstitium and perivascular space, whereby fluid and solutes may enter or exit the interstitium, are consistent with the experimental data. Simulations provide an upper limit for superficial convective velocity magnitude (approximately $$v$$ v  = 50 μm min−1), a useful finding for researchers developing techniques to measure interstitial bulk flow. Conclusions For the large molecules of interest in neuropathology, bulk flow may be an important mechanism of interstitial transport. Further work is warranted to investigate the potential for bulk flow

Epoxyeicosanoids as mediators of neurogenic vasodilation in cerebral vessels

Epoxyeicosatrienoic acids (EETs) are potent vasodilators produced from arachidonic acid by cytochrome P-450 (CYP) epoxygenases and metabolized to vicinal diols by soluble epoxide hydrolase (sEH). In the brain, EETs are produced by astrocytes and the vascular endothelium and are involved in the control of cerebral blood flow (CBF). Recent evidence, however, suggests that epoxygenases and sEH are present in perivascular vasodilator nerve fibers innervating the cerebral surface vasculature. In the present study, we tested the hypothesis that EETs are nerve-derived relaxing factors in the cerebral circulation. We first traced these fibers by retrograde labeling in the rat to trigeminal ganglia (TG) and sphenopalatine ganglia (SPG). We then examined the expression of CYP epoxygenases and sEH in these ganglia. RT-PCR and Western blot analysis identified CYP2J3 and CYP2J4 epoxygenase isoforms and sEH in both TG and SPG, and immunofluorescence double labeling identified CYP2J and sEH immunoreactivity in neuronal cell bodies of both ganglia. To evaluate the functional role of EETs in neurogenic vasodilation, we elicited cortical hyperemia by electrically stimulating efferent cerebral perivascular nerve fibers and by chemically stimulating oral trigeminal fibers with capsaicin. Cortical blood flow responses were monitored by laser-Doppler flowmetry. Local administration to the cortical surface of the putative EET antagonist 14,15-epoxyeicosa-5(Z)-enoic acid (30 μmol/l) attenuated CBF responses to electrical and chemical stimulation. These results suggest that EETs are produced by perivascular nerves and play a role in neurogenic vasodilation of the cerebral vasculature. The findings have important implications to such clinical conditions as migraine, vasospasm after subarachnoid hemorrhage, and stroke

The effect of aquaporin-4 mis-localization on Aβ deposition in mice

The reduced clearance of amyloid-β (Aβ) is thought to contribute to the development of the pathology associated with Alzheimer's disease (AD), which is characterized by the deposition of Aβ plaques. Previous studies have shown that Aβ is cleared via the glymphatic system, a brain-wide network of perivascular pathways that supports the exchange between cerebrospinal fluid and interstitial fluid within the brain. Such exchange is dependent upon the water channel aquaporin-4 (AQP4), localized at astrocytic endfeet. While prior studies have shown that both the loss and mislocalization of AQP4 slow Aβ clearance and promote Aβ plaque formation, the relative impact of the loss or mislocalization of AQP4 on Aβ deposition has never been directly compared. In this study, we evaluated how the deposition of Aβ plaques within the 5XFAD mouse line is impacted by either Aqp4 gene deletion or the loss of AQP4 localization in the α-syntrophin (Snta1) knockout mouse. We observed that both the absence (Aqp4 KO) and mislocalization (Snta1 KO) of AQP4 significantly increases the parenchymal Aβ plaque and microvascular Aβ deposition across the brain, when compared with 5XFAD littermate controls. Further, the mislocalization of AQP4 had a more pronounced impact on Aβ plaque deposition than did global Aqp4 gene deletion, perhaps pointing to a key role that mislocalization of perivascular AQP4 plays in AD pathogenesis