Fluid transport in the brain

Cerebrospinal Fluid Pressure Measurement and Infusion Studies Using Lumbar Puncture

Lumbar puncture provides an easy way of accessing the subarachnoid space. Measuring of the opening cerebrospinal fluid pressure is the most commonly used method of evaluating intracranial pressure but provides basic snapshot information only. Further insights into cerebrospinal fluid dynamics can be obtained through infusion studies, which rely on measurement of the degree of pressure change in response to addition of fluid volume into the subarachnoid space. The authors describe applications of these 2 techniques pertinent to a practicing neuroradiologist, who may be asked to assist with fluoroscopy-guided lumbar puncture in patients with increased body mass index or difficult spine anatomy.

Association of Glymphatic Function With Clinical Characteristics in Patients With Clinical and Asymptomatic Creutzfeldt-Jakob Disease

BACKGROUND AND OBJECTIVES

Abnormal glymphatic system-related proteins have been identified in a small-scale pathologic study of patients with Creutzfeldt-Jakob disease (CJD). However, it remains unclear whether glymphatic dysfunction occurs in vivo in patients with CJD and whether this decline begins during the preclinical stage. This study aimed to investigate the relationship between glymphatic dysfunction and clinical characteristics in patients with CJD, as well as potential glymphatic impairment in preclinical CJD.

METHODS

This prospective cohort study recruited patients with CJD and healthy controls (HCs) from the Department of Neurology at Xuanwu Hospital, Capital Medical University, Beijing, China, from 2018 to 2022. In addition, a family with preclinical genetic CJD carrying the G114V pathogenic variant was followed over 6 years with 3 evaluations. All participants underwent diffusion tensor imaging along the perivascular space (DTI-ALPS) to measure glymphatic function in vivo and 18F-fludeoxyglucose-PET to identify CJD-related metabolic patterns. Associations between the DTI-ALPS index and Medical Research Council Prion Disease Rating Scale (MRC-PDRS) score were evaluated using multiple linear regression.

RESULTS

We enrolled 35 patients with CJD (mean age 59.6 ± 10.7 years, 40% female, with the time from onset to glymphatic dysfunction assessment averaging 39% of the total disease course), 28 age-matched and sex-matched HCs, and a family with preclinical genetic CJD consisting of 7 carriers and 7 noncarriers. Patients with CJD exhibited lower DTI-ALPS values compared with HCs (p < 0.001). Partial correlation analyses revealed significant correlations between the DTI-ALPS index and MRC-PDRS score (r = 0.346, p = 0.049) and disease progression (r = -0.468, p = 0.006), but not with disease duration or cognitive severity after adjusting for age and sex. Multivariate linear analysis demonstrated that poorer MRC-PDRS scores (β = 0.702, p = 0.014) were associated with a lower DTI-ALPS index. The DTI-ALPS index of asymptomatic G114V carriers showed no significant difference compared with noncarriers. However, a preclinical CJD case exhibited an 8.2% decrease in the DTI-ALPS index 3.3 years before onset. No significant correlation was found between regional metabolic standardized uptake value ratios and DTI-ALPS index.

DISCUSSION

Our study indicates that glymphatic dysfunction is associated with CJD severity and disease progression. Glymphatic dysfunction may occur in the preclinical stage, but these findings should be interpreted with caution because they are based on individual findings.

Lipid nanoparticles and transcranial focused ultrasound enhance the delivery of SOD1 antisense oligonucleotides to the murine brain for ALS therapy

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease with extremely limited therapeutic options. One key pathological feature of ALS is the abnormal accumulation of misfolded proteins within motor neurons. Hence, reducing the burden of misfolded protein has emerged as a promising therapeutic approach. Antisense oligonucleotides (ASOs) have the potential to effectively silence proteins with gain-of-function mutations, such as superoxide dismutase 1 (SOD1). However, ASO delivery to the central nervous system (CNS) is hindered by poor blood-brain barrier (BBB) penetration and the invasiveness of intrathecal administration. In the current study, we demonstrate effective systemic delivery of a next-generation SOD1 ASO (Tofersen) into the brain of wildtype and G93A-SOD1 transgenic C57BL/6 mice using calcium phosphate lipid nanoparticles (CaP lipid NPs). We show that transcranial focused ultrasound (FUS) with intravenously administered microbubbles can significantly enhance ASO-loaded nanoparticle delivery into the mouse brain. Magnetic resonance imaging (MRI) and immunohistological analysis showed reduced SOD1 expression in the FUS-exposed brain regions and increased motor neuron count in the spinal cord of treated mice suggesting decreased motor neuron degeneration. Importantly, the BBB opening was transient without evidence of structural changes, neuroinflammation or damage to the brain tissue, indicating that the treatment is well tolerated. Overall, our results highlight FUS-assisted nanoparticle delivery of ASOs as a promising non-invasive therapeutic strategy for the treatment of ALS and CNS diseases more broadly.

A brief history of the development of transcranial tissue Doppler ultrasound

This article documents the early development of the first transcranial Doppler (TCD)-based ultrasound system for continuous monitoring of brain tissue pulsations (BTPs). Transcranial tissue Doppler (TCTD) uses a lightweight, wearable single-element ultrasound probe to track tissue motion perpendicular to the skin's surface, providing tissue displacement estimates along a single beam line. Feasibility tests using an adapted TCD system confirmed that brain tissue motion data can be obtained from existing TCD hardware. Brain Tissue Velocimetry (Brain TV), a TCTD data acquisition system, was then developed to provide a lightweight and portable means of continuously recording TCTD data in real-time. Brain TV measurements are synchronized to a 3-lead electrocardiogram and can be recorded alongside other physiological measurements, such as blood pressure, heart rate and end-tidal carbon dioxide. We have shown that Brain TV is able to record BTPs from sample depths ranging from 22 to 80 mm below the probe's surface and from multiple positions on the head. Studies in healthy volunteers, stroke patients and ultrasound phantom brain models demonstrate how TCTD might provide insights into the relationships between physiological measurements and brain tissue motion and show promise for rapid clinical assessment and continuous monitoring of BTPs.

Immune system activation and cognitive impairment in arterial hypertension

Chronic arterial hypertension disrupts the integrity of the cerebral microvasculature, doubling the risk of age-related dementia. Despite sufficient antihypertensive therapy in still a significant proportion of individuals blood pressure lowering alone does not preserve cognitive health. Accumulating evidence highlights the role of inflammatory mechanisms in the pathogenesis of hypertension. In this review, we introduce a temporal framework to explore how early immune system activation and interactions at neurovascular-immune interfaces pave the way to cognitive impairment. The overall paradigm suggests that prohypertensive stimuli induce mechanical stress and systemic inflammatory responses that shift peripheral and meningeal immune effector mechanisms toward a proinflammatory state. Neurovascular-immune interfaces in the brain include a dysfunctional blood-brain barrier, crossed by peripheral immune cells; the perivascular space, in which macrophages respond to cerebrospinal fluid- and blood-derived immune regulators; and the meningeal immune reservoir, particularly T cells. Immune responses at these interfaces bridge peripheral and neurovascular unit inflammation, directly contributing to impaired brain perfusion, clearance of toxic metabolites, and synaptic function. We propose that deep immunophenotyping in biofluids together with advanced neuroimaging could aid in the translational determination of sequential immune and brain endotypes specific to arterial hypertension. This could close knowledge gaps on how and when immune system activation transits into neurovascular dysfunction and cognitive impairment. In the future, targeting specific immune mechanisms could prevent and halt hypertension disease progression before clinical symptoms arise, addressing the need for new interventions against one of the leading threats to cognitive health.

Relaxation-exchange magnetic resonance imaging (REXI): a non-invasive imaging method for evaluating trans-barrier water exchange in the choroid plexus

BACKGROUND

The choroid plexus (CP) plays a crucial role in cerebrospinal fluid (CSF) production and brain homeostasis. However, non-invasive imaging techniques to assess its function remain limited. This study was conducted to develop a novel, contrast-agent-free MRI technique, termed relaxation-exchange magnetic resonance imaging (REXI), for evaluating CP-CSF water transport, a potential biomarker of CP function.

METHODS

REXI utilizes the inherent and large difference in magnetic resonance transverse relaxation times (T2s) between CP tissue (e.g., blood vessels and epithelial cells) and CSF. It uses a filter block to remove most CP tissue magnetization (shorter T2), a mixing block for CP-CSF water exchange with mixing time tm, and a detection block with multi-echo acquisition to determine the CP/CSF component fraction after exchange. The REXI pulse sequence was implemented on a 9.4 T preclinical MRI scanner. For validation of REXI's ability to measure exchange, we conducted preliminary tests on urea-water proton-exchange phantoms with various pH levels. We measured the steady-state water efflux rate from CP to CSF in rats and tested the sensitivity of REXI in detecting CP dysfunction induced by the carbonic anhydrase inhibitor acetazolamide.

RESULTS

REXI pulse sequence successfully captured changes in the proton exchange rate (from short-T2 component to long-T2 component [i.e., ksl]) of urea-water phantoms at varying pH, demonstrating its sensitivity to exchange processes. In rat CP, REXI significantly suppressed the CP tissue signal, reducing the short-T2 fraction (fshort) from 0.44 to 0.23 (p < 0.0001), with significant recovery to 0.28 after a mixing time of 400 ms (p = 0.014). The changes in fshort at various mixing times can be accurately described by a two-site exchange model, yielding a steady-state water efflux rate from CP to CSF (i.e., kbc) of 0.49 s-1. A scan-rescan experiment demonstrated that REXI had excellent reproducibility in measuring kbc (intraclass correlation coefficient = 0.90). Notably, acetazolamide-induced CSF reduction resulted in a 66% decrease in kbc within rat CP.

CONCLUSIONS

This proof-of-concept study demonstrates the feasibility of REXI for measuring trans-barrier water exchange in the CP, offering a promising biomarker for future assessments of CP function.

Blood-brain barrier permeable β-blockers association with Alzheimer's disease cerebrospinal fluid biomarkers levels in non-demented individuals

β-blockers that easily cross the blood-brain barrier (BBB) seem to diminish the risk of Alzheimer's disease (AD), hypothetically facilitating waste clearance. However, their effect on AD pathophysiological markers is unknown. We compared cerebrospinal fluid (CSF) AD biomarker levels among non-demented individuals taking low, intermediate, or high BBB permeable β-blockers in two samples (ADNI: n = 216; EPAD: n = 79). We found that CSF amyloid-β levels were higher in individuals taking highly permeable β-blockers in the ADNI sample. This result was not replicated in EPAD, in which diminished levels of pTau181 and tTau were observed. These data suggest that β-blockers may impact AD pathophysiology.

Blood osmolytes such as sugar can drive brain fluid flows in a poroelastic model

The glymphatic system of fluid flow through brain tissue may clear amyloid-β during sleep and as such underlie the need for sleep. Dysfunctional glymphatic transport has been implicated in pathological conditions ranging from stroke and dementia to psychiatric illnesses. To date, the fastest observed in-vivo brain flows have been reported after the manipulation of blood osmotic pressures. Surprisingly, the brain seems to shrink while receiving more influx. Though influx of an incompressible fluid might expand the tissue, no physical theory for these observations has been proposed. We here present a minimal mathematical model of brain pressure, deformation, and fluid flows due to vascular osmotic pressures. The model is based on Darcy flow, linear poroelasticity theory and conservation of mass. We propose that a screened Poisson equation holds for interstitial pressure because vascular filtration corresponds to fluid divergence. The model resolves the apparent paradox of combined fluid influx with tissue shrinkage by showing that fluid absorption into the blood can drive both. In this model, small glucose concentration differences between plasma and brain can drive brain flow velocities observed in recent in-vivo assays. Osmosis may therefore drive brain fluid flow under physiological conditions and provide an explanation for the known correlations between diabetes and dementia.

Reimagining the meninges from a neuroimmune perspective: a boundary, but not peripheral

Recent advances in neuroscience have transformed our understanding of the meninges, the layers surrounding the central nervous system (CNS). Two key findings have advanced our understanding: researchers identified cranial bone marrow as a reservoir for meningeal immune cells, and rediscovered a brain lymphatic system. Once viewed merely as a protective barrier, the meninges are now recognized as a dynamic interface crucial for neuroimmune interactions. This shift in perspective highlights their unique role in maintaining CNS balance, shaping brain development, and regulating responses to injury and disease. This review synthesizes the latest insights into meningeal anatomy and function, with a focus on newly identified structures such as dural-associated lymphoid tissues (DALT) and arachnoid cuff exit (ACE) points. We also examine the diverse immune cell populations within the meninges and their interactions with the CNS, underscoring the emerging view of the meninges as active participants in brain immunity. Finally, we outline critical unanswered questions about meningeal immunity, proposing directions for future research. By addressing these knowledge gaps, we aim to deepen our understanding of the meninges' role in brain health and disease, potentially paving the way for novel therapeutic approaches.

Characterising the power spectrum dynamics of the non-REM to REM sleep transition

The transition from non-rapid eye movement (NREM) to rapid eye movement (REM) sleep is considered a transitional or intermediate stage (IS), characterised by high amplitude spindles in the frontal cortex and theta activity in the occipital cortex. Early reports in rats showed an IS lasting from 1 to 5 s, but recent studies suggested a longer duration of this stage of up to 20 s. To further characterise the IS, we analysed its spectral characteristics on electrocorticogram (ECoG) recordings of the olfactory bulb (OB), primary motor (M1), primary somatosensory (S1), and secondary visual cortex (V2) in 12 Wistar male adult rats. By comparing the IS with consolidated NREM/REM epochs, our results reveal that the IS has specific power spectral patterns that fall out of the NREM and REM sleep state power distribution. Specifically, the main findings were that sigma (11-16 Hz) power in OB, M1, S1, and V2 increased during the IS compared with NREM and REM sleep, which started first in the frontal part of the brain (OB -54 s, M1 -53 s) prior to the last spindle occurrence. The beta band (17-30 Hz) power showed a similar pattern to that of the sigma band, starting -54 s before the last spindle occurrence in the M1 cortex. Notably, sigma infraslow coupling (~0.02 Hz) increased during the IS but occurred at a slower frequency (~0.01 Hz) compared with NREM sleep. Thus, we argue that the NREM to REM transition contains its own local spectral profile, in accordance with previous reports, and is more extended than described previously.

Astrocyte aquaporin mediates a tonic water efflux maintaining brain homeostasis

Brain water homeostasis not only provides a physical protection, but also determines the diffusion of chemical molecules key for information processing and metabolic stability. As a major type of glia in brain parenchyma, astrocytes are the dominant cell type expressing aquaporin water channel. How astrocyte aquaporin contributes to brain water homeostasis in basal physiology remains to be understood. We report that astrocyte aquaporin 4 (AQP4) mediates a tonic water efflux in basal conditions. Acute inhibition of astrocyte AQP4 leads to intracellular water accumulation as optically resolved by fluorescence-translated imaging in acute brain slices, and in vivo by fiber photometry in mobile mice. We then show that aquaporin-mediated constant water efflux maintains astrocyte volume and osmotic equilibrium, astrocyte and neuron Ca2+ signaling, and extracellular space remodeling during optogenetically induced cortical spreading depression. Using diffusion-weighted magnetic resonance imaging (DW-MRI), we observed that in vivo inhibition of AQP4 water efflux heterogeneously disturbs brain water homeostasis in a region-dependent manner. Our data suggest that astrocyte aquaporin, though bidirectional in nature, mediates a tonic water outflow to sustain cellular and environmental equilibrium in brain parenchyma.

Enhancing glymphatic fluid transport by pan-adrenergic inhibition suppresses epileptogenesis in male mice

Epileptogenesis is the process whereby the previously normally functioning brain begins to generate spontaneous, unprovoked seizures. Status epilepticus (SE), which entails a massive release of neuronal glutamate and other neuroactive substances, is one of the best-known triggers of epileptogenesis. We here asked whether pharmacologically promoting glymphatic clearance during or after SE is beneficial and able to attenuate the subsequent epileptogenesis. We induced SE in adult male mice by intrahippocampal kainic acid (KA) infusion. Acute administration of a cocktail of adrenergic receptor antagonists (propranolol, prazosin, and atipamezole: PPA), enhanced glymphatic flow and effectively reduced the severity of spontaneous seizures in the chronic phase. The PPA treatment also reduced reactive gliosis and inhibited the loss of polarized expression of AQP4 water channels in the vascular endfeet of astrocytes. Administration of PPA after cessation of SE (30 hours post KA) also effectively suppressed epileptogenesis and improved outcome. Conversely, mice with constitutively low glymphatic transport due to genetic deletion of the aquaporin 4 (AQP4) water channel showed exacerbation of KA-induced epileptogenesis. We conclude that the pharmacological modulation of glymphatic fluid transport may represent a potential strategy to dampen epileptogenesis and the occurrence of spontaneous seizures following KA-induced SE.

Nasal lymphatic obstruction of CSF drainage as a possible cause of Alzheimer's disease and dementia

Alzheimer's disease, the most common form of dementia among older adults, slowly destroys memory and thinking skills. In recent years, scientists have made tremendous progress in understanding Alzheimer's disease, still, they do not yet fully understand what causes the disease. This article proposes a novel etiology for Alzheimer's disease. Our hypothesis developed from a review of nuclear medicine scans, in which the authors observed a significant increase in nasal turbinate vasodilation and blood pooling in patients with hypertension, sleep apnea, diabetes and/or obesity, all risk factors for Alzheimer's disease. The authors propose that nasal turbinate vasodilation and resultant blood pooling lead to the obstruction of normal nasal lymphatic clearance of cerebrospinal fluid and its waste products from the brain. The nasal turbinate vasodilation, due to increased parasympathetic activity, occurs alongside the well-established increased sympathetic activity of the cardiovascular system as seen in patients with hypertension. The increased parasympathetic activity is likely due to an autonomic imbalance secondary to the increase in worldwide consumption of highly processed food associated with dysregulation of the glucose regulatory system. The authors' hypothesis offers a novel mechanism and a new paradigm for the etiology of Alzheimer's disease and helps explain the rapid worldwide rise in the disease and other dementias which are expected to double in the next 20 years. This new paradigm provides compelling evidence for the modulation of the parasympathetic nervous system as a novel treatment strategy for Alzheimer's disease and other degenerative brain diseases, specifically targeting nasal turbinate lymphatic flow.

Astrocyte TrkB promotes brain injury and edema formation in ischemic stroke

Following ischemic stroke astrocytes undergo rapid molecular and functional changes that may accentuate tissue damage. In this study we identified the neurotrophin receptor TrkB in astrocytes as a key promoter of acute CNS injury in ischemic stroke. In fact, TrkB protein was strongly upregulated in astrocytes after human and experimental stroke, and transgenic mice lacking astrocyte TrkB displayed significantly smaller lesion volume, lower brain atrophy and better motor performance than control animals after transient middle cerebral artery occlusion. Neuropathological studies evidenced that edema directly correlated with astrogliosis and was limited in transgenic mice. Importantly, adaptive levels of the water channel AQP4 was astrocyte TrkB-dependent as AQP4 upregulation after stroke did not occur in mice lacking astrocyte TrkB. In vitro experiments with wild-type and/or TrkB-deficient astrocytes highlighted TrkB-dependent upregulation of AQP4 via activation of HIF1-alpha under hypoxia. Collectively, our observations indicate that TrkB signaling in astrocytes contributes to the development of edema and worsens cerebral ischemia.

Waste clearance shapes aging brain health

Brain health is intimately connected to fluid flow dynamics that cleanse the brain of potentially harmful waste material. This system is regulated by vascular dynamics, the maintenance of perivascular spaces, neural activity during sleep, and lymphatic drainage in the meningeal layers. However, aging can impinge on each of these layers of regulation, leading to impaired brain cleansing and the emergence of various age-associated neurological disorders, including Alzheimer's and Parkinson's diseases. Understanding the intricacies of fluid flow regulation in the brain and how this becomes altered with age could reveal new targets and therapeutic strategies to tackle age-associated neurological decline.

Endothelial Piezo1 channel mediates mechano-feedback control of brain blood flow

Hyperemia in response to neural activity is essential for brain health. A hyperemic response delivers O2 and nutrients, clears metabolic waste, and concomitantly exposes cerebrovascular endothelial cells to hemodynamic forces. While neurovascular research has primarily centered on the front end of hyperemia-neuronal activity-to-vascular response-the mechanical consequences of hyperemia have gone largely unexplored. Piezo1 is an endothelial mechanosensor that senses hyperemia-associated forces. Using genetic mouse models and pharmacologic approaches to manipulate endothelial Piezo1 function, we evaluated its role in blood flow control and whether it impacts cognition. We provide evidence of a built-in brake system that sculpts hyperemia, and specifically show that Piezo1 activation triggers a mechano-feedback system that promotes blood flow recovery to baseline. Further, genetic Piezo1 modification led to deficits in complementary memory tasks. Collectively, our findings establish a role for endothelial Piezo1 in cerebral blood flow regulation and a role in its behavioral sequelae.

Alzheimer's disease and sleep disorders: A bidirectional relationship

Alzheimer's disease (AD) is the most prevalent dementia, pathologically featuring abnormal accumulation of amyloid-β (Aβ) and hyperphosphorylated tau, while sleep, divided into rapid eye movement sleep (REM) and nonrapid eye movement sleep (NREM), plays a key role in consolidating social and spatial memory. Emerging evidence has revealed that sleep disorders such as circadian disturbances and disruption of neuronal rhythm activity are considered as both candidate risks and consequence of AD, suggesting a bidirectional relationship between sleep and AD. This review will firstly grasp basic knowledge of AD pathogenesis, then highlight macrostructural and microstructural alteration of sleep along with AD progression, explain the interaction between accumulation of Aβ and hyperphosphorylated tau, which are two critical neuropathological processes of AD, as well as neuroinflammation and sleep, and finally introduce several methods of sleep enhancement as strategies to reduce AD-associated neuropathology. Although theories about the bidirectional relationship and relevant therapeutic methods in mice have been well developed in recent years, the knowledge in human is still limited. More studies on how to effectively ameliorate AD pathology in patients by sleep enhancement and what specific roles of sleep play in AD are needed.

Angiotensin-converting enzyme inhibitors: a therapeutic option for controlling blood pressure associated with delayed cognitive processing speed

Antihypertensive treatment (AT) is essential for preventing hypertension-related cognitive decline. The goals of this observational study were to compare cognitive performance (CP) between non-hypertensive (NH) volunteers and hypertensive patients and to evaluate the correlation between CP and antihypertensive drugs (AHD). Three groups were constituted: NH (n = 30) [group 1], hypertensive with systolic blood pressure (SBP) < 140 mmHg and diastolic blood pressure (DBP) < 90 mmHg (n = 54) [group 2] and hypertensive with SBP ≥ 140 or DBP ≥ 90 (n = 31) [group 3]. To analyze the cognitive domains, a neuropsychological battery was applied and the raw performance values in these tests were transformed into z-scores. The domain was considered impaired if it presented a z-score below -1.5 SD. Compared to group 1, both groups of hypertensive were older (51 [ ± 12] years) and showed a worse CP in episodic memory (p = 0.014), language (p = 0.003) and processing speed (PS) [p = 0.05]. Angiotensin-converting enzyme inhibitors (ACEi) and angiotensin receptor blockers (ARB) were the most used AHD (46.3%, p = 0.01 [group 2] and 64.5%, p = 0.005 [group 3]) and showed correlations with PS. Linear regression models revealed a negative association of PS with the use of ACEi (β = -0.230, p = 0.004), but not with the use of ARB (β = 0.208, p = 0.008). The effect of AT on cognition appears to go beyond the search for lower blood pressure targets and also includes the mechanism of action of AHD on the brain, so that additional benefits may possibly be achieved with simple adaptations in the treatment regimen, particularly in patients without clinically manifest cognitive impairment.

Restoration of cervical lymphatic vessel function in aging rescues cerebrospinal fluid drainage

Cervical lymphatic vessels (cLVs) have been shown to drain solutes and cerebrospinal fluid (CSF) from the brain. However, their hydrodynamical properties have never been evaluated in vivo. Here, we developed two-photon optical imaging with particle tracking in vivo of CSF tracers (2P-OPTIC) in superficial and deep cLVs of mice, characterizing their flow and showing that the major driver is intrinsic pumping by contraction of the lymphatic vessel wall. Moreover, contraction frequency and flow velocity were reduced in aged mice, which coincided with a reduction in smooth muscle actin expression. Slowed flow in aged mice was rescued using topical application of prostaglandin F2α, a prostanoid that increases smooth muscle contractility, which restored lymphatic function in aged mice and enhanced central nervous system clearance. We show that cLVs are important regulators of CSF drainage and that restoring their function is an effective therapy for improving clearance in aging.

Why do central nervous system barriers host a diverse immune landscape?

The meninges in vertebrates comprise three layers (dura, arachnoid, pia mater), representing an important barrier surrounding and protecting the central nervous system (CNS). The most exterior CNS barrier, the dura mater, is unique because it resembles a peripheral tissue. It hosts a rich immune landscape, lymphatic vessels, and fenestrated vasculature, allowing microbes and other threats from the blood to extravasate into the meninges, potentially reaching the underlying CNS. The highly specialized large venous drainage system in the dura is especially susceptible to infection. Here, we explore specializations in the CNS barrier system from an anatomical and immunological perspective and posit that the dura mater evolved an elaborate innate and adaptive immune system in specific locations within it to protect underlying CNS tissue against invading pathogens.

Polystyrene nanoplastics induced learning and memory impairments in mice by damaging the glymphatic system

The excessive usage of nanoplastics (NPs) has posed a serious threat to the ecological environment and human health, which can enter the brain and then result in neurotoxicity. However, research on the neurotoxic effects of NPs based on different exposure routes and modifications of functional groups is lacking. In this study, the neurotoxicity induced by NPs was studied using polystyrene nanoplastics (PS-NPs) of different modifications (PS, PS-COOH, and PS-NH2). It was found that PS-NH2 through intranasal administration (INA) exposure route exhibited the greatest accumulation in the mice brain after exposure for 7 days. After the mice were exposed to PS-NH2 by INA means for 28 days, the exploratory ability and spatial learning ability were obviously damaged in a dose-dependent manner. Further analysis indicated that these damages induced by PS-NH2 were closely related to the decreased ability of glymphatic system to clear β-amyloid (Aβ) and phosphorylated Tau (P-Tau) proteins, which was ascribed to the loss of aquaporin-4 (AQP4) polarization in the astrocytic endfeet. Moreover, the loss of AQP4 polarization might be regulated by the NF-κB pathway. Our current study establishes the connection between the neurotoxicity induced by PS-NPs and the glymphatic system dysfunction for the first time, which will contribute to future research on the neurotoxicity of NPs.

The role of endothelial frequency in the cerebral blood flow control during neonatal asphyxia: a retrospective longitudinal study

BACKGROUND

Cerebral blood flow dynamics can be explored through analysis of endothelial frequencies. Our hypothesis posits a disparity in endothelial activity among neonates with perinatal asphyxia, stratified by the presence or absence of neuronal lesions.

METHODS

We conducted a retrospective longitudinal study involving newborns treated with hypothermia for moderate to severe asphyxia. Participants were grouped based on the presence or absence of neuronal damage to investigate temporal endothelial involvement in cerebral blood flow regulation. Regional cerebral oxygen saturation (rScO2) was measured using near-infrared spectroscopy (NIRS), and temporal series were analyzed in the frequency domain, utilizing the original frequency of the INVOS™ device.

RESULTS

The study included 88 patients, with 53% (47/88) being male and 33% (29/88) demonstrating brain lesions on magnetic resonance imaging. Among them, 86% (76/88) had a gestational age exceeding 37 weeks according to the Ballard scale, and 81% (71/88) had a birth weight exceeding 2500 g. Cohen's d effect size was calculated to assess differences in endothelial frequency between groups, indicating a small effect size based on cerebral MRI findings (Cohen's d values for Day 2 = 0.2351 and Day 3 = 0.2325).

CONCLUSION

NIRS represents a valuable tool for monitoring cerebral autoregulation in neonates affected by perinatal asphyxia, underscoring the utility of assessing endothelial frequency or energy on rScO2 measured by NIRS using the original INVOS™ device frequency.

Heart rate variability impairment during sleep in Veterans with REM sleep behavior disorder, traumatic brain injury, and posttraumatic stress disorder: An early potential window into autonomic dysfunction?

Individuals with comorbid REM sleep behavior disorder (RBD) and neurotrauma (defined by traumatic brain injury and post-traumatic stress disorder) have an earlier age of RBD symptom onset, increased RBD-related symptom severity and more neurological features indicative of prodromal synucleinopathy compared to RBD only. An early sign of neurodegenerative condition is autonomic dysfunction, which we sought to evaluate by examining heart rate variability during sleep. Participants with overnight polysomnography were recruited from the VA Portland Health Care System. Veterans without neurotrauma or RBD (controls; n=19), with RBD only (RBD, n=14), and with RBD and neurotrauma (RBD+NT, n=19) were evaluated. Eligible 5-minute NREM and REM epochs without apneas/hypopneas, microarousals, and ectopic beats were analyzed for frequency and time domain (e.g. low frequency power, LF; high frequency power, HF; root mean square of successive RR intervals, RMSSD; % of RR intervals that vary ≥50 ms, pNN50) heart rate variability outcomes. Heart rate did not significantly differ between groups in any sleep stage. Time domain and frequency domain variables (e.g., LF power, HF power, RMSSD, and pNN50) were significantly reduced in the RBD and RBD+NT groups compared to controls and RBD only during NREM sleep. There were no group differences detected during REM sleep. These data suggest significant reductions in heart rate variability during NREM sleep in RBD+NT participants, suggesting greater autonomic dysfunction compared to controls or RBD alone. Heart rate variability during sleep may be an early, promising biomarker, yielding mechanistic insight for diagnosis and prognosis of early neurodegeneration in this vulnerable population.

STATEMENT OF SIGNIFICANCE

Comorbid REM sleep behavior disorder (RBD) and neurotrauma (NT, traumatic brain injury + post-traumatic stress disorder; RBD+NT) is associated with increased neurodegenerative symptom burden and worsened health. Sleep and autonomic function are integrally and bidirectionally related to neurodegenerative processes. In the current study, we sought to determine if early signs of autonomic dysfunction, measured via heart rate variability (HRV), were present during sleep in comorbid RBD+NT compared to RBD only and controls. Our data show reduced time and frequency domain HRV during NREM sleep in RBD+NT Veterans compared to RBD only and controls. These data contribute evidence that participants with RBD and comorbid NT demonstrate significantly worse autonomic dysfunction compared to age/sex matched participants with RBD alone.

Glymphatic System in Preterm Neonates: Developmental Insights Following Birth Asphyxia

BACKGROUND

Birth asphyxia (BA) and germinal matrix hemorrhage-intraventricular hemorrhage (GMH-IVH) are common clinical events in preterm neonates. However, their effects on the glymphatic system (GS) development in preterm neonates remain arcane.

PURPOSE

To evaluate the developmental trajectory of the GS, and to investigate the effects of BA and GMH-IVH on GS function in preterm neonates.

STUDY TYPE

Prospective.

POPULATION

Two independent datasets, prospectively acquired internal dataset (including 99 preterm neonates, 40 female, mean [standard deviation] gestational age (GA) at birth, 29.95 [2.63] weeks) and the developing Human Connectome Project (dHCP) dataset (including 81 preterm neonates, 29 female, median [interquartile range] GA at birth, 32.71 [4.28] weeks).

FIELD STRENGTH/SEQUENCE

3.0 T MRI and diffusion-weighted spin-echo planar imaging sequence.

ASSESSMENT

The diffusion-weighted images were preprocessed in volumetric space using the FMRIB Software Library and diffusion along the perivascular space (DTI-ALPS) index was accessed to evaluate GS function.

STATISTICAL TESTS

Two sample t tests, one-way analysis of variance followed by least-significant difference (LSD) post hoc analysis, chi-squared tests, and Pearson's correlation analysis. Significance level: P < 0.05.

RESULTS

In prospectively acquired internal dataset, preterm neonates with BA exhibited a significant lower DTI-ALPS index than those without BA (0.98 ± 0.08 vs. 1.08 ± 0.07, T = -5.89); however, GMH-IVH did not exert significant influences on the DTI-ALPS index (P = 0.83 and 0.27). The DTI-ALPS index increased significantly at postmenstrual age ranging from 25 to 34 weeks (r = 0.38) and then plateaued after 34 weeks (P = 0.35), which we also observed in the dHCP dataset.

DATA CONCLUSION

BA rather than GMH-IVH serves as the major influencing factor in the development of GS in preterm neonates. Moreover, as GS development follows a nonlinear trajectory, we recommend close monitoring of GS development in preterm neonates with a GA less than 34 weeks.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY: Stage 2.

Glymphatic clearance is enhanced during sleep

We here revisited the concept that glymphatic clearance is enhanced by sleep and anesthesia. Utilizing dynamic magnetic resonance imaging (MRI), single photon emission computed tomography (SPECT), and fluorescent fiber photometry, we report brain glymphatic clearance is enhanced by both sleep and anesthesia, and sharply suppressed by wakefulness. Another key finding was that less tracer enters the brains of awake animals and that brain clearance across different brain states can only be compared after adjusting for the injected tracer dose.

Influencing factors of glymphatic system during perioperative period

The glymphatic system is a functional cerebrospinal fluid circulatory system that uses peri-arterial space for inflow of cerebrospinal fluid and peri-venous space for efflux of cerebrospinal fluid from brain parenchyma. This brain-wide fluid transport pathway facilitates the exchange between cerebrospinal fluid and interstitial fluid and clears metabolic waste from the metabolically active brain. Multiple lines of work show that the glymphatic system is crucial to normal brain functions, and the dysfunction of the glymphatic system is closely associated with various neurological disorders, including aging, neurodegeneration, and acute brain injury. Currently, it is common to explore the functional and molecular mechanisms of the glymphatic system based on animal models. The function of glymphatic system during perioperative period is affected by many factors such as physiological, pathological, anesthetic and operative methods. To provide a reference for the interpretation of the results of glymphatic system studies during perioperative period, this article comprehensively reviews the physiological and pathological factors that interfere with the function of the glymphatic system during perioperative period, investigates the effects of anesthetic drugs on glymphatic system function and the potential underlying mechanisms, describes operative methods that interfere with the function of the glymphatic system, and potential intervention strategies based on the glymphatic system. Future, these variables should be taken into account as critical covariates in the design of functional studies on the glymphatic system.

Response to Commentary on "Structural characterization of SLYM - a 4th meningeal membrane" by Julie Siegenthaler and Christer Betsholtz

Histological studies have for decades documented that each of the classical meningeal membranes contains multiple fibroblast layers with distinct cellular morphology. Particularly, the sublayers of the arachnoid membranes have received attention due to their anatomical complexity. Early studies found that tracers injected into the cerebrospinal fluid (CSF) do not distribute freely but are restricted by the innermost sublayer of the arachnoid membrane. The existence of restrictions on CSF movement and the subdivision of the subarachnoid space into several distinct compartments have recently been confirmed by in vivo 2-photon studies of rodents, as well as macroscopic imaging of pigs and magnetic resonance imaging of human brain. Based on in vivo imaging and immunophenotyping characterization, we identified the structural basis for this compartmentalization of the subarachnoid space, which we term 'Subarachnoid lymphatic-like membrane', SLYM. The SLYM layer engages the subarachnoid vasculature as it approaches the brain parenchyma, demarcating a roof over pial perivascular spaces. Functionally, the separation of pial periarterial and perivenous spaces in the larger subarachnoid space is critical for the maintenance of unidirectional glymphatic clearance. In light of its close apposition to the pial surface and to the brain perivascular fluid exit points, the SLYM also provides a primary locus for immune surveillance of the brain. Yet, the introduction of SLYM, in terms of its anatomic distinction and hence functional specialization, has met resistance. Its critics assert that SLYM has been described in the literature by other terms, including the inner arachnoid membrane, the interlaminate membrane, the outer pial layer, the intermediate lamella, the pial membrane, the reticular layer of the arachnoid membrane or, more recently, BFB2-3. We argue that our conception of SLYM as an anatomically and functionally distinct construct is both necessary and warranted since its functional roles are wholly distinct from those of the overlying arachnoid barrier layer. Our terminology also lends clarity to a complex anatomy that has hitherto been ill-described. In that regard, we also note the lack of specificity of DPP4, which has recently been introduced as a 'selected defining marker' of the arachnoid barrier layer. We note that DPP4 labels fibroblasts in all meningeal membranes as well as in the trabecula arachnoides and the vascular adventitial layers, thus obviating its utility in meningeal characterization. Instead, we report a set of glymphatic-associated proteins that serve to accurately specify SLYM and distinguish it from its adjacent yet functionally distinct membranes.

Cerebrospinal fluid flow extends to peripheral nerves further unifying the nervous system

Cerebrospinal fluid (CSF) is responsible for maintaining brain homeostasis through nutrient delivery and waste removal for the central nervous system (CNS). Here, we demonstrate extensive CSF flow throughout the peripheral nervous system (PNS) by tracing distribution of multimodal 1.9-nanometer gold nanoparticles, roughly the size of CSF circulating proteins, infused within the lateral cerebral ventricle (a primary site of CSF production). CSF-infused 1.9-nanometer gold transitions from CNS to PNS at root attachment/transition zones and distributes through the perineurium and endoneurium, with ultimate delivery to axoplasm of distal peripheral nerves. Larger 15-nanometer gold fails to transit from CNS to PNS and instead forms "dye-cuffs," as predicted by current dogma of CSF restriction within CNS, identifying size limitations in central to peripheral flow. Intravenous 1.9-nanometer gold is unable to cross the blood-brain/nerve barrier. Our findings suggest that CSF plays a consistent role in maintaining homeostasis throughout the nervous system with implications for CNS and PNS therapy and neural drug delivery.

Therapeutic approaches to CNS diseases via the meningeal lymphatic and glymphatic system: prospects and challenges

The brain has traditionally been considered an "immune-privileged" organ lacking a lymphatic system. However, recent studies have challenged this view by identifying the presence of the glymphatic system and meningeal lymphatic vessels (MLVs). These discoveries offer new opportunities for waste clearance and treatment of central nervous system (CNS) diseases. Various strategies have been developed based on these pathways, including modulation of glymphatic system function, enhancement of meningeal lymphatic drainage, and utilization of these routes for drug delivery. Consequently, this review explores the developmental features and physiological roles of the cerebral lymphatic system as well as its significance in various CNS disorders. Notably, strategies for ameliorating CNS diseases have been discussed with a focus on enhancing glymphatic system and MLVs functionality through modulation of physiological factors along with implementing pharmacological and physical treatments. Additionally, emphasis is placed on the potential use of the CNS lymphatic system in drug delivery while envisioning future directions in terms of mechanisms, applications, and translational research.

Chronic sleep fragmentation impairs brain interstitial clearance in young wildtype mice

Accumulating evidence shows that most chronic neurological diseases have a link with sleep disturbances, and that patients with chronically poor sleep undergo an accelerated cognitive decline. Indeed, a single-night of sleep deprivation may increase metabolic waste levels in cerebrospinal fluid. However, it remains unknown how chronic sleep disturbances in isolation from an underlying neurological disease may affect the glymphatic system. Clearance of brain interstitial waste by the glymphatic system occurs primarily during sleep, driven by multiple oscillators including arterial pulsatility, and vasomotion. Herein, we induced sleep fragmentation in young wildtype mice and assessed the effects on glymphatic activity and cognitive functions. Chronic sleep fragmentation reduced glymphatic function and impaired cognitive functions in healthy mice. A mechanistic analysis showed that the chronic sleep fragmentation suppressed slow vasomotion, without altering cardiac-driven pulsations. Taken together, results of this study document that chronic sleep fragmentation suppresses brain metabolite clearance and impairs cognition, even in the absence of disease.

A comprehensive review on modeling aspects of infusion-based drug delivery in the brain

Brain disorders represent an ever-increasing health challenge worldwide. While conventional drug therapies are less effective due to the presence of the blood-brain barrier, infusion-based methods of drug delivery to the brain represent a promising option. Since these methods are mechanically controlled and involve multiple physical phases ranging from the neural and molecular scales to the brain scale, highly efficient and precise delivery procedures can significantly benefit from a comprehensive understanding of drug-brain and device-brain interactions. Behind these interactions are principles of biophysics and biomechanics that can be described and captured using mathematical models. Although biomechanics and biophysics have received considerable attention, a comprehensive mechanistic model for modeling infusion-based drug delivery in the brain has yet to be developed. Therefore, this article reviews the state-of-the-art mechanistic studies that can support the development of next-generation models for infusion-based brain drug delivery from the perspective of fluid mechanics, solid mechanics, and mathematical modeling. The supporting techniques and database are also summarized to provide further insights. Finally, the challenges are highlighted and perspectives on future research directions are provided. STATEMENT OF SIGNIFICANCE: Despite the immense potential of infusion-based drug delivery methods for bypassing the blood-brain barrier and efficiently delivering drugs to the brain, achieving optimal drug distribution remains a significant challenge. This is primarily due to our limited understanding of the complex interactions between drugs and the brain that are governed by principles of biophysics and biomechanics, and can be described using mathematical models. This article provides a comprehensive review of state-of-the-art mechanistic studies that can help to unravel the mechanism of drug transport in the brain across the scales, which underpins the development of next-generation models for infusion-based brain drug delivery. More broadly, this review will serve as a starting point for developing more effective treatments for brain diseases and mechanistic models that can be used to study other soft tissue and biomaterials.

Diffusion magnetic resonance imaging of cerebrospinal fluid dynamics: Current techniques and future advancements

Cerebrospinal fluid (CSF) plays a critical role in metabolic waste clearance from the brain, requiring its circulation throughout various brain pathways, including the ventricular system, subarachnoid spaces, para-arterial spaces, interstitial spaces, and para-venous spaces. The complexity of CSF circulation has posed a challenge in obtaining noninvasive measurements of CSF dynamics. The assessment of CSF dynamics throughout its various circulatory pathways is possible using diffusion magnetic resonance imaging (MRI) with optimized sensitivity to incoherent water movement across the brain. This review presents an overview of both established and emerging diffusion MRI techniques designed to measure CSF dynamics and their potential clinical applications. The discussion offers insights into the optimization of diffusion MRI acquisition parameters to enhance the sensitivity and specificity of diffusion metrics on underlying CSF dynamics. Lastly, we emphasize the importance of cautious interpretations of diffusion-based imaging, especially when differentiating between tissue- and fluid-related changes or elucidating structural versus functional alterations.

Mouse brain contains age-dependent extraparenchymal granular structures and astrocytes, both reactive to natural IgM antibodies, linked to the fissura magna

BACKGROUND

Mouse brains can contain specific polyglucosan aggregates known as Periodic Acid-Schiff (PAS)-granules. Generated in astrocytes, these granules increase with age and exhibit neo-epitopes of carbohydrate nature that are recognized by natural IgM antibodies (IgMs). The existence of neoepitopes on PAS granules suggests the presence of neoepitopes in other brain structures, and this is investigated here. To this end, brain sections from SAMP8 and ICR-CD1 mice were examined at different ages.

RESULTS

We have identified two novel structures that, apart from PAS granules, are recognized by natural IgMs. On one side, IgM reactive (IgM+) granular structures which are placed in the longitudinal fissure, the quadrigeminal cistern, and a region that extends from the quadrigeminal cistern to the interpeduncular cistern. This last region, located between the telencephalon and both the mesencephalon and diencephalon, is designated henceforth as the fissura magna, as it is indeed a fissure and the largest in the brain. As all these regions are extraparenchymal (EP), the IgM+ granules found in these zones have been named EP granules. These EP granules are mainly associated with fibroblasts and are not stained with PAS. On the other side, some IgM+ astrocytes have been found in the glia limitans, near the above-mentioned fissures. Remarkably, EP granules are more prevalent at younger ages, while the number of IgM+ astrocytes increases with age, similarly to the already described evolution of PAS granules.

CONCLUSIONS

The present work reports the presence of two brain-related structures that, apart from PAS granules, contain neo-epitopes of carbohydrate nature, namely EP granules and IgM+ astrocytes. We suggest that EP granules, associated to fibroblasts, may be part of a physiological function in brain clearance or brain-CSF immune surveillance, while both PAS granules and IgM+ astrocytes may be related to the increasing accumulation of harmful materials that occurs with age and linked to brain protective mechanisms. Moreover, the specific localisation of these EP granules and IgM+ astrocytes suggest the importance of the fissura magna in these brain-related cleaning and immune functions. The overall results reinforce the possible link between the fissura magna and the functioning of the glymphatic system.

Nasal turbinate lymphatic obstruction: a proposed new paradigm in the etiology of essential hypertension

Hypertension affects an estimated 1.3 billion people worldwide and is considered the number one contributor to mortality via stroke, heart failure, renal failure, and dementia. Although the physiologic mechanisms leading to the development of essential hypertension are poorly understood, the regulation of cerebral perfusion has been proposed as a primary cause. This article proposes a novel etiology for essential hypertension. Our hypothesis developed from a review of nuclear medicine scans, where the authors observed a significantly abnormal increase in nasal turbinate vasodilation in hypertensive patients using quantitative region of interest analysis. The authors propose that nasal turbinate vasodilation and resultant blood pooling obstruct the flow of cerebrospinal fluid passing through nasal turbinate lymphatics, thereby increasing intracranial pressure. The authors discuss the glymphatic/lymphatic clearance system which is impaired with age, and at which time hypertension also develops. The increased intracranial pressure leads to compensatory hypertension via Cushing's mechanism, i.e., the selfish brain hypothesis. The nasal turbinate vasodilation, due to increased parasympathetic activity, occurs simultaneously along with the well-established increased sympathetic activity of the cardiovascular system. The increased parasympathetic activity is likely due to an autonomic imbalance secondary to the increase in worldwide consumption of processed food. This hypothesis explains the rapid worldwide rise in essential hypertension in the last 50 years and offers a novel mechanism and a new paradigm for the etiology of essential hypertension. This new paradigm offers compelling evidence for the modulation of parasympathetic nervous system activity as a novel treatment strategy, specifically targeting nasal turbinate regulation, to treat diseases such as hypertension, idiopathic intracranial hypertension, and degenerative brain diseases. The proposed mechanism of essential hypertension presented in this paper is a working hypothesis and confirmatory studies will be needed.

Pediatric Hemispheric High-Grade Gliomas and H3.3-G34 Mutation: A Review of the Literature on Biological Features and New Therapeutic Strategies

Pediatric high-grade glioma (pHGG) encompasses a wide range of gliomas with different genomic, epigenomic, and transcriptomic features. Almost 50% of pHGGs present a mutation in genes coding for histone 3, including the subtype harboring the H3.3-G34 mutation. In this context, histone mutations are frequently associated with mutations in TP53 and ATRX, along with PDGFRA and NOTCH2NL amplifications. Moreover, the H3.3-G34 histone mutation induces epigenetic changes in immune-related genes and exerts modulatory functions on the microenvironment. Also, the functionality of the blood-brain barrier (BBB) has an impact on treatment response. The prognosis remains poor with conventional treatments, thus eliciting the investigation of additional and alternative therapies. Promising molecular targets include PDGFRA amplification, BRAF mutation, EGFR amplification, NF1 loss, and IDH mutation. Considering that pHGGs harboring the H3.3-G34R mutation appear to be more susceptible to immunotherapies (ITs), different options have been recently explored, including immune checkpoint inhibitors, antibody mediated IT, and Car-T cells. This review aims to summarize the knowledge concerning cancer biology and cancer-immune cell interaction in this set of pediatric gliomas, with a focus on possible therapeutic options.

Challenges and Future Perspectives in Modeling Neurodegenerative Diseases Using Organ-on-a-Chip Technology

Neurodegenerative diseases (NDDs) affect more than 50 million people worldwide, posing a significant global health challenge as well as a high socioeconomic burden. With aging constituting one of the main risk factors for some NDDs such as Alzheimer's disease (AD) and Parkinson's disease (PD), this societal toll is expected to rise considering the predicted increase in the aging population as well as the limited progress in the development of effective therapeutics. To address the high failure rates in clinical trials, legislative changes permitting the use of alternatives to traditional pre-clinical in vivo models are implemented. In this regard, microphysiological systems (MPS) such as organ-on-a-chip (OoC) platforms constitute a promising tool, due to their ability to mimic complex and human-specific tissue niches in vitro. This review summarizes the current progress in modeling NDDs using OoC technology and discusses five critical aspects still insufficiently addressed in OoC models to date. Taking these aspects into consideration in the future MPS will advance the modeling of NDDs in vitro and increase their translational value in the clinical setting.

Anesthesia Blunts Carbon Dioxide Effects on Glymphatic Cerebrospinal Fluid Dynamics in Mechanically Ventilated Rats

BACKGROUND

Impaired glymphatic clearance of cerebral metabolic products and fluids contribute to traumatic and ischemic brain edema and neurodegeneration in preclinical models. Glymphatic perivascular cerebrospinal fluid flow varies between anesthetics possibly due to changes in vasomotor tone and thereby in the dynamics of the periarterial cerebrospinal fluid (CSF)-containing space. To better understand the influence of anesthetics and carbon dioxide levels on CSF dynamics, this study examined the effect of periarterial size modulation on CSF distribution by changing blood carbon dioxide levels and anesthetic regimens with opposing vasomotor influences: vasoconstrictive ketamine-dexmedetomidine (K/DEX) and vasodilatory isoflurane.

METHODS

End-tidal carbon dioxide (ETco2) was modulated with either supplemental inhaled carbon dioxide to reach hypercapnia (Etco2, 80 mmHg) or hyperventilation (Etco2, 20 mmHg) in tracheostomized and anesthetized female rats. Distribution of intracisternally infused radiolabeled CSF tracer 111In-diethylamine pentaacetate was assessed for 86 min in (1) normoventilated (Etco2, 40 mmHg) K/DEX; (2) normoventilated isoflurane; (3) hypercapnic K/DEX; and (4) hyperventilated isoflurane groups using dynamic whole-body single-photon emission tomography. CSF volume changes were assessed with magnetic resonance imaging.

RESULTS

Under normoventilation, cortical CSF tracer perfusion, perivascular space size around middle cerebral arteries, and intracranial CSF volume were higher under K/DEX compared with isoflurane (cortical maximum percentage of injected dose ratio, 2.33 [95% CI, 1.35 to 4.04]; perivascular size ratio 2.20 [95% CI, 1.09 to 4.45]; and intracranial CSF volume ratio, 1.90 [95% CI, 1.33 to 2.71]). Under isoflurane, tracer was directed to systemic circulation. Under K/DEX, the intracranial tracer distribution and CSF volume were uninfluenced by hypercapnia compared with normoventilation. Intracranial CSF tracer distribution was unaffected by hyperventilation under isoflurane despite a 28% increase in CSF volume around middle cerebral arteries.

CONCLUSIONS

K/DEX and isoflurane overrode carbon dioxide as a regulator of CSF flow. K/DEX could be used to preserve CSF space and dynamics in hypercapnia, whereas hyperventilation was insufficient to increase cerebral CSF perfusion under isoflurane.

EDITOR’S PERSPECTIVE

CO2 as an engine for neurofluid flow: Exploring the coupling between vascular reactivity, brain clearance, and changes in tissue properties

The brain relies on an effective clearance mechanism to remove metabolic waste products for the maintenance of homeostasis. Recent studies have focused on elucidating the forces that drive the motion of cerebrospinal fluid (CSF), responsible for removal of these waste products. We demonstrate that vascular responses evoked using controlled manipulations of partial pressure of carbon dioxide (PaCO2) levels, serve as an endogenous driver of CSF clearance from the brain. To demonstrate this, we retrospectively surveyed our database, which consists of brain metastases patients from whom blood oxygen level-dependent (BOLD) images were acquired during targeted hypercapnic and hyperoxic respiratory challenges. We observed a correlation between CSF inflow signal around the fourth ventricle and CO2-induced changes in cerebral blood volume. By contrast, no inflow signal was observed in response to the nonvasoactive hyperoxic stimulus, validating our measurements. Moreover, our results establish a link between the rate of the hemodynamic response (to elevated PaCO2) and peritumoral edema load, which we suspect may affect CSF flow, consequently having implications for brain clearance. Our expanded perspective on the factors involved in neurofluid flow underscores the importance of considering both cerebrovascular responses, as well as the brain mechanical properties, when evaluating CSF dynamics in the context of disease processes.

Regulation of brain fluid volumes and pressures: basic principles, intracranial hypertension, ventriculomegaly and hydrocephalus

The principles of cerebrospinal fluid (CSF) production, circulation and outflow and regulation of fluid volumes and pressures in the normal brain are summarised. Abnormalities in these aspects in intracranial hypertension, ventriculomegaly and hydrocephalus are discussed. The brain parenchyma has a cellular framework with interstitial fluid (ISF) in the intervening spaces. Framework stress and interstitial fluid pressure (ISFP) combined provide the total stress which, after allowing for gravity, normally equals intracerebral pressure (ICP) with gradients of total stress too small to measure. Fluid pressure may differ from ICP in the parenchyma and collapsed subarachnoid spaces when the parenchyma presses against the meninges. Fluid pressure gradients determine fluid movements. In adults, restricting CSF outflow from subarachnoid spaces produces intracranial hypertension which, when CSF volumes change very little, is called idiopathic intracranial hypertension (iIH). Raised ICP in iIH is accompanied by increased venous sinus pressure, though which is cause and which effect is unclear. In infants with growing skulls, restriction in outflow leads to increased head and CSF volumes. In adults, ventriculomegaly can arise due to cerebral atrophy or, in hydrocephalus, to obstructions to intracranial CSF flow. In non-communicating hydrocephalus, flow through or out of the ventricles is somehow obstructed, whereas in communicating hydrocephalus, the obstruction is somewhere between the cisterna magna and cranial sites of outflow. When normal outflow routes are obstructed, continued CSF production in the ventricles may be partially balanced by outflow through the parenchyma via an oedematous periventricular layer and perivascular spaces. In adults, secondary hydrocephalus with raised ICP results from obvious obstructions to flow. By contrast, with the more subtly obstructed flow seen in normal pressure hydrocephalus (NPH), fluid pressure must be reduced elsewhere, e.g. in some subarachnoid spaces. In idiopathic NPH, where ventriculomegaly is accompanied by gait disturbance, dementia and/or urinary incontinence, the functional deficits can sometimes be reversed by shunting or third ventriculostomy. Parenchymal shrinkage is irreversible in late stage hydrocephalus with cellular framework loss but may not occur in early stages, whether by exclusion of fluid or otherwise. Further studies that are needed to explain the development of hydrocephalus are outlined.

Mouse brain elastography changes with sleep/wake cycles, aging, and Alzheimer's disease

Understanding the physiological processes in aging and how neurodegenerative disorders affect cognitive function is a high priority for advancing human health. One specific area of recently enabled research is the in vivo biomechanical state of the brain. This study utilized reverberant optical coherence elastography, a high-resolution elasticity imaging method, to investigate stiffness changes during the sleep/wake cycle, aging, and Alzheimer's disease in murine models. Four-dimensional scans of 44 wildtype mice, 13 mice with deletion of aquaporin-4 water channel, and 12 mice with Alzheimer-related pathology (APP/PS1) demonstrated that (1) cortical tissue became softer (on the order of a 10% decrease in shear wave speed) when young wildtype mice transitioned from wake to anesthetized, yet this effect was lost in aging and with mice overexpressing amyloid-β or lacking the water channel AQP4. (2) Cortical stiffness increased with age in all mice lines, but wildtype mice exhibited the most prominent changes as a function of aging. The study provides novel insight into the brain's biomechanics, the constraints of fluid flow, and how the state of brain activity affects basic properties of cortical tissues.

Trigeminal ganglion neurons are directly activated by influx of CSF solutes in a migraine model

Classical migraine patients experience aura, which is transient neurological deficits associated with cortical spreading depression (CSD), preceding headache attacks. It is not currently understood how a pathological event in cortex can affect peripheral sensory neurons. In this study, we show that cerebrospinal fluid (CSF) flows into the trigeminal ganglion, establishing nonsynaptic signaling between brain and trigeminal cells. After CSD, ~11% of the CSF proteome is altered, with up-regulation of proteins that directly activate receptors in the trigeminal ganglion. CSF collected from animals exposed to CSD activates trigeminal neurons in naïve mice in part by CSF-borne calcitonin gene-related peptide (CGRP). We identify a communication pathway between the central and peripheral nervous system that might explain the relationship between migrainous aura and headache.

Shifting from ependyma to choroid plexus epithelium and the changing expressions of aquaporin-1 and aquaporin-4

The cells of the choroid plexus (CP) epithelium are specialized ependymal cells (ECs) but have distinct properties. The CP cells and ECs form single-cell sheets contiguous to each other at a transitional zone. The CP is underlined by a basal lamina and has barrier properties, whereas the ECs do not. The basal lamina of the CP is continuous with the glia limitans superficialis and, consequently, the CP stroma is continuous with the meninges along entering blood vessels. The CP has previously been reported to express aquaporin-1 (AQP1) mostly apically, and ECs show mostly basolateral aquaporin-4 (AQP4) expression. Recent evidence in various systems has shown that in changing conditions the expression and distribution of AQP4 can be modified, involving phosphorylation and calmodulin-triggered translocation. Studies on the human CP revealed that AQP4 is also expressed in some CP cells, which is likely to be increased during ageing based on mouse data. Moreover, subependymal astrocytic processes in the ependyma-CP transition, forming a glial plate around blood vessels and facing the CP stroma, were strongly positive for AQP4. We propose that the increased AQP4 expression might be a compensatory mechanism for the observed reduction in CSF production in the ageing human brain. The high AQP4 density in the transition zone might facilitate the transport of water into and out of the CP stroma and serve as a drainage and clearing pathway for metabolites in the CNS.

Four-dimensional flow MRI for quantitative assessment of cerebrospinal fluid dynamics: Status and opportunities

Neurological disorders can manifest with altered neurofluid dynamics in different compartments of the central nervous system. These include alterations in cerebral blood flow, cerebrospinal fluid (CSF) flow, and tissue biomechanics. Noninvasive quantitative assessment of neurofluid flow and tissue motion is feasible with phase contrast magnetic resonance imaging (PC MRI). While two-dimensional (2D) PC MRI is routinely utilized in research and clinical settings to assess flow dynamics through a single imaging slice, comprehensive neurofluid dynamic assessment can be limited or impractical. Recently, four-dimensional (4D) flow MRI (or time-resolved three-dimensional PC with three-directional velocity encoding) has emerged as a powerful extension of 2D PC, allowing for large volumetric coverage of fluid velocities at high spatiotemporal resolution within clinically reasonable scan times. Yet, most 4D flow studies have focused on blood flow imaging. Characterizing CSF flow dynamics with 4D flow (i.e., 4D CSF flow) is of high interest to understand normal brain and spine physiology, but also to study neurological disorders such as dysfunctional brain metabolite waste clearance, where CSF dynamics appear to play an important role. However, 4D CSF flow imaging is challenged by the long T1 time of CSF and slower velocities compared with blood flow, which can result in longer scan times from low flip angles and extended motion-sensitive gradients, hindering clinical adoption. In this work, we review the state of 4D CSF flow MRI including challenges, novel solutions from current research and ongoing needs, examples of clinical and research applications, and discuss an outlook on the future of 4D CSF flow.

Changes in Astroglial Water Flow in the Pre-amyloid Phase of the STZ Model of AD Dementia

Alzheimer's disease (AD) is an age-dependent neurodegenerative disease that is typically sporadic and has a high social and economic cost. We utilized the intracerebroventricular administration of streptozotocin (STZ), an established preclinical model for sporadic AD, to investigate hippocampal astroglial changes during the first 4 weeks post-STZ, a period during which amyloid deposition has yet to occur. Astroglial proteins aquaporin 4 (AQP-4) and connexin-43 (Cx-43) were evaluated, as well as claudins, which are tight junction (TJ) proteins in brain barriers, to try to identify changes in the glymphatic system and brain barrier during the pre-amyloid phase. Glial commitment, glucose hypometabolism and cognitive impairment were characterized during this phase. Astroglial involvement was confirmed by an increase in glial fibrillary acidic protein (GFAP); concurrent proteolysis was also observed, possibly mediated by calpain. Levels of AQP-4 and Cx-43 were elevated in the fourth week post-STZ, possibly accelerating the clearance of extracellular proteins, since these proteins actively participate in the glymphatic system. Moreover, although we did not see a functional disruption of the blood-brain barrier (BBB) at this time, claudin 5 (present in the TJ of the BBB) and claudin 2 (present in the TJ of the blood-cerebrospinal fluid barrier) were reduced. Taken together, data support a role for astrocytes in STZ brain damage, and suggest that astroglial dysfunction accompanies or precedes neuronal damage in AD.

Intermittent Metabolic Switching and Vascular Cognitive Impairment

Intermittent fasting (IF), a dietary pattern alternating between eating and fasting periods within a 24-hour cycle, has garnered recognition for its potential to enhance both healthspan and lifespan in animal models and humans. It also shows promise in alleviating age-related diseases, including neurodegeneration. Vascular cognitive impairment (VCI) spans a severity range from mild cognitive deficits to severe cognitive deficits and loss of function in vascular dementia. Chronic cerebral hypoperfusion has emerged as a significant contributor to VCI, instigating vascular pathologies such as microbleeds, blood-brain barrier dysfunction, neuronal loss, and white matter lesions. Preclinical studies in rodents strongly suggest that IF has the potential to attenuate pathological mechanisms, including excitotoxicity, oxidative stress, inflammation, and cell death pathways in VCI models. Hence, this supports evaluating IF in clinical trials for both existing and at-risk VCI patients. This review compiles existing data supporting IF's potential in treating VCI-related vascular and neuronal pathologies, emphasizing the mechanisms by which IF may mitigate these issues. Hence providing a comprehensive overview of the available data supporting IF's potential in treating VCI by emphasizing the underlying mechanisms that make IF a promising intervention for VCI.

An Overview on the Physiopathology of the Blood-Brain Barrier and the Lipid-Based Nanocarriers for Central Nervous System Delivery

The state of well-being and health of our body is regulated by the fine osmotic and biochemical balance established between the cells of the different tissues, organs, and systems. Specific districts of the human body are defined, kept in the correct state of functioning, and, therefore, protected from exogenous or endogenous insults of both mechanical, physical, and biological nature by the presence of different barrier systems. In addition to the placental barrier, which even acts as a linker between two different organisms, the mother and the fetus, all human body barriers, including the blood-brain barrier (BBB), blood-retinal barrier, blood-nerve barrier, blood-lymph barrier, and blood-cerebrospinal fluid barrier, operate to maintain the physiological homeostasis within tissues and organs. From a pharmaceutical point of view, the most challenging is undoubtedly the BBB, since its presence notably complicates the treatment of brain disorders. BBB action can impair the delivery of chemical drugs and biopharmaceuticals into the brain, reducing their therapeutic efficacy and/or increasing their unwanted bioaccumulation in the surrounding healthy tissues. Recent nanotechnological innovation provides advanced biomaterials and ad hoc customized engineering and functionalization methods able to assist in brain-targeted drug delivery. In this context, lipid nanocarriers, including both synthetic (liposomes, solid lipid nanoparticles, nanoemulsions, nanostructured lipid carriers, niosomes, proniosomes, and cubosomes) and cell-derived ones (extracellular vesicles and cell membrane-derived nanocarriers), are considered one of the most successful brain delivery systems due to their reasonable biocompatibility and ability to cross the BBB. This review aims to provide a complete and up-to-date point of view on the efficacy of the most varied lipid carriers, whether FDA-approved, involved in clinical trials, or used in in vitro or in vivo studies, for the treatment of inflammatory, cancerous, or infectious brain diseases.

Neurosurgery and the glymphatic system

The discovery of the glymphatic system has fundamentally altered our comprehension of cerebrospinal fluid transport and the removal of waste from brain metabolism. In the past decade, since its initial characterization, research on the glymphatic system has surged exponentially. Its potential implications for central nervous system disorders have sparked significant interest in the field of neurosurgery. Nonetheless, ongoing discussions and debates persist regarding the concept of the glymphatic system, and our current understanding largely relies on findings from experimental animal studies. This review aims to address several key inquiries: What methodologies exist for evaluating glymphatic function in humans today? What is the current evidence supporting the existence of a human glymphatic system? Can the glymphatic system be considered distinct from the meningeal-lymphatic system? What is the human evidence for glymphatic-meningeal lymphatic system failure in neurosurgical diseases? Existing literature indicates a paucity of techniques available for assessing glymphatic function in humans. Thus far, intrathecal contrast-enhanced magnetic resonance imaging (MRI) has shown the most promising results and have provided evidence for the presence of a glymphatic system in humans, albeit with limitations. It is, however, essential to recognize the interconnection between the glymphatic and meningeal lymphatic systems, as they operate in tandem. There are some human studies demonstrating deteriorations in glymphatic function associated with neurosurgical disorders, enriching our understanding of their pathophysiology. However, the translation of this knowledge into clinical practice is hindered by the constraints of current glymphatic imaging modalities.

Central nervous system-derived extracellular vesicles: the next generation of neural circulating biomarkers?

The central nervous system (CNS) is integrated by glial and neuronal cells, and both release extracellular vesicles (EVs) that participate in CNS homeostasis. EVs could be one of the best candidates to operate as nanosized biological platforms for analysing multidimensional bioactive cargos, which are protected during systemic circulation of EVs. Having a window into the molecular level processes that are happening in the CNS could open a new avenue in CNS research. This raises a particular point of interest: can CNS-derived EVs in blood serve as circulating biomarkers that reflect the pathological status of neurological diseases? L1 cell adhesion molecule (L1CAM) is a widely reported biomarker to identify CNS-derived EVs in peripheral blood. However, it has been demonstrated that L1CAM is also expressed outside the CNS. Given that principal data related to neurodegenerative diseases, such as multiple sclerosis, amyotrophic lateral sclerosis, Parkinson's disease and Alzheimer's disease were obtained using L1CAM-positive EVs, efforts to overcome present challenges related to its specificity are required. In this sense, other surface biomarkers for CNS-derived EVs, such as glutamate aspartate transporter (GLAST) and myelin oligodendrocyte glycoprotein (MOG), among others, have started to be used. Establishing a panel of EV biomarkers to analyse CNS-derived EVs in blood could increase the specificity and sensitivity necessary for these types of studies. This review covers the main evidence related to CNS-derived EVs in cerebrospinal fluid and blood samples of patients with neurological diseases, focusing on the reported biomarkers and the technical possibilities for their isolation. EVs are emerging as a mirror of brain physiopathology, reflecting both localized and systemic changes. Therefore, when the technical hindrances for EV research and clinical applications are overcome, novel disease-specific panels of EV biomarkers would be discovered to facilitate transformation from traditional medicine to personalized medicine.

Strategies for enhanced gene delivery to the central nervous system

The delivery of genes to the central nervous system (CNS) has been a persistent challenge due to various biological barriers. The blood-brain barrier (BBB), in particular, hampers the access of systemically injected drugs to parenchymal cells, allowing only a minimal percentage (<1%) to pass through. Recent scientific insights highlight the crucial role of the extracellular space (ECS) in governing drug diffusion. Taking into account advancements in vectors, techniques, and knowledge, the discussion will center on the most notable vectors utilized for gene delivery to the CNS. This review will explore the influence of the ECS - a dynamically regulated barrier-on drug diffusion. Furthermore, we will underscore the significance of employing remote-control technologies to facilitate BBB traversal and modulate the ECS. Given the rapid progress in gene editing, our discussion will also encompass the latest advances focused on delivering therapeutic editing in vivo to the CNS tissue. In the end, a brief summary on the impact of Artificial Intelligence (AI)/Machine Learning (ML), ultrasmall, soft endovascular robots, and high-resolution endovascular cameras on improving the gene delivery to the CNS will be provided.