Nanoscale exploration of the extracellular space in the live brain by combining single carbon nanotube tracking and super-resolution imaging analysis

Magnetic carbon nanotubes modified with proteins and hydrophilic monomers: Cytocompatibility, in-vitro toxicity assays and permeation across biological interfaces

Carbon nanotubes are promising materials for biomedical applications like delivery systems and tissue scaffolds. In this paper, magnetic carbon nanotubes (M-CNTs) covered with bovine serum albumin (M-CNTs-BSA) or functionalized with hydrophilic monomers (M-CNTs-HL) were synthesized, characterized, and evaluated concerning their interaction with Caco-2 cells. There is no comparison between these two types of functionalization, and this study aimed to verify their influence on the material's interaction with the cells. Different concentrations of the nanotubes were applied to investigate cytotoxicity, cell metabolism, oxidative stress, apoptosis, and capability to cross biomimetic barriers. The materials showed cytocompatibility up to 100 μg mL-1 and a hemolysis rate below 2 %. Nanotubes' suspensions were allowed to permeate Caco-2 monolayers for up to 8 h under the effect of the magnetic field. Magnetic nanoparticles associated with the nanotubes allowed estimation of permeation through the monolayers, with values ranging from 0.50 to 7.19 and 0.27 to 9.30 × 10-3 μg (equivalent to 0.43 to 6.22 and 0.23 to 9.54 × 10-2 % of the initially estimated mass of magnetic nanoparticles) for cells exposed and non-exposed to the magnets, respectively. Together, these results support that the developed materials are promising for applications in biomedical and biotechnological fields.

Mapping Extracellular Space Features of Viral Encephalitis to Evaluate the Proficiency of Anti-Viral Drugs

The extracellular space (ECS) is an important barrier against viral attack on brain cells, and dynamic changes in ECS microstructure characteristics are closely related to the progression of viral encephalitis in the brain and the efficacy of antiviral drugs. However, mapping the precise morphological and rheological features of the ECS in viral encephalitis is still challenging so far. Here, a robust approach is developed using single-particle diffusional fingerprinting of quantum dots combined with machine learning to map ECS features in the brain and predict the efficacy of antiviral encephalitis drugs. These results demonstrated that this approach can characterize the microrheology and geometry of the brain ECS at different stages of viral infection and identify subtle changes induced by different drug treatments. This approach provides a potential platform for drug proficiency assessment and is expected to offer a reliable basis for the clinical translation of drugs.

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.

Nanoreporter Identifies Lysosomal Storage Disease Lipid Accumulation Intracranially

Dysregulated lipid metabolism contributes to neurodegenerative pathologies and neurological decline in lysosomal storage disorders as well as more common neurodegenerative diseases. Niemann-Pick type A (NPA) is a fatal neurodegenerative lysosomal storage disease characterized by abnormal sphingomyelin accumulation in the endolysosomal lumen. The ability to monitor abnormalities in lipid homeostasis intracranially could improve basic investigations and the development of effective treatment strategies. We investigated the carbon nanotube-based detection of intracranial lipid content. We found that the near-infrared emission of a carbon nanotube-based lipid sensor responds to lipid accumulation in neuronal and in vivo models of NPA. The nanosensor detected lipid accumulation intracranially in an acid sphingomyelinase knockout mouse via noninvasive near-infrared spectroscopy. This work indicates a tool to improve drug development processes in NPA, other lysosomal storage diseases, and neurodegenerative diseases.

On the equivalence of binary phase masks optimized for localization or detection in extended depth-of-field localization microscopy

Binary annular masks have recently been proposed to extend the depth of field (DoF) of single-molecule localization microscopy. A strategy for designing optimal masks has been introduced based on maximizing the emitter localization accuracy, expressed in terms of Fisher information, over a targeted DoF range. However, the complete post-processing pipeline to localize a single emitter consists of two successive steps: detection, where the regions containing emitters are determined, and localization, where the sub-pixel position of each detected emitter is estimated. Phase masks usually optimize only this second step. The presence of a phase mask also affecting detection, the purpose of this paper is to quantify and mitigate this effect. Using a rigorous framework built from a detection-oriented information theoretical criterion (Bhattacharyya distance), we demonstrate that in most cases of practical significance, annular binary phase masks maximizing Fisher information also maximize the detection probability. This result supports the common design practice consisting of optimizing a phase mask by maximizing Fisher information only.

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.

Glutamate-Transporter Unbinding in Probabilistic Synaptic Environment Facilitates Activation of Distant NMDA Receptors

Once outside the synaptic cleft, the excitatory neurotransmitter glutamate is rapidly bound by its high-affinity transporters, which are expressed in abundance on the surface of perisynaptic astroglia. While this binding and the subsequent uptake of glutamate constrain excitatory transmission mainly within individual synapses, there is growing evidence for the physiologically important extrasynaptic actions of glutamate. However, the mechanistic explanation and the scope of such actions remain obscure. Furthermore, a significant proportion of glutamate molecules initially bound by transporters could be released back into the extracellular space before being translocated into astrocytes. To understand the implications of such effects, we simulated the release, diffusion, and transporter and receptor interactions of glutamate molecules in the synaptic environment. The latter was represented via trial-by-trial stochastic generation of astroglial and neuronal elements in the brain neuropil (overlapping spheroids of varied sizes), rather than using the 'average' morphology, thus reflecting the probabilistic nature of neuropil architectonics. Our simulations predict significant activation of high-affinity receptors, such as receptors of the NMDA type, at distances beyond half-micron from the glutamate release site, with glutamate-transporter unbinding playing an important role. These theoretical predictions are consistent with recent glutamate imaging data, thus lending support to the concept of significant volume-transmitted actions of glutamate in the brain.

Local diffusion in the extracellular space of the brain

The brain extracellular space (ECS) is a vast interstitial reticulum of extreme morphological complexity, composed of narrow gaps separated by local expansions, enabling interconnected highways between neural cells. Constituting on average 20% of brain volume, the ECS is key for intercellular communication, and understanding its diffusional properties is of paramount importance for understanding the brain. Within the ECS, neuroactive substances travel predominantly by diffusion, spreading through the interstitial fluid and the extracellular matrix scaffold after being focally released. The nanoscale dimensions of the ECS render it unresolvable by conventional live tissue compatible imaging methods, and historically diffusion of tracers has been used to indirectly infer its structure. Novel nanoscopic imaging techniques now show that the ECS is a highly dynamic compartment, and that diffusivity in the ECS is more heterogeneous than anticipated, with great variability across brain regions and physiological states. Diffusion is defined primarily by the local ECS geometry, and secondarily by the viscosity of the interstitial fluid, including the obstructive and binding properties of the extracellular matrix. ECS volume fraction and tortuosity both strongly determine diffusivity, and each can be independently regulated e.g. through alterations in glial morphology and the extracellular matrix composition. Here we aim to provide an overview of our current understanding of the ECS and its diffusional properties. We highlight emerging technological advances to respectively interrogate and model diffusion through the ECS, and point out how these may contribute in resolving the remaining enigmas of the ECS.

Transport in the Brain Extracellular Space: Diffusion, but Which Kind?

The mechanisms of transport of substances in the brain parenchyma have been a hot topic in scientific discussion in the past decade. This discussion was triggered by the proposed glymphatic hypothesis, which assumes a directed flow of cerebral fluid within the parenchyma, in contrast to the previous notion that diffusion is the main mechanism. However, when discussing the issue of “diffusion or non-diffusion”, much less attention was given to the question that diffusion itself can have a different character. In our opinion, some of the recently published results do not fit into the traditional understanding of diffusion. In this regard, we outline the relevant new theoretical approaches on transport processes in complex random media such as concepts of diffusive diffusivity and time-dependent homogenization, which expands the understanding of the forms of transport of substances based on diffusion.

Increased Extrasynaptic Glutamate Escape in Stochastically Shaped Probabilistic Synaptic Environment

Excitatory synapses in the brain are often surrounded by nanoscopic astroglial processes that express high-affinity glutamate transporters at a high surface density. This ensures that the bulk of glutamate leaving the synaptic cleft is taken up for its subsequent metabolic conversion and replenishment in neurons. Furthermore, variations in the astroglial coverage of synapses can thus determine to what extent glutamate released into the synaptic cleft could activate its receptors outside the cleft. The biophysical determinants of extrasynaptic glutamate actions are complex because they involve a competition between transporters and target receptors of glutamate in the tortuous space of synaptic environment. To understand key spatiotemporal relationships between the extrasynaptic landscapes of bound and free glutamate, we explored a detailed Monte Carlo model for its release, diffusion, and uptake. We implemented a novel representation of brain neuropil in silico as a space filled with randomly scattered, overlapping spheres (spheroids) of distributed size. The parameters of perisynaptic space, astroglial presence, and glutamate transport were constrained by the empirical data obtained for the 'average' environment of common cortical synapses. Our simulations provide a glimpse of the perisynaptic concentration landscapes of free and transporter-bound glutamate relationship, suggesting a significant tail of space-average free glutamate within 3 ms post-release.

Near-Infrared Carbon Nanotube Tracking Reveals the Nanoscale Extracellular Space around Synapses

We provide evidence of a local synaptic nanoenvironment in the brain extracellular space (ECS) lying within 500 nm of postsynaptic densities. To reveal this brain compartment, we developed a correlative imaging approach dedicated to thick brain tissue based on single-particle tracking of individual fluorescent single wall carbon nanotubes (SWCNTs) in living samples and on speckle-based HiLo microscopy of synaptic labels. We show that the extracellular space around synapses bears specific properties in terms of morphology at the nanoscale and inner diffusivity. We finally show that the ECS juxta-synaptic region changes its diffusion parameters in response to neuronal activity, indicating that this nanoenvironment might play a role in the regulation of brain activity.

The glymphatic system: Current understanding and modeling

We review theoretical and numerical models of the glymphatic system, which circulates cerebrospinal fluid and interstitial fluid around the brain, facilitating solute transport. Models enable hypothesis development and predictions of transport, with clinical applications including drug delivery, stroke, cardiac arrest, and neurodegenerative disorders like Alzheimer’s disease. We sort existing models into broad categories by anatomical function: Perivascular flow, transport in brain parenchyma, interfaces to perivascular spaces, efflux routes, and links to neuronal activity. Needs and opportunities for future work are highlighted wherever possible; new models, expanded models, and novel experiments to inform models could all have tremendous value for advancing the field.

On Some Current Challenges in High-Resolution Optical Bioimaging

In this Perspective we propose our current point of view and a suggestive roadmap on the field of high-resolution optical microscopy dedicated to bioimaging. Motivated by biological applications, researchers have indeed devised an impressive amount of strategies to address the diverse constraints of imaging and studying biological matter down to the molecular scale, making this interdisciplinary research field a vibrant forum for creativity. Throughout the discussion, we highlight several striking recent successes in this quest. We also identify some next challenges still ahead to apprehend biological questions in increasingly complex living organisms for integrative studies in a minimally invasive manner.

Toward dynamic, anisotropic, high-resolution, and functional measurement in the brain extracellular space

Diffusion of substances in the brain extracellular space (ECS) is important for extrasynaptic communication, extracellular ionic homeostasis, drug delivery, and metabolic waste clearance. However, substance diffusion is largely constrained by the geometry of brain ECS and the extracellular matrix. Investigating the diffusion properties of substances not only reveals the structural information of the brain ECS but also advances the understanding of intercellular signaling of brain cells. Among different techniques for substance diffusion measurement, the optical imaging method is sensitive and straightforward for measuring the dynamics and distribution of fluorescent molecules or sensors and has been used for molecular diffusion measurement in the brain. We mainly discuss recent advances of optical imaging-enabled measurements toward dynamic, anisotropic, high-resolution, and functional aspects of the brain ECS diffusion within the last 5 to 10 years. These developments are made possible by advanced imaging, such as light-sheet microscopy and single-particle tracking in tissue, and new fluorescent biosensors for neurotransmitters. We envision future efforts to map the ECS diffusivity across the brain under healthy and diseased conditions to guide the therapeutic delivery and better understand neurochemical transmissions that are relevant to physiological signaling and functions in brain circuits.

When Super-Resolution Localization Microscopy Meets Carbon Nanotubes

We recently assisted in a revolution in the realm of fluorescence microscopy triggered by the advent of super-resolution techniques that surpass the classic diffraction limit barrier. By providing optical images with nanometer resolution in the far field, super-resolution microscopy (SRM) is currently accelerating our understanding of the molecular organization of bio-specimens, bridging the gap between cellular observations and molecular structural knowledge, which was previously only accessible using electron microscopy. SRM mainly finds its roots in progress made in the control and manipulation of the optical properties of (single) fluorescent molecules. The flourishing development of novel fluorescent nanostructures has recently opened the possibility of associating super-resolution imaging strategies with nanomaterials’ design and applications. In this review article, we discuss some of the recent developments in the field of super-resolution imaging explicitly based on the use of nanomaterials. As an archetypal class of fluorescent nanomaterial, we mainly focus on single-walled carbon nanotubes (SWCNTs), which are photoluminescent emitters at near-infrared (NIR) wavelengths bearing great interest for biological imaging and for information optical transmission. Whether for fundamental applications in nanomaterial science or in biology, we show how super-resolution techniques can be applied to create nanoscale images “in”, “of” and “with” SWCNTs.

Effect of Particle Size and Surface Charge on Nanoparticles Diffusion in the Brain White Matter

Purpose

Brain disorders have become a serious problem for healthcare worldwide. Nanoparticle-based drugs are one of the emerging therapies and have shown great promise to treat brain diseases. Modifications on particle size and surface charge are two efficient ways to increase the transport efficiency of nanoparticles through brain-blood barrier; however, partly due to the high complexity of brain microstructure and limited visibility of Nanoparticles (NPs), our understanding of how these two modifications can affect the transport of NPs in the brain is insufficient.

Methods

In this study, a framework, which contains a stochastic geometric model of brain white matter (WM) and a mathematical particle tracing model, was developed to investigate the relationship between particle size/surface charge of the NPs and their effective diffusion coefficients (D) in WM.

Results

The predictive capabilities of this method have been validated using published experimental tests. For negatively charged NPs, both particle size and surface charge are positively correlated with D before reaching a size threshold. When Zeta potential (Zp) is less negative than -10 mV, the difference between NPs’ D in WM and pure interstitial fluid (IF) is limited.

Conclusion

A deeper understanding on the relationships between particle size/surface charge of NPs and their D in WM has been obtained. The results from this study and the developed modelling framework provide important tools for the development of nano-drugs and nano-carriers to cure brain diseases.

Super-Resolution Radial Fluctuations (SRRF) nanoscopy in the near infrared

Super resolution microscopy methods have been designed to overcome the physical barrier of the diffraction limit and push the resolution to nanometric scales. A recently developed super resolution technique, super-resolution radial fluctuations (SRRF) [Nature communications, 7, 12471 (2016)10.1038/ncomms12471], has been shown to super resolve images taken with standard microscope setups without fluorophore localization. Herein, we implement SRRF on emitters in the near-infrared (nIR) range, single walled carbon nanotubes (SWCNTs), whose fluorescence emission overlaps with the biological transparency window. Our results open the path for super-resolving SWCNTs for biomedical imaging and sensing applications.

Super-resolving Microscopy in Neuroscience

Fluorescence imaging techniques play a pivotal role in our understanding of the nervous system. The emergence of various super-resolution microscopy methods and specialized fluorescent probes enables direct insight into neuronal structure and protein arrangements in cellular subcompartments with so far unmatched resolution. Super-resolving visualization techniques in neurons unveil a novel understanding of cytoskeletal composition, distribution, motility, and signaling of membrane proteins, subsynaptic structure and function, and neuron-glia interaction. Well-defined molecular targets in autoimmune and neurodegenerative disease models provide excellent starting points for in-depth investigation of disease pathophysiology using novel and innovative imaging methodology. Application of super-resolution microscopy in human brain samples and for testing clinical biomarkers is still in its infancy but opens new opportunities for translational research in neurology and neuroscience. In this review, we describe how super-resolving microscopy has improved our understanding of neuronal and brain function and dysfunction in the last two decades.

Poly-ion complex micelles effectively deliver CoA-conjugated CPT1A inhibitors to modulate lipid metabolism in brain cells

Carnitine palmitoyltransferase 1A (CPT1A) is a central player in lipid metabolism, catalyzing the first step to fatty acid oxidation (FAO). Inhibiting CPT1A, especially in the brain, can have several pharmacological benefits, such as in treating obesity and brain cancer. C75-CoA is a strong competitive inhibitor of CPT1A. However, due to its negatively charged nature, it has low cellular permeability. Herein, we report the use of poly-ion complex (PIC) micelles to deliver the specific CPT1A inhibitors (±)-, (+)-, and (-)-C75-CoA into U87MG glioma cells and GT1-7 neurons. PIC micelles were formed through charge-neutralization of the cargo with the cationic side chain of PEG-poly{N-[N'-(2-aminoethyl)-2-aminoethyl]aspartamide} (PEG-PAsp(DET)), forming particles with 55 to 65 nm diameter. Upon short-term incubation with cells, the micelle-encapsulated CPT1A inhibitors resulted in up to 5-fold reduction of ATP synthesis compared to the free drug, without an apparent decline in cell viability. Micelle treatment showed a discernible decrease in 14C-palmitate oxidation into CO2 and acid-soluble metabolites, confirming that the substantial lowering of ATP production has resulted from FAO inhibition. Micelle treatment also diminished IC50 by 2 to 4-fold over the free drug-treated U87MG after long-term incubation. To measure the cellular uptake of these CoA-adduct loaded PIC micelles, we synthesized a fluorescent CoA derivative and prepared Fluor-CoA micelles which showed efficient internalization in the cell lines, both in 2D and 3D culture models, especially in neurons where uptake reached up to 3-fold over the free dye. Our results starkly demonstrate that the PIC micelles are a promising delivery platform for anionic inhibitors of CPT1A in glioma cells and neurons, laying the groundwork for future research or clinical applications.

Super-resolution fluorescence imaging of extracellular environments

The extracellular matrix (ECM) is an important biophysical environment that plays a role in a number of physiological processes. The ECM is highly dynamic, with changes occurring as local, nanoscale, physicochemical variations in physical confinement and chemistry from the perspective of biological molecules. The length and time scale of ECM dynamics are challenging to measure with current spectroscopic techniques. Super-resolution fluorescence microscopy has the potential to probe local, nanoscale, physicochemical variations in the ECM. Here, we review super-resolution imaging and analysis methods and their application to study model nanoparticles and biomolecules within synthetic ECM hydrogels and the brain extracellular space (ECS). We provide a perspective of future directions for the field that can move super-resolution imaging of the ECM towards more biomedically-relevant samples. Overall, super-resolution imaging is a powerful tool that can increase our understanding of extracellular environments at new spatiotemporal scales to reveal ECM processes at the molecular-level.

Rapid Generation of Neurospheres from Hippocampal Neurons Using Extracellular-Matrix-Mimetic Scaffolds

3D models of brain organoids represent an innovative and promising tool in neuroscience studies. However, the process of neurosphere formation in vitro remains complicated and is not always very effective. This is largely due to the lack of growth factors, guidance cues, and scaffold structures commonly found in tissues. Here we present a new, simple, and efficient method for generating neurospheres using scaffolds composed of electrospun nylon fibers with a diameter of 40-180 nm, which makes them similar to the brain extracellular matrix (ECM) components. Several main advantages of the proposed method should be highlighted. The method is fast, and the biomaterial consumption is low. Also, the resulting neurospheres are attached to the scaffold nanofibers. This not only provides the experimental convenience but also suggests that the resulting organoid models can potentially demonstrate fundamentally new properties, being closer to the nervous tissue in vivo. We demonstrate the influence of the fibrous scaffold structure on the formation, morphology, and composition of neurospheres and confirm adequate functional activity of the cellular components of these spheroids. The proposed approach can be further used for drug screening, modeling of neurodevelopmental, neurodegenerative disorders, and, potentially, therapeutic tissue engineering.

Buffering by Transporters Can Spare Geometric Hindrance in Controlling Glutamate Escape

The surface of astrocyte processes that often surround excitatory synapses is packed with high-affinity glutamate transporters, largely preventing extrasynaptic glutamate escape. The shape and prevalence of perisynaptic astroglia vary among brain regions, in some cases providing a complete isolation of synaptic connections from the surrounding tissue. The perception has been that the geometry of perisynaptic environment is therefore essential to preventing extrasynaptic glutamate escape. To understand to what degree this notion holds, we modelled brain neuropil as a space filled with a scatter of randomly sized, overlapping spheres representing randomly shaped cellular elements and intercellular lumen. Simulating release and diffusion of glutamate molecules inside the interstitial gaps in this medium showed that high-affinity transporters would efficiently constrain extrasynaptic spread of glutamate even when diffusion passages are relatively open. We thus estimate that, in the hippocampal or cerebellar neuropil, the bulk of glutamate released by a synaptic vesicle is rapidly bound by transporters (or high-affinity target receptors) mainly in close proximity of the synaptic cleft, whether or not certain physiological or pathological events change local tissue geometry.

Multiple Particle Tracking Detects Changes in Brain Extracellular Matrix and Predicts Neurodevelopmental Age

Brain extracellular matrix (ECM) structure mediates many aspects of neural development and function. Probing structural changes in brain ECM could thus provide insights into mechanisms of neurodevelopment, the loss of neural function in response to injury, and the detrimental effects of pathological aging and neurological disease. We demonstrate the ability to probe changes in brain ECM microstructure using multiple particle tracking (MPT). We performed MPT of colloidally stable polystyrene nanoparticles in organotypic rat brain slices collected from rats aged 14-70 days old. Our analysis revealed an inverse relationship between nanoparticle diffusive ability in the brain extracellular space and age. Additionally, the distribution of effective ECM pore sizes in the cortex shifted to smaller pores throughout development. We used the raw data and features extracted from nanoparticle trajectories to train a boosted decision tree capable of predicting chronological age with high accuracy. Collectively, this work demonstrates the utility of combining MPT with machine learning for measuring changes in brain ECM structure and predicting associated complex features such as chronological age. This will enable further understanding of the roles brain ECM play in development and aging and the specific mechanisms through which injuries cause aberrant neuronal function. Additionally, this approach has the potential to develop machine learning models capable of detecting the presence of injury or indicating the extent of injury based on changes in the brain microenvironment microstructure.

Characterizing extracellular diffusion properties using diffusion-weighted MRS of sucrose injected in mouse brain

Brain water and some critically important energy metabolites, such as lactate or glucose, are present in both intracellular and extracellular spaces (ICS/ECS) at significant levels. This ubiquitous nature makes diffusion MRI/MRS data sometimes difficult to interpret and model. While it is possible to glean information on the diffusion properties in ICS by measuring the diffusion of purely intracellular endogenous metabolites (such as NAA), the absence of endogenous markers specific to ECS hampers similar analyses in this compartment. In past experiments, exogenous probes have therefore been injected into the brain to assess their apparent diffusion coefficient (ADC) and thus estimate tortuosity in ECS. Here, we use a similar approach in mice by injecting sucrose, a well-known ECS marker, in either the lateral ventricles or directly in the prefrontal cortex. For the first time, we propose a thorough characterization of ECS diffusion properties encompassing (1) short-range restriction by looking at signal attenuation at high b values, (2) tortuosity and long-range restriction by measuring ADC time-dependence at long diffusion times and (3) microscopic anisotropy by performing double diffusion encoding (DDE) measurements. Overall, sucrose diffusion behavior is strikingly different from that of intracellular metabolites. Acquisitions at high b values not only reveal faster sucrose diffusion but also some sensitivity to restriction, suggesting that the diffusion in ECS is not fully Gaussian at high b. The time evolution of the ADC at long diffusion times shows that the tortuosity regime is not reached yet in the case of sucrose, while DDE experiments suggest that it is not trapped in elongated structures. No major difference in sucrose diffusion properties is reported between the two investigated routes of injection and brain regions. These original experimental insights should be useful to better interpret and model the diffusion signal of molecules that are distributed between ICS and ECS compartments.

Near-infrared nanoscopy with carbon-based nanoparticles for the exploration of the brain extracellular space

Understanding the physiology and pathology of the brain requires detailed knowledge of its complex structures as well as dynamic internal processes at very different scales from the macro down to the molecular dimensions. A major yet poorly described brain compartment is the brain extracellular space (ECS). Signalling molecules rapidly diffuse through the brain ECS which is complex and dynamic structure at numerous lengths and time scales. In recent years, characterization of the ECS using nanomaterials has made remarkable progress, including local analysis of nanoscopic dimensions and diffusivity as well as local chemical sensing. In particular, carbon nanomaterials combined with advanced optical technologies, biochemical and biophysical analysis, offer novel promises for understanding the ECS morphology as well as neuron connectivity and neurochemistry. In this review, we present the state-of-the-art in this quest, which mainly focuses on a type of carbon nanomaterial, single walled carbon nanotubes, as fluorescent nanoprobes to unveil the ECS features in the nanometre domain.

Computationally-efficient spatiotemporal correlation analysis super-resolves anomalous diffusion

Anomalous diffusion dynamics in confined nanoenvironments govern the macroscale properties and interactions of many biophysical and material systems. Currently, it is difficult to quantitatively link the nanoscale structure of porous media to anomalous diffusion within them. Fluorescence correlation spectroscopy super-resolution optical fluctuation imaging (fcsSOFI) has been shown to extract nanoscale structure and Brownian diffusion dynamics within gels, liquid crystals, and polymers, but has limitations which hinder its wider application to more diverse, biophysically-relevant datasets. Here, we parallelize the least-squares curve fitting step on a GPU improving computation times by up to a factor of 40, implement anomalous diffusion and two-component Brownian diffusion models, and make fcsSOFI more accessible by packaging it in a user-friendly GUI. We apply fcsSOFI to simulations of the protein fibrinogen diffusing in polyacrylamide of varying matrix densities and super-resolve locations where slower, anomalous diffusion occurs within smaller, confined pores. The improvements to fcsSOFI in speed, scope, and usability will allow for the wider adoption of super-resolution correlation analysis to diverse research topics.

Real-Time Dissecting the Dynamics of Drug Transportation in the Live Brain

Brain diseases are becoming a more and more serious threat to human health. Many critical properties of the transport mechanisms of drugs in live brains remain poorly understood. In this work, single-particle tracking was used to dissect the transport dynamics of wheat germ agglutinin (WGA) in live brain and characterize the geometry and rheology of the extracellular space (ECS). The results revealed that the movements of WGA were influenced by the specific-binding molecules and the nature of the ECS. We further analyzed the mobility behaviors of WGA globally and quantitatively and found that movement of WGA in brain cells of acute slices was an active transport process associated with actin filaments and microtubules. This work paves the way for studies aiming at characterizing the biophysics of drug transport in the context of live brains, which may contribute to developing potential new therapeutic applications for brain diseases.

Governing Transport Principles for Nanotherapeutic Application in the Brain

Neurological diseases account for a significant portion of the global disease burden. While research efforts have identified potential drugs or drug targets for neurological diseases, most therapeutic platforms are still ineffective at reaching the target location selectively and with high yield. Restricted transport, including passage across the blood-brain barrier, through the brain parenchyma, and into specific cells, is a major cause of ineffective therapeutic delivery. However, nanotechnology is a promising, tailorable platform for overcoming these transport barriers and improving therapeutic delivery to the brain. We provide a transport-oriented analysis of nanotechnology's ability to navigate these transport barriers in the brain. We also provide an opinion on the need for technology development for increasing our capacity to characterize and quantify nanoparticle passage through each transport barrier. Finally, we highlight the importance of incorporating the effect of disease, metabolic state, and regional dependencies to better understand transport of nanotherapeutics in the brain.

Current Techniques for Investigating the Brain Extracellular Space

The brain extracellular space (ECS) is a continuous reticular compartment that lies between the cells of the brain. It is vast in extent relative to its resident cells, yet, at the same time the nano- to micrometer dimensions of its channels and reservoirs are commonly finer than the smallest cellular structures. Our conventional view of this compartment as largely static and of secondary importance for brain function is rapidly changing, and its active dynamic roles in signaling and metabolite clearance have come to the fore. It is further emerging that ECS microarchitecture is highly heterogeneous and dynamic and that ECS geometry and diffusional properties directly modulate local diffusional transport, down to the nanoscale around individual synapses. The ECS can therefore be considered an extremely complex and diverse compartment, where numerous physiological events are unfolding in parallel on spatial and temporal scales that span orders of magnitude, from milliseconds to hours, and from nanometers to centimeters. To further understand the physiological roles of the ECS and identify new ones, researchers can choose from a wide array of experimental techniques, which differ greatly in their applicability to a given sample and the type of data they produce. Here, we aim to provide a basic introduction to the available experimental techniques that have been applied to address the brain ECS, highlighting their main characteristics. We include current gold-standard techniques, as well as emerging cutting-edge modalities based on recent super-resolution microscopy. It is clear that each technique comes with unique strengths and limitations and that no single experimental method can unravel the unknown physiological roles of the brain ECS on its own.

Synucleinopathy alters nanoscale organization and diffusion in the brain extracellular space through hyaluronan remodeling

In recent years, exploration of the brain extracellular space (ECS) has made remarkable progress, including nanoscopic characterizations. However, whether ECS precise conformation is altered during brain pathology remains unknown. Here we study the nanoscale organization of pathological ECS in adult mice under degenerative conditions. Using electron microscopy in cryofixed tissue and single nanotube tracking in live brain slices combined with super-resolution imaging analysis, we find enlarged ECS dimensions and increased nanoscale diffusion after α-synuclein-induced neurodegeneration. These animals display a degraded hyaluronan matrix in areas close to reactive microglia. Furthermore, experimental hyaluronan depletion in vivo reduces dopaminergic cell loss and α-synuclein load, induces microgliosis and increases ECS diffusivity, highlighting hyaluronan as diffusional barrier and local tissue organizer. These findings demonstrate the interplay of ECS, extracellular matrix and glia in pathology, unraveling ECS features relevant for the α-synuclein propagation hypothesis and suggesting matrix manipulation as a disease-modifying strategy.

The nanoscale organisation of the brain extracellular space can be studied in vivo. Here, the authors investigate how it changes in response to α-synuclein pathology, and identify interactions between microglia and the extracellular matrix.

Fluorescent sp3 Defect-Tailored Carbon Nanotubes Enable NIR-II Single Particle Imaging in Live Brain Slices at Ultra-Low Excitation Doses

Cellular and tissue imaging in the second near-infrared window (NIR-II, ~1000–1350 nm) is advantageous for in vivo studies because of low light extinction by biological constituents at these wavelengths. However, deep tissue imaging at the single molecule sensitivity has not been achieved in the NIR-II window due to lack of suitable bio-probes. Single-walled carbon nanotubes have emerged as promising near-infrared luminescent molecular bio-probes; yet, their inefficient photoluminescence (quantum yield ~1%) drives requirements for sizeable excitation doses (~1–10 kW/cm²) that are significantly blue-shifted from the NIR-II region (<850 nm) and may thus ultimately compromise live tissue. Here, we show that single nanotube imaging can be achieved in live brain tissue using ultralow excitation doses (~0.1 kW/cm²), an order of magnitude lower than those currently used. To accomplish this, we synthesized fluorescent sp³-defect tailored (6,5) carbon nanotubes which, when excited at their first order excitonic transition (~985 nm) fluoresce brightly at ~1160 nm. The biocompatibility of these functionalized nanotubes, which are wrapped by encapsulation agent (phospholipid-polyethylene glycol), is demonstrated using standard cytotoxicity assays. Single molecule photophysical studies of these biocompatible nanotubes allowed us to identify the optimal luminescence properties in the context of biological imaging.