Local diffusion in the extracellular space of the brain

Functional abnormalities of the glymphatic system in cognitive disorders

Various pathological mechanisms represent distinct therapeutic targets for cognitive disorders, but a balance between clearance and production is essential for maintaining the stability of the brain's internal environment. Thus, the glymphatic system may represent a common pathway by which to address cognitive disorders. Using the established model of the glymphatic system as our foundation, this review disentangles and analyzes the components of its clearance mechanism, including the initial inflow of cerebrospinal fluid, the mixing of cerebrospinal fluid with interstitial fluid, and the outflow of the mixed fluid and the clearance. Each section summarizes evidence from experimental animal models and human studies, highlighting the normal physiological properties of key structures alongside their pathological manifestations in cognitive disorders. The same pathologic manifestations of different cognitive disorders appearing in the glymphatic system and the same upstream influences are main points of interest of this review. We conclude this article by discussing new findings and outlining the limitations identified in current research progress.

Live STED imaging of functional neuroanatomy

In the mammalian brain, a large network of excitable and modulatory cells efficiently processes, analyzes and stores vast amounts of information. The brain's anatomy influences the flow of neural information between neurons and glia, from which all thought, emotion and action arises. Consequently, one of the grand challenges in neuroscience is to uncover the finest structural details of the brain in the context of its overall architecture. Recent developments in microscopy and biosensors have enabled the investigation of brain microstructure and function with unprecedented specificity and resolution, dendritic spines being an exemplary case, which has provided deep insights into neuronal mechanisms of higher brain function, such as learning and memory. As diffraction-limited light microscopy methods cannot resolve the fine details of brain cells (the 'anatomical ground truth'), electron microscopy is used instead to contextualize functional signals. This approach can be quite unsatisfying given the fragility and dynamic nature of the structures under investigation. We have recently developed a method for combining super-resolution stimulated emission depletion microscopy with functional measurements in brain slices, offering nanoscale resolution in functioning brain structures. We describe how to concurrently perform morphological and functional imaging with a confocal STED microscope. Specifically, the procedure guides the user on how to record astrocytic Ca2+ signals at tripartite synapses, outlining a framework for analyzing structure-function relationships of brain cells at nanoscale resolution. The imaging requires 2-3 h and the image analysis between 2 h and 2 d.

Fluid flow and amyloid transport and aggregation in the brain interstitial space

The driving mechanisms at the base of the clearance of biological wastes in the brain interstitial space (ISS) are still poorly understood and an actively debated subject. A complete comprehension of the processes that lead to the aggregation of amyloid proteins in such environment, hallmark of the onset and progression of Alzheimer's disease, is of crucial relevance. Here we employ combined computational fluid dynamics and molecular dynamics techniques to uncover the role of fluid flow and proteins transport in the brain ISS. Our work identifies diffusion as the principal mechanism for amyloid-β proteins clearance, whereas fluid advection may lead transport for larger molecular bodies, like amyloid-β aggregates or extracellular vesicles. We also clearly quantify the impact of large nascent prefibrils on the fluid flowing and shearing. Finally, we show that, even in the irregular brain interstitial space (ISS), hydrodynamic interactions enhance amyloid-β aggregation at all stages of the aggregation pathway. Our results are key to understand the role of fluid flow and solvent-solute interplay on therapeutics like antibodies acting in the brain ISS.

Myocardial fibrosis is associated with brain microstructural alterations in patients with heart failure: A diffusion MRI study

PURPOSE

This study aimed to examine the changes in brain diffusion in patients with heart failure (HF) by measuring the apparent diffusion coefficient (ADC) and to investigate the relationship between these changes and cardiac injury features.

METHODS

The study included 49 patients with HF and 39 healthy controls. Cognitive performance was assessed using the Mini-Mental State Examination (MMSE) and the Montreal Cognitive Assessment (MoCA). All participants underwent a brain DWI scan, followed by cardiac MR in patients with HF. The ADC values in different regions of the brain were estimated using region-of-interest analysis. Cardiac MR was used to evaluate left ventricular function and the volume of late gadolinium enhancement (LGE). Group differences in ADC values were analysed using generalised linear mixed modelling. Correlation analysis was used to explore the associations between brain diffusion alterations, cognitive performance, and cardiac injury features.

RESULTS

Patients with HF showed significantly increased ADC values in several brain regions, including the frontal, parietal, temporal, occipital lobes, cingulate cortex, hippocampus, cerebellum, and deep nuclei. ADC values in the right hippocampus, right amygdala, and bilateral insula were negatively correlated with MoCA scores. ADC values in the left inferior occipital gyrus (r = 0.570, P<0.001) and lobule VI of the cerebellar hemisphere (r = 0.560, P = 0.001) were correlated with LGE volume.

CONCLUSION

These findings suggest that specific brain regions in patients with HF experience microstructural changes associated with cognitive impairment. Moreover, chronic brain microstructural alterations may be related to myocardial fibrosis in patients with HF.

Intracellular α-synuclein assemblies are sufficient to alter nanoscale diffusion in the striatal extracellular space

α-synucleinopathies progression involves the spread of α-synuclein aggregates through the extracellular space (ECS). Single-particle tracking studies showed that α-synuclein-induced neurodegeneration increases ECS molecular diffusivity. To disentangle the consequences of neuronal loss versus α-synuclein-positive intracellular assemblies formation, we performed near-infrared single-particle tracking to characterise ECS rheology in the striatum of mouse models of α-synucleinopathies. We showed that intracellular α-synuclein assemblies, without neurodegeneration, suffice to alter nanoscale diffusion in the striatal ECS.

A perspective: neuraxial therapeutics in pain management: now and future

The neuraxial delivery of drugs for the management of pain and other spinal pathologies is widely employed and is the subject of a large volume of ongoing research with several thousand papers appearing in the past 5 years alone on neuraxial delivery. Several learned texts have been recently published. A number of considerations have contributed to this widespread interest in the development of the use of neuraxial therapeutics to manage pain. In the following section, major topics relevant to spinal encoding and in the use of neuraxial therapeutics are considered by the Frontiers in Pain Research editors of the research topic: "Neuraxial Therapeutics in Pain Management: Now and Future". This paper seeks to serve as a perspective to encourage the submission of manuscripts reflecting research in this exciting area.

Numerical simulation study of nanoparticle diffusion in gray matter

Purpose

Nanomedicine-based approaches have shown great potential in the treatment of central nervous system diseases. However, the fate of nanoparticles (NPs) within the brain parenchyma has not received much attention. The complexity of the microstructure of the brain and the invisibility of NPs make it difficult to study NP transport within the grey matter. Moreover, regulation of NP delivery is not fully understood.

Methods

2D interstitial system (ISS) models reflecting actual extracellular space (ECS) were constructed. A particle tracing model was used to simulate the diffusion of the NPs. The effect of NP size on NP diffusion was studied using numerical simulations. The diffusion of charged NPs was explored by comparing experimental and numerical simulation data, and the effect of cell membrane potential on the diffusion of charged NPs was further studied.

Results

The model was verified using previously published experimental data. Small NPs could diffuse efficiently into the ISS. The diffusion of charged NPs was hindered in the ISS. Changes in cell membrane potential had little effect on NP diffusion.

Conclusion

This study constructed 2D brain ISS models that reflected the actual ECS and simulated the diffusion of NPs within it. The study found that uncharged small NPs could effectively diffuse within the ISS and that the cell membrane potential had a limited effect on the diffusion of charged NPs. The model and findings of this study can aid the design of nanomedicines and nanocarriers for the diagnosis and treatment of brain diseases.

Barriers of the CNS transfer rate dynamics in patients with vascular cognitive impairment and dementia

Background

Advances in in vivo MRI techniques enable cerebral barrier transfer rates (K trans ) measurement in patients with vascular cognitive impairment and dementia (VCID). However, a consensus has not been reached on the dynamic contribution and importance of cerebral barrier abnormalities to the differential diagnosis of dementia subtypes. Our goal was to investigate the dynamics of blood-brain barrier (BBB) and blood-CSF barrier (BCSFB) K trans in patients with VCID longitudinally and determine the effect of aging.

Methods

We studied subjects at two time points over two years; they were 65.5 years of age (SD = 15.94, M/F = 24/14) at the first visit. We studied 38 patients, 18 of whom had two visits. We calculated the BBB and BCSFB K trans with dynamic contrast-enhanced T1 MR, and we used 1H-MR spectroscopy to measure N-acetylaspartate (NAA) levels in the white matter as a marker of injury. In addition, we measured CSF levels of active-matrix metalloproteinase-3 (MMP3) as an inflammatory biomarker to aid in patient clustering.

Results

Longitudinal BBB measurements revealed variable dynamic behavior: after two years, the BBB K trans increased in 55% of patients and decreased in the remaining 45% unpredictably. We did not find a significant linear model of BBB K trans versus age for VCID. For healthy controls, the model was K trans = 0.0014 + 0.0002 × age, which was significant (p = 0.046). VCID patients showed a reduction in BCSFB K trans compared to healthy controls (p = 0.01). Combining NAA, CSF MMP3, and K trans in a clustering analysis separated patients into groups.

Conclusion

These results suggest that BBB K trans in VCID is dynamic and BCSFB K trans reduced by age. By combining inflammatory biomarkers with BBB K trans data, it is possible to separate VCID patients into distinct groups with different underlying pathologies.

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.

In vivo bioluminescence imaging of the intracerebral fibroin-controlled AAV-α-synuclein diffusion for monitoring the central nervous system and peripheral expression

Among the several animal models of α-synucleinopathies, the well-known viral vector-mediated delivery of wild-type or mutated (A53T) α-synuclein requires new tools to increase the lesion in mice and follow up in vivo expression. To this end, we developed a bioluminescent expression reporter of the human A53T-α-synuclein gene using the NanoLuc system into an AAV2/9, embedded or not in a fibroin solution to stabilise its expression in space and time. We first verified the expression of the fused protein in vitro on transfected cells by bioluminescence and Western blotting. Next, two groups of C57Bl6Jr mice were unilaterally injected with the AAV-NanoLuc-human-A53T-α-synuclein above the substantia nigra combined (or not) with fibroin. We first show that the in vivo cerebral bioluminescence signal was more intense in the presence of fibroin. Using immunohistochemistry, we find that the human-A53T-α-synuclein protein is more restricted to the ipsilateral side with an overall greater magnitude of the lesion when fibroin was added. However, we also detected a bioluminescence signal in peripheral organs in both conditions, confirmed by the presence of viral DNA corresponding to the injected AAV in the liver using qPCR.

Novel extracellular matrix architecture on excitatory neurons revealed by HaloTag-HAPLN1

The brain's extracellular matrix (ECM) regulates neuronal plasticity and animal behavior. ECM staining shows an aggregated pattern in a net-like structure around a subset of neurons and diffuse staining in the interstitial matrix. However, understanding the structural features of ECM deposition across various neuronal types and subcellular compartments remains limited. To visualize the organization pattern and assembly process of the hyaluronan-scaffolded ECM in the brain, we fused a HaloTag to HAPLN1, which links hyaluronan and proteoglycans. Expression or application of the probe enables us to identify spatial and temporal regulation of ECM deposition and heterogeneity in ECM aggregation among neuronal populations. Dual-color birthdating shows the ECM assembly process in culture and in vivo. Sparse expression in vivo reveals novel forms of ECM architecture around excitatory neurons and developmentally regulated dendritic ECM. Overall, our study uncovers extensive structural features of the brain' ECM, suggesting diverse roles in regulating neuronal plasticity.

Asteroid impact: the potential of astrocytes to modulate human neural networks within organoids

Astrocytes are a vital cellular component of the central nervous system that impact neuronal function in both healthy and pathological states. This includes intercellular signals to neurons and non-neuronal cells during development, maturation, and aging that can modulate neural network formation, plasticity, and maintenance. Recently, human pluripotent stem cell-derived neural aggregate cultures, known as neurospheres or organoids, have emerged as improved experimental platforms for basic and pre-clinical neuroscience compared to traditional approaches. Here, we summarize the potential capability of using organoids to further understand the mechanistic role of astrocytes upon neural networks, including the production of extracellular matrix components and reactive signaling cues. Additionally, we discuss the application of organoid models to investigate the astrocyte-dependent aspects of neuropathological diseases and to test astrocyte-inspired technologies. We examine the shortcomings of organoid-based experimental platforms and plausible improvements made possible by cutting-edge neuroengineering technologies. These advancements are expected to enable the development of improved diagnostic strategies and high-throughput translational applications regarding neuroregeneration.

Effects of Ischemic Stroke on Interstitial Fluid Clearance in Mouse Brain: a Bead Study

The clearance of brain interstitial fluid (ISF) is important in maintaining brain homeostasis. ISF clearance impairment leads to toxic material accumulation in the brain, and ischemic stroke could impair ISF clearance. The present study investigates ISF clearance under normal and ischemic conditions. The carboxylate-modified FluoSpheres beads (0.04 μm in diameter) were injected into the striatum. Sham or transient middle cerebral artery occlusion surgeries were performed on the mice. The brain sections were immunostained with cell markers, and bead distribution at various time points was examined with a confocal microscope. Primary mouse neuronal cultures were incubated with the beads to explore in vitro endocytosis.  Two physiological routes for ISF clearance were identified. The main one was to the lateral ventricle (LV) through the cleft between the striatum and the corpus callosum (CC)/external capsule (EC), where some beads were captured by the ependymal macrophages and choroid plexus. An alternative and minor route was to the subarachnoid space through the CC/EC and the cortex, where some of the beads were endocytosed by neurons. After ischemic stroke, a significant decrease in the main route and an increase in the minor route were observed. Additionally, microglia/macrophages engulfed the beads in the infarction. In conclusion, we report that the physiological clearance of ISF and beads mainly passes through the cleft between the CC/EC and striatum into the LV, or alternatively through the cortex into the subarachnoid space. Stroke delays the main route but enhances the minor route, and microglia/macrophages engulf the beads in the infarction. Ischemic stroke impairs the clearance of brain interstitial fluid/beads. Under physiological conditions, the main route ( ① ) of interstitial fluid clearance is to the lateral ventricle, and the minor one ( ② ) is to the subarachnoid space. Ischemic stroke weakens the main route ( ① ), enhances the minor one ( ② ), and leads to microglial/macrophage phagocytosis within the infarction ( ③ ).

Shadow imaging for panoptical visualization of brain tissue in vivo

Progress in neuroscience research hinges on technical advances in visualizing living brain tissue with high fidelity and facility. Current neuroanatomical imaging approaches either require tissue fixation (electron microscopy), do not have cellular resolution (magnetic resonance imaging) or only give a fragmented view (fluorescence microscopy). Here, we show how regular light microscopy together with fluorescence labeling of the interstitial fluid in the extracellular space provide comprehensive optical access in real-time to the anatomical complexity and dynamics of living brain tissue at submicron scale. Using several common fluorescence microscopy modalities (confocal, light-sheet and 2-photon microscopy) in mouse organotypic and acute brain slices and the intact mouse brain in vivo, we demonstrate the value of this straightforward 'shadow imaging' approach by revealing neurons, microglia, tumor cells and blood capillaries together with their complete anatomical tissue contexts. In addition, we provide quantifications of perivascular spaces and the volume fraction of the extracellular space of brain tissue in vivo.

Sheet and void porous media models for brain interstitial space

The interstitial space (ISS) component of brain extracellular space resembles an unconsolidated porous medium. Previous analysis of the diffusion of small molecules in this domain shows that the typical porosity is 0.2 and typical tortuosity 1.6. An ensemble of cubic cells separated by uniform sheets of ISS cannot generate the measured tortuosity, even if some of the tortuosity value is attributed to interstitial viscosity, so more complex models are needed. Here two models are analysed: the corner cubic void (CCV) and the edge tunnel void (ETV). Both models incorporate dead spaces formed from local expansions of the ISS to increase geometrical tortuosity. Using Monte Carlo simulation of diffusion it is found that in the range of normal porosities, the square of the tortuosity is a linear function of the ratio of void to sheet volumes for the CCV model and this model can generate the experimentally observed tortuosities. For abnormally high porosities, however, the linear relation fails. The ETV model shows a quartic functional relation and can only generate the observed tortuosity if interstitial viscosity is present. The CCV model is used to analyse the recently described changes in porosity between asleep and awake brain states.