Astrocytic Endfeet Research Articles

Gaps in the wall of a perivascular space act as valves to produce a directed flow of cerebrospinal fluid: a hoop-stress model.

The flow of cerebrospinal fluid (CSF) along perivascular spaces (PVSs) is an important part of the brain's system for clearing metabolic waste. Astrocyte endfeet bound the PVSs of penetrating arteries, separating them from brain extracellular space. Gaps between astrocyte endfeet might provide a low-resistance pathway for fluid transport across the wall. Recent studies suggest that the astrocyte endfeet function as valves that rectify the CSF flow, producing the net flow observed in pial PVSs by changing the size of the gaps in response to pressure changes. In this study, we quantify this rectification based on three features of the PVSs: the quasi-circular geometry, the deformable endfoot wall, and the pressure oscillation inside. We provide an analytical model, based on the thin-shell hoop-stress approximation, and predict a pumping efficiency of about 0.4, which would contribute significantly to the observed flow. When we add the flow resistance of the extracellular space (ECS) to the model, we find an increased net flow during sleep, due to the known increase in ECS porosity (decreased flow resistance) compared to that in the awake state. We corroborate our analytical model with three-dimensional fluid-solid interaction simulations.

Role of aquaporin-4 polarization in extracellular solute clearance

Waste from the brain has been shown to be cleared via the perivascular spaces through the so-called glymphatic system. According to this model the cerebrospinal fluid (CSF) enters the brain in perivascular spaces of arteries, crosses the astrocyte endfoot layer, flows through the parenchyma collecting waste that is subsequently drained along veins. Glymphatic clearance is dependent on astrocytic aquaporin-4 (AQP4) water channels that are highly enriched in the endfeet. Even though the polarized expression of AQP4 in endfeet is thought to be of crucial importance for glymphatic CSF influx, its role in extracellular solute clearance has only been evaluated using non-quantitative fluorescence measurements. Here we have quantitatively evaluated clearance of intrastriatally infused small and large radioactively labeled solutes in mice lacking AQP4 (Aqp4–/–) or lacking the endfoot pool of AQP4 (Snta1–/–). We confirm that Aqp4–/– mice show reduced clearance of both small and large extracellular solutes. Moreover, we find that the Snta1–/– mice have reduced clearance only for the 500 kDa [3H]dextran, but not 0.18 kDa [3H]mannitol suggesting that polarization of AQP4 to the endfeet is primarily important for clearance of large, but not small molecules. Lastly, we observed that clearance of 500 kDa [3H]dextran increased with age in adult mice. Based on our quantitative measurements, we confirm that presence of AQP4 is important for clearance of extracellular solutes, while the perivascular AQP4 localization seems to have a greater impact on clearance of large versus small molecules.

Long-range inhibitory neurons mediate cortical neurovascular coupling

To meet the high energy demands of brain function, cerebral blood flow (CBF) parallels changes in neuronal activity by a mechanism known as neurovascular coupling (NVC). However, which neurons play a role in mediating NVC is not well understood. Here, we identify in mice and humans a specific population of cortical GABAergic neurons that co-express neuronal nitric oxide synthase and tachykinin receptor 1 (Tacr1). Through whole-tissue clearing, we demonstrate that Tacr1 neurons extend local and long-range projections across functionally connected cortical areas. We show that whisker stimulation elicited Tacr1 neuron activity in the barrel cortex through feedforward excitatory pathways. Additionally, through optogenetic experiments, we demonstrate that Tacr1 neurons are instrumental in mediating CBF through the relaxation of mural cells in a similar fashion to whisker stimulation. Finally, by electron microscopy, we observe that Tacr1 processes contact astrocytic endfeet. These findings suggest that Tacr1 neurons integrate cortical activity to mediate NVC.

TIMP1 protects against blood-brain barrier disruption after subarachnoid haemorrhage by inhibiting ubiquitination of astrocytic β1-integrin

BackgroundAstrocytes regulate blood-brain barrier (BBB) integrity, whereas subarachnoid haemorrhage (SAH) results in astrocyte dysregulation and BBB disruption. Here, we explored the involvement of tissue inhibitor of matrix metalloprotease-1 (TIMP1) in...

Multisensory gamma stimulation promotes glymphatic clearance of amyloid

The glymphatic movement of fluid through the brain removes metabolic waste1–4. Noninvasive 40 Hz stimulation promotes 40 Hz neural activity in multiple brain regions and attenuates pathology in mouse models of Alzheimer’s disease5–8. Here we show that multisensory gamma stimulation promotes the influx of cerebrospinal fluid and the efflux of interstitial fluid in the cortex of the 5XFAD mouse model of Alzheimer’s disease. Influx of cerebrospinal fluid was associated with increased aquaporin-4 polarization along astrocytic endfeet and dilated meningeal lymphatic vessels. Inhibiting glymphatic clearance abolished the removal of amyloid by multisensory 40 Hz stimulation. Using chemogenetic manipulation and a genetically encoded sensor for neuropeptide signalling, we found that vasoactive intestinal peptide interneurons facilitate glymphatic clearance by regulating arterial pulsatility. Our findings establish novel mechanisms that recruit the glymphatic system to remove brain amyloid.

Aquaporin-4 and transient receptor potential vanilloid 4 balance in early postnatal neurodevelopment.

In the adult brain, the water channel aquaporin-4 (AQP4) is expressed in astrocyte endfoot, in supramolecular assemblies, called "Orthogonal Arrays of Particles" (OAPs) together with the transient receptor potential vanilloid 4 (TRPV4), finely regulating the cell volume. The present study aimed at investigating the contribution of AQP4 and TRPV4 to CNS early postnatal development using WT and AQP4 KO brain and retina and neuronal stem cells (NSCs), as an in vitro model of astrocyte differentiation. Western blot analysis showed that, differently from AQP4 and the glial cell markers, TRPV4 was downregulated during CNS development and NSC differentiation. Blue native/SDS-PAGE revealed that AQP4 progressively organized into OAPs throughout the entire differentiation process. Fluorescence quenching assay indicated that the speed of cell volume changes was time-related to NSC differentiation and functional to their migratory ability. Calcium imaging showed that the amplitude of TRPV4 Ca2+ transient is lower, and the dynamics are changed during differentiation and suppressed in AQP4 KO NSCs. Overall, these findings suggest that early postnatal neurodevelopment is subjected to temporally modulated water and Ca2+ dynamics likely to be those sustaining the biochemical and physiological mechanisms responsible for astrocyte differentiation during brain and retinal development.

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

The glymphatic system transports cerebrospinal fluid (CSF) into the brain via arterial perivascular spaces and removes interstitial fluid from the brain along perivenous spaces and white matter tracts. This directional fluid flow supports the clearance of metabolic wastes produced by the brain. Glymphatic fluid transport is facilitated by aquaporin-4 (AQP4) water channels, which are enriched in the astrocytic vascular endfeet comprising the outer boundary of the perivascular space. Yet, prior studies of AQP4 function have relied on genetic models, or correlated altered AQP4 expression with glymphatic flow in disease states. Herein, we sought to pharmacologically manipulate AQP4 function with the inhibitor AER-271 to assess the contribution of AQP4 to glymphatic fluid transport in mouse brain. Administration of AER-271 inhibited glymphatic influx as measured by CSF tracer infused into the cisterna magna and inhibited increases in the interstitial fluid volume as measured by diffusion-weighted MRI. Furthermore, AER-271 inhibited glymphatic efflux as assessed by an in vivo clearance assay. Importantly, AER-271 did not affect AQP4 localization to the astrocytic endfeet, nor have any effect in AQP4 deficient mice. Since acute pharmacological inhibition of AQP4 directly decreased glymphatic flow in wild-type but not in AQP4 deficient mice, we foresee AER-271 as a new tool for manipulation of the glymphatic system in rodent brain.

Astrocyte Regulation of Cerebral Blood Flow in Health and Disease.

Astrocytes play an important role in controlling microvascular diameter and regulating local cerebral blood flow (CBF) in several physiological and pathological scenarios. Neurotransmitters released from active neurons evoke Ca2+ increases in astrocytes, leading to the release of vasoactive metabolites of arachidonic acid (AA) from astrocyte endfeet. Synthesis of prostaglandin E2 (PGE2) and epoxyeicosatrienoic acids (EETs) dilate blood vessels while 20-hydroxyeicosatetraenoic acid (20-HETE) constricts vessels. The release of K+ from astrocyte endfeet also contributes to vasodilation or constriction in a concentration-dependent manner. Whether astrocytes exert a vasodilation or vasoconstriction depends on the local microenvironment, including the metabolic status, the concentration of Ca2+ reached in the endfoot, and the resting vascular tone. Astrocytes also contribute to the generation of steady-state vascular tone. Tonic release of both 20-HETE and ATP from astrocytes constricts vascular smooth muscle cells, generating vessel tone, whereas tone-dependent elevations in endfoot Ca2+ produce tonic prostaglandin dilators to limit the degree of constriction. Under pathological conditions, including Alzheimer's disease, epilepsy, stroke, and diabetes, disruption of normal astrocyte physiology can compromise the regulation of blood flow, with negative consequences for neurological function.

Aquaporin-4-IgG-seropositive neuromyelitis optica spectrum disorders: progress of experimental models based on disease pathogenesis.

Neuromyelitis optica spectrum disorders are neuroinflammatory demyelinating disorders that lead to permanent visual loss and motor dysfunction. To date, no effective treatment exists as the exact causative mechanism remains unknown. Therefore, experimental models of neuromyelitis optica spectrum disorders are essential for exploring its pathogenesis and in screening for therapeutic targets. Since most patients with neuromyelitis optica spectrum disorders are seropositive for IgG autoantibodies against aquaporin-4, which is highly expressed on the membrane of astrocyte endfeet, most current experimental models are based on aquaporin-4-IgG that initially targets astrocytes. These experimental models have successfully simulated many pathological features of neuromyelitis optica spectrum disorders, such as aquaporin-4 loss, astrocytopathy, granulocyte and macrophage infiltration, complement activation, demyelination, and neuronal loss; however, they do not fully capture the pathological process of human neuromyelitis optica spectrum disorders. In this review, we summarize the currently known pathogenic mechanisms and the development of associated experimental models in vitro, ex vivo, and in vivo for neuromyelitis optica spectrum disorders, suggest potential pathogenic mechanisms for further investigation, and provide guidance on experimental model choices. In addition, this review summarizes the latest information on pathologies and therapies for neuromyelitis optica spectrum disorders based on experimental models of aquaporin-4-IgG-seropositive neuromyelitis optica spectrum disorders, offering further therapeutic targets and a theoretical basis for clinical trials.

Exposure to Low-Intensity Blast Increases Clearance of Brain Amyloid Beta.

The long-term effects of exposure to blast overpressure are an important health concern in military personnel. Increase in amyloid beta (Aβ) has been documented after non-blast traumatic brain injury (TBI) and may contribute to neuropathology and an increased risk for Alzheimer's disease. We have shown that Aβ levels decrease following exposure to a low-intensity blast overpressure event. To further explore this observation, we examined the effects of a single 37 kPa (5.4 psi) blast exposure on brain Aβ levels, production, and clearance mechanisms in the acute (24 h) and delayed (28 days) phases post-blast exposure in an experimental rat model. Aβ and, notably, the highly neurotoxic detergent soluble Aβ42 form, was reduced at 24 h but not 28 days after blast exposure. This reduction was not associated with changes in the levels of Aβ oligomers, expression levels of amyloid precursor protein (APP), or increase in enzymes involved in the amyloidogenic cleavage of APP, the β- and ϒ-secretases BACE1 and presenilin-1, respectively. The levels of ADAM17 α-secretase (also known as tumor necrosis factor α-converting enzyme) decreased, concomitant with the reduction in brain Aβ. Additionally, significant increases in brain levels of the endothelial transporter, low-density related protein 1 (LRP1), and enhancement in co-localization of aquaporin-4 (AQP4) to perivascular astrocytic end-feet were observed 24 h after blast exposure. These findings suggest that exposure to low-intensity blast may enhance endothelial clearance of Aβ by LRP1-mediated transcytosis and alter AQP4-aided glymphatic clearance. Collectively, the data demonstrate that low-intensity blast alters enzymatic, transvascular, and perivascular clearance of Aβ.

Tumor-specific migration routes of xenotransplanted human glioblastoma cells in mouse brain

The migration of neural progenitor cells (NPCs) to their final destination during development follows well-defined pathways, such as along blood vessels. Cells originating from the highly malignant tumor glioblastoma (GBM) seem to exploit similar routes for infiltrating the brain parenchyma. In this report, we have examined the migration of GBM cells using three-dimensional high-resolution confocal microscopy in brain tumors derived from eight different human GBM cell lines xenografted into immunodeficient mice. The primary invasion routes identified were long-distance migration along white matter tracts and local migration along blood vessels. We found that GBM cells in the majority of tumors (6 out of 8) did not exhibit association with blood vessels. These tumors, derived from low lamin A/C expressing GBM cells, were comparatively highly diffusive and invasive. Conversely, in 2 out of 8 tumors, we noted perivascular invasion and displacement of astrocyte end-feet. These tumors exhibited less diffusive migration, grew as solid tumors, and were distinguished by elevated expression of lamin A/C. We conclude that the migration pattern of glioblastoma is distinctly tumor cell-specific. Furthermore, the ability to invade the confined spaces within white matter tracts may necessitate low expression of lamin A/C, contributing to increased nuclear plasticity. This study highlights the role of GBM heterogeneity in driving the aggressive growth of glioblastoma.

A Closer Look at the Perivascular Unit in the Development of Enlarged Perivascular Spaces in Obesity, Metabolic Syndrome, and Type 2 Diabetes Mellitus.

The recently described perivascular unit (PVU) resides immediately adjacent to the true capillary neurovascular unit (NVU) in the postcapillary venule and contains the normal-benign perivascular spaces (PVS) and pathological enlarged perivascular spaces (EPVS). The PVS are important in that they have recently been identified to be the construct and the conduit responsible for the delivery of metabolic waste from the interstitial fluid to the ventricular cerebrospinal fluid for disposal into the systemic circulation, termed the glymphatic system. Importantly, the outermost boundary of the PVS is lined by protoplasmic perivascular astrocyte endfeet (pvACef) that communicate with regional neurons. As compared to the well-recognized and described neurovascular unit (NVU) and NVU coupling, the PVU is less well understood and remains an emerging concept. The primary focus of this narrative review is to compare the similarities and differences between these two units and discuss each of their structural and functional relationships and how they relate not only to brain homeostasis but also how they may relate to the development of multiple clinical neurological disease states and specifically how they may relate to obesity, metabolic syndrome, and type 2 diabetes mellitus. Additionally, the concept and importance of a perisynaptic astrocyte coupling to the neuronal synapses with pre- and postsynaptic neurons will also be considered as a perisynaptic unit to provide for the creation of the information transfer in the brain via synaptic transmission and brain homeostasis. Multiple electron microscopic images and illustrations will be utilized in order to help explain these complex units.

Single-molecule imaging of aquaporin-4 array dynamics in astrocytes.

Aquaporin-4 (AQP4) facilitates water transport across astrocytic membranes in the brain, forming highly structured nanometric arrays. AQP4 has a central role in regulating cerebrospinal fluid (CSF) circulation and facilitating the clearance of solutes from the extracellular space of the brain. Adrenergic signaling has been shown to modulate the volume of the extracellular space of the brain via AQP4 localized at the end-feet of astrocytes, but the mechanisms by which AQP4 regulates CSF inflow and outflow in the brain remain elusive. Using advanced imaging techniques, including super-resolution microscopy and single-molecule tracking, we investigated the hypothesis that β-adrenergic receptor activation induces cellular changes that regulate AQP4 array size and mobility, thus influencing water transport in the brain. We report that the β-adrenergic agonist, isoproterenol hydrochloride, decreases AQP4 array size and enhances its membrane mobility, while hyperosmotic conditions induce the formation of larger, less mobile arrays. These findings reveal that AQP4 arrays are dynamic structures, responsive to adrenergic signals and osmotic changes, highlighting a novel regulatory mechanism of water transport in the brain. Our results provide insights into the molecular control of CSF circulation and extracellular brain space volume, laying the groundwork for understanding the relationship between astrocyte water transport, sleep physiology, and neurodegeneration.

Analysis of potassium ion diffusion from neurons to capillaries: Effects of astrocyte endfeet geometry.

Neurovascular coupling (NVC) refers to a local increase in cerebral blood flow in response to increased neuronal activity. Mechanisms of communication between neurons and blood vessels remain unclear. Astrocyte endfeet almost completely cover cerebral capillaries, suggesting that astrocytes play a role in NVC by releasing vasoactive substances near capillaries. An alternative hypothesis is that direct diffusion through the extracellular space of potassium ions (K+ ) released by neurons contributes to NVC. Here, the goal is to determine whether astrocyte endfeet present a barrier to K+ diffusion from neurons to capillaries. Two simplified 2D geometries of extracellular space, clefts between endfeet, and perivascular space are used: (i) a source 1μm from a capillary; (ii) a neuron 15 μm from a capillary. K+ release is modelled as a step increase in [K+ ] at the outer boundary of the extracellular space. The time-dependent diffusion equation is solved numerically. In the first geometry, perivascular [K+ ] approaches its final value within 0.05 s. Decreasing endfeet cleft width or increasing perivascular space width slows the rise in [K+ ]. In the second geometry, the increase in perivascular [K+ ] occurs within 0.5s and is insensitive to changes in cleft width or perivascular space width. Predicted levels of perivascular [K+ ] are sufficient to cause vasodilation, and the rise time is within the time for flow increase in NVC. These results suggest that direct diffusion of K+ through the extracellular space is a possible NVC signalling mechanism.

Interplay between the glymphatic system and neurotoxic proteins in Parkinson's disease and related disorders: current knowledge and future directions.

Parkinson's disease is a common neurodegenerative disorder that is associated with abnormal aggregation and accumulation of neurotoxic proteins, including α-synuclein, amyloid-β, and tau, in addition to the impaired elimination of these neurotoxic protein. Atypical parkinsonism, which has the same clinical presentation and neuropathology as Parkinson's disease, expands the disease landscape within the continuum of Parkinson's disease and related disorders. The glymphatic system is a waste clearance system in the brain, which is responsible for eliminating the neurotoxic proteins from the interstitial fluid. Impairment of the glymphatic system has been proposed as a significant contributor to the development and progression of neurodegenerative disease, as it exacerbates the aggregation of neurotoxic proteins and deteriorates neuronal damage. Therefore, impairment of the glymphatic system could be considered as the final common pathway to neurodegeneration. Previous evidence has provided initial insights into the potential effect of the impaired glymphatic system on Parkinson's disease and related disorders; however, many unanswered questions remain. This review aims to provide a comprehensive summary of the growing literature on the glymphatic system in Parkinson's disease and related disorders. The focus of this review is on identifying the manifestations and mechanisms of interplay between the glymphatic system and neurotoxic proteins, including loss of polarization of aquaporin-4 in astrocytic endfeet, sleep and circadian rhythms, neuroinflammation, astrogliosis, and gliosis. This review further delves into the underlying pathophysiology of the glymphatic system in Parkinson's disease and related disorders, and the potential implications of targeting the glymphatic system as a novel and promising therapeutic strategy.

Dopamine and noradrenaline activate spinal astrocyte endfeet via D1-like receptors.

Astrocytes, the most abundant glial cells in the central nervous system, respond to a wide variety of neurotransmitters binding to metabotropic receptors. Here, we investigated the intracellular calcium responses of spinal cord astrocytes to dopamine and noradrenaline, two catecholamines released by specific descending pathways. In a slice preparation from the spinal cord of neonatal mice, puff application of dopamine resulted in intracellular calcium responses that remained in the endfeet. Noradrenaline induced stronger responses that also started in the endfeet but spread to neighbouring compartments. The intracellular calcium responses were unaffected by blocking neuronal activity or inhibiting various neurotransmitter receptors, suggesting a direct effect of dopamine and noradrenaline on astrocytes. The intracellular calcium responses induced by noradrenaline and dopamine were inhibited by the D1 receptor antagonist SCH 23390. We assessed the functional consequences of these astrocytic responses by examining changes in arteriole diameter. Puff application of dopamine or noradrenaline resulted in vasoconstriction of spinal arterioles. However, blocking D1 receptors or manipulating astrocytic intracellular calcium levels did not abolish the vasoconstrictions, indicating that the observed intracellular calcium responses in astrocyte endfeet were not responsible for the vascular changes. Our findings demonstrate a compartmentalized response of spinal cord astrocytes to catecholamines and expand our understanding of astrocyte-neurotransmitter interactions and their potential roles in the physiology of the central nervous system.

Lineage and geometry of MRI‐visible perivenous and periarterial spaces in white matter confirmed on immunohistochemical microscopy

AbstractBackgroundMRI‐visible perivascular spaces (mrPVS) in the brain, typically found in white matter and assumed to be pathologically enlarged though historically considered a benign finding, have been more recently associated with normal aging as well as with various disease states, including traumatic brain injury, sleep apnea, Alzheimer’s Disease (AD), Parkinson’s Disease, and cerebral amyloid angiopathy. One putative cause for their pathological enlargement has been damage to the parenchyma surrounding the PVS, bounded by the astrocytic endfoot, caused by an increase in vascular pressure or pulsatility. Arteries are more pulsatile than veins, and thus it is expected that mrPVS, presumed to be enlarged, will be predominately around arteries. This pre and post mortem MRI + immunohistochemistry (IHC) study sought to clarify the lineage and spatial distribution of PVS visible at the resolution of MRI in aged individuals who were cognitively normal (C), mildly cognitively impaired (MCI), had clinical signs of dementia and mixed AD and vascular disease at pathology (AD/VD), and had pathology‐confirmed AD only (AD).MethodDetailed imaging and IHC acquisition, as well as MRI and IHC processing and registration methods, can be found at [https://pubmed.ncbi.nlm.nih.gov/36343095/ and https://pubmed.ncbi.nlm.nih.gov/36054513/]. Each MRI was inspected for PVS by an experienced rater (JH) and trichrome and smooth muscle actin (SMA) IHC of the corresponding vessels were judged to be arteries or veins based on SMA positivity and morphology (DLS).Result35 mrPVS over 19 subjects’ post mortem MRIs were unequivocally identified on trichrome and SMA. Of these, 23 surrounded arteries (SMA+, Fig. 1), and 12 surrounded veins (faint/thin/uneven SMA, Fig. 2). Mean distance (range) to the cortical ribbon from mrPVS surrounding arteries and veins were 2.8mm[0‐8mm] and 5.4mm[2‐14mm], respectively.ConclusionPVS large enough to be visible on MRI are more frequently arterial if nearer the cortical ribbon; mrPVS nearer the ventricle are more likely to be collecting veins. As collecting veins nearer the ventricles and arterioles in cortex tend to have higher pulse pressures, this finding is congruent with the hypothesis that PVS may enlarge as a result of higher vascular pressures common in older age and cerebrovascular disease.

Lineage and geometry of MRI‐visible perivenous and periarterial spaces in white matter confirmed on immunohistochemical microscopy

AbstractBackgroundMRI‐visible perivascular spaces (mrPVS) in the brain, typically found in white matter and assumed to be pathologically enlarged though historically considered a benign finding, have been more recently associated with normal aging as well as with various disease states, including traumatic brain injury, sleep apnea, Alzheimer’s Disease (AD), Parkinson’s Disease, and cerebral amyloid angiopathy. One putative cause for their pathological enlargement has been damage to the parenchyma surrounding the PVS, bounded by the astrocytic endfoot, caused by an increase in vascular pressure or pulsatility. Arteries are more pulsatile than veins, and thus it is expected that mrPVS, presumed to be enlarged, will be predominately around arteries. This pre and post mortem MRI + immunohistochemistry (IHC) study sought to clarify the lineage and spatial distribution of PVS visible at the resolution of MRI in aged individuals who were cognitively normal (C), mildly cognitively impaired (MCI), had clinical signs of dementia and mixed AD and vascular disease at pathology (AD/VD), and had pathology‐confirmed AD only (AD).MethodDetailed imaging and IHC acquisition, as well as MRI and IHC processing and registration methods, can be found at [https://pubmed.ncbi.nlm.nih.gov/36343095/ and https://pubmed.ncbi.nlm.nih.gov/36054513/]. Each MRI was inspected for PVS by an experienced rater (JH) and trichrome and smooth muscle actin (SMA) IHC of the corresponding vessels were judged to be arteries or veins based on SMA positivity and morphology (DLS).Result35 mrPVS over 19 subjects’ post mortem MRIs were unequivocally identified on trichrome and SMA. Of these, 23 surrounded arteries (SMA+, Fig. 1), and 12 surrounded veins (faint/thin/uneven SMA, Fig. 2). Mean distance (range) to the cortical ribbon from mrPVS surrounding arteries and veins were 2.8mm[0‐8mm] and 5.4mm[2‐14mm], respectively.ConclusionPVS large enough to be visible on MRI are more frequently arterial if nearer the cortical ribbon; mrPVS nearer the ventricle are more likely to be collecting veins. As collecting veins nearer the ventricles and arterioles in cortex tend to have higher pulse pressures, this finding is congruent with the hypothesis that PVS may enlarge as a result of higher vascular pressures common in older age and cerebrovascular disease.

Associations between glymphatic function and BBB water exchange rate and perfusion in a Latinx cohort

AbstractBackgroundThe glymphatic system is thought to clear waste products from the brain by facilitating the exchange of interstitial fluid (ISF) with cerebrospinal fluid (CSF) through aquaporin‐4 (AQP4) water channels present in astrocyte endfeet. A recent non‐invasive MRI method, diffusion tensor image analysis along the perivascular space (DTI‐ALPS), has been proposed for evaluating brain clearance function and has shown reduced glymphatic function in various neurological disorders such as Alzheimer’s disease. However, the role of microvascular perfusion and blood‐brain barrier (BBB) permeability in the glymphatic system remains unclear. In this study, we used a novel diffusion‐prepared arterial spin labeling (DP‐ASL) MRI technique to measure BBB water exchange rate (kw) and cerebral blood flow (CBF) and studied their relationships with the DTI‐ALPS score.Method44 Latinx subjects (20 male; age = 65.6±5.6 years) were recruited from the USC MarkVCID study. Imaging parameters for DP‐ASL were: resolution = 3.5×3.5×8mm3, 10 slices, scan time 10 mins. kw was calculated from the ratio of intra‐ and extra‐vascular perfusion signals with total‐general‐variation (TGV) regularized single‐pass approximation (SPA) modeling and CBF was calculated from PLD = 1800ms data using one‐compartment Kety model. kw and CBF maps were normalized into the MNI space and regional analysis was performed in the frontal/temporal/parietal lobe, and subcortical regions. Association between ALPS score and CBF/kw values were studied by linear regressions. P<0.05 was considered as significant.ResultFig.1 shows kw and CBF maps from three representative subjects with increasing ALPS scores (top to bottom). ALPS was significantly correlated with kw in MTL (β = 1.7e‐3,P = 0.04) and a positive trend was observed between ALPS and whole brain average kw (P = 0.11). While significant correlations were found between ALPS and CBF in multiple brain regions including frontal lobe (β = 7e‐3,P = 0.009), temporal lobe (β = 7e‐3,P = 0.002), parietal lobe (β = 4e‐3,P = 0.03), MTL (β = 8e‐3,P = 0.03), ACC (β = 6e‐3,P = 0.005), PCC (β = 4e‐3,P = 0.02), precuneus (β = 4e‐3,P = 0.04) and whole brain (β = 8e‐3,P = 0.01). Fig.2 shows scatter plots of ALPS versus kw (A) and CBF in MTL and whole brain, respectively.ConclusionThis is the first study in Latinx population demonstrating positive correlations between ALPS score and both CBF and kw, suggesting a possible involvement of blood flow and BBB water exchange rate in the glymphatic system.

Alterations of Astrocytic Aquaporin 4 expression and distribution in Down Syndrome with Alzheimer’s Disease onset

AbstractBackgroundAstrocytic Aquaporin 4 (AQP4) is a water channel that is typically expressed in astrocytic endfeet, associated with the blood brain barrier (BBB). This association is thought to facilitate the removal of waste products (e.g. beta amyloid (Aβ)) via the ‘glymphatic system’. With Aβ plaque deposition in Alzheimer’s Disease (AD), AQP4 becomes dysregulated observed as a shift in distribution from astrocytic endfeet to astrocytic processes, resulting in inefficient Aβ removal. Here, AQP4 expression and distribution within astrocytes was characterized via immunohistochemistry in human brain tissue from people with Down Syndrome without AD (DS) and with AD (DSAD). We hypothesized that AQP4 protein level would be higher and distributed along astrocytic processes as opposed to the BBB in DSAD relative to DS.MethodAQP4 was evaluated by immunohistochemistry and quantified using QuPath (% loads, positive pixel count) for both frontal (FC) and occipital cortex (OC) in the following groups: DS (n = 17, age range = 0‐51) and DSAD (n = 20, age range = 46‐70). In addition, AQP4 distribution was classified by a blinded observer who annotated distinct AQP4 positive astrocytic morphologies, listed as follows: In WM, ‘Star,’ ‘Webbed,’ and ‘Stippled,’ and in GM, ‘Bushy,’ ‘Branched,’ ‘Globular,’ ‘Endfoot/Vascular Associated.’ResultWe show that AQP4 protein staining was higher in GM DS with AD neuropathology than in DS without AD neuropathology for both FC and OC (FC p = 0.0007; OC p = 0.0034). In addition, we observed a shift from ‘star’ shaped distribution within DS astrocytes to more ‘webbed’ and ‘stippled’ shapes in DSAD; this was not observed in OC. We found a shift in GM morphologies from ‘bushy’ morphology of AQP4 immunostaining in DS to more ‘Branched and Globular’ shapes in DSAD in OC whereas there is a shift from a ‘Bushy’ morphology in DS to more ‘Endfoot/Vascular Associated’ morphologies in the DSAD FC.ConclusionAQP4 expression is higher in DSAD than DS and appears to show a spectrum of astrocyte morphologies with distinct patterns based upon spatial location (FC/OC and GM/WM). Taken together, AQP4 dysfunction and shift to abnormal spatial location may reduce Aβ clearance through the glymphatic system, leading to enhanced plaque formation in DSAD.