Calcium diffusion enhanced after cleavage of negatively charged components of brain extracellular matrix by chondroitinase ABC

Toward Precise Modeling of Dopamine Release Kinetics: Comparison and Validation of Kinetic Models Using Voltammetry

Dopamine (DA) is a neurotransmitter present within the animal brain that is responsible for a wide range of physiologic functions, including motivation, reward, and movement control. Changes or dysfunction in the dynamics of DA release are thought to play a pivotal role in regulating various physiological and behavioral processes, as well as leading to neuropsychiatric diseases. Therefore, it is of fundamental interest to neuroscientists to understand and accurately model the kinetics that govern dopaminergic neurotransmission. In the past several decades, many mathematical models have been proposed to attempt to capture the biologic parameters that govern dopaminergic kinetics, with each model seeking to improve upon a previous model. In this review, each of these models are derived, and the ability of each model to properly fit two fast-scan cyclic voltammetry (FSCV) data sets will be demonstrated and discussed. The dopamine oxidation current in both FSCV data sets exhibits hang-up and overshoot behaviors, which have traditionally been difficult for mathematical models to capture. We show that more recent models are better able to model DA release that exhibits these behaviors but that no single model is clearly the best. Rather, models should be selected based on their mathematical properties to best fit the FSCV data one is trying to model. Developing such differential equation models to describe the kinetics of DA release from the synapse confers significant applications both for advancing scientific understanding of DA neurotransmission and for advancing clinical ability to treat neuropsychiatric diseases.

Astrocytes require perineuronal nets to maintain synaptic homeostasis in mice

Perineuronal nets (PNNs) are densely packed extracellular matrices that cover the cell body of fast-spiking inhibitory neurons. PNNs stabilize synapses inhibiting synaptic plasticity. Here we show that synaptic terminals of fast-spiking interneurons localize to holes in the PNNs in the adult mouse somatosensory cortex. Approximately 95% of holes in the PNNs contain synapses and astrocytic processes expressing Kir4.1, glutamate and GABA transporters. Hence, holes in the PNNs contain tripartite synapses. In the adult mouse brain, PNN degradation causes an expanded astrocytic coverage of the neuronal somata without altering the axon terminals. The loss of PNNs impairs astrocytic transmitter and potassium uptake, resulting in the spillage of glutamate into the extrasynaptic space. Our data show that PNNs and astrocytes cooperate to contain synaptically released signals in physiological conditions. Their combined action is altered in mouse models of Alzheimer's disease and epilepsy where PNNs are disrupted.

Assessing translational applicability of perineuronal net dysfunction in Alzheimer's disease across species

In the context of aging and age-associated neurodegenerative disorders, the brain's extracellular matrix (ECM) serves as a critical regulator for neuronal health and cognitive function. Within the extracellular space, proteoglycans and their glycosaminoglycan attachments play essential roles in forming, stabilizing, and protecting neural circuits throughout neurodevelopment and adulthood. Recent studies in rodents reveal that chondroitin sulfate-glycosaminoglycan (CS-GAG) containing perineuronal nets (PNNs) exhibit both structural and compositional differences throughout the brain. While animal studies are illuminating, additional research is required to translate these interregional PNN/CS-GAG variations to human brain tissue. In this perspective article, we first investigate the translational potential for interregional CS-GAG variances across species as novel targets for region-specific therapeutic development. We specifically focus on the observation that alterations in brain PNN-associated CS-GAGs have been linked with the progression of Alzheimer's disease (AD) neuropathology in humans, but these changes have not been fully recapitulated in rodent models of this disease. A second highlight of this perspective article investigates whether AD-associated shifts in CS-GAGs in humans may be dependent on region-specific baseline differences in CS-GAG sulfation patterning. The current findings begin to disentangle the intricate relationships between the interregional differences in brain PNN/CS-GAG matrices across species, while emphasizing the need to better understand the close relationship between dementia and changes in brain CS-GAG sulfation patterns in patients with AD and related dementias.

Astrocyte-Neuronal Communication and Its Role in Stroke

Astrocytes are the most abundant glial cells in the central nervous system. These cells are an important hub for intercellular communication. They participate in various pathophysiological processes, including synaptogenesis, metabolic transformation, scar production, and blood-brain barrier repair. The mechanisms and functional consequences of astrocyte-neuron signaling are more complex than previously thought. Stroke is a disease associated with neurons in which astrocytes also play an important role. Astrocytes respond to the alterations in the brain microenvironment after stroke, providing required substances to neurons. However, they can also have harmful effects. In this review, we have summarized the function of astrocytes, their association with neurons, and two paradigms of the inflammatory response, which suggest that targeting astrocytes may be an effective strategy for treating stroke.

Simulations of active zone structure and function at mammalian NMJs predict that loss of calcium channels alone is not sufficient to replicate LEMS effects

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune-mediated neuromuscular disease thought to be caused by autoantibodies against P/Q-type voltage-gated calcium channels (VGCCs), which attack and reduce the number of VGCCs within transmitter release sites (active zones; AZs) at the neuromuscular junction (NMJ), resulting in neuromuscular weakness. However, patients with LEMS also have antibodies to other neuronal proteins, and about 15% of patients with LEMS are seronegative for antibodies against VGCCs. We hypothesized that a reduction in the number of P/Q-type VGCCs alone is not sufficient to explain LEMS effects on transmitter release. Here, we used a computational model to study a variety of LEMS-mediated effects on AZ organization and transmitter release constrained by electron microscopic, pharmacological, immunohistochemical, voltage imaging, and electrophysiological observations. We show that models of healthy AZs can be modified to predict the transmitter release and short-term facilitation characteristics of LEMS and that in addition to a decrease in the number of AZ VGCCs, disruption in the organization of AZ proteins, a reduction in AZ number, a reduction in the amount of synaptotagmin, and the compensatory expression of L-type channels outside the remaining AZs are important contributors to LEMS-mediated effects on transmitter release. Furthermore, our models predict that antibody-mediated removal of synaptotagmin in combination with disruption in AZ organization alone could mimic LEMS effects without the removal of VGCCs (a seronegative model). Overall, our results suggest that LEMS pathophysiology may be caused by a collection of pathological alterations to AZs at the NMJ, rather than by a simple loss of VGCCs.NEW & NOTEWORTHY We used a computational model of the active zone (AZ) in the mammalian neuromuscular junction to investigate Lambert-Eaton myasthenic syndrome (LEMS) pathophysiology. This model suggests that disruptions in presynaptic active zone organization and protein content (particularly synaptotagmin), beyond the simple removal of presynaptic calcium channels, play an important role in LEMS pathophysiology.

Understanding Neuropeptide Transmission in the Brain by Optical Uncaging and Release

Neuropeptides are abundant and essential signaling molecules in the nervous system involved in modulating neural circuits and behavior. Neuropeptides are generally released extrasynaptically and signal via volume transmission through G-protein-coupled receptors (GPCR). Although substantive functional roles of neuropeptides have been discovered, many questions on neuropeptide transmission remain poorly understood, including the local diffusion and transmission properties in the brain extracellular space. To address this challenge, intensive efforts are required to develop advanced tools for releasing and detecting neuropeptides with high spatiotemporal resolution. Because of the rapid development of biosensors and materials science, emerging tools are beginning to provide a better understanding of neuropeptide transmission. In this perspective, we summarize the fundamental advances in understanding neuropeptide transmission over the past decade, highlight the tools for releasing neuropeptides with high spatiotemporal solution in the brain, and discuss open questions and future directions in the field.

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.

Microglia as hackers of the matrix: sculpting synapses and the extracellular space

Microglia shape the synaptic environment in health and disease, but synapses do not exist in a vacuum. Instead, pre- and postsynaptic terminals are surrounded by extracellular matrix (ECM), which together with glia comprise the four elements of the contemporary tetrapartite synapse model. While research in this area is still just beginning, accumulating evidence points toward a novel role for microglia in regulating the ECM during normal brain homeostasis, and such processes may, in turn, become dysfunctional in disease. As it relates to synapses, microglia are reported to modify the perisynaptic matrix, which is the diffuse matrix that surrounds dendritic and axonal terminals, as well as perineuronal nets (PNNs), specialized reticular formations of compact ECM that enwrap neuronal subsets and stabilize proximal synapses. The interconnected relationship between synapses and the ECM in which they are embedded suggests that alterations in one structure necessarily affect the dynamics of the other, and microglia may need to sculpt the matrix to modify the synapses within. Here, we provide an overview of the microglial regulation of synapses, perisynaptic matrix, and PNNs, propose candidate mechanisms by which these structures may be modified, and present the implications of such modifications in normal brain homeostasis and in disease.

Perineuronal Net Dynamics in the Pathophysiology of Epilepsy

Perineuronal nets (PNNs) are condensed extracellular matrix (ECM) assemblies of polyanionic chondroitin sulfate proteoglycans, hyaluronan, and tenascins that primarily wrap around GABAergic parvalbumin (PV) interneurons. During development, PNN formation terminates the critical period of neuroplasticity, a process that can be reversed by experimental disruption of PNNs. Perineuronal nets also regulate the intrinsic properties of the enclosed PV neurons thereby maintaining their inhibitory activity. Recent studies have implicated PNNs in central nervous system diseases as well as PV neuron dysfunction; consequently, they have further been associated with altered inhibition, particularly in the genesis of epilepsy. A wide range of seizure presentations in human and rodent models exhibit ECM remodeling with PNN disruption due to elevated protease activity. Inhibition of PNN proteolysis reduces seizure activity suggesting that PNN degrading enzymes may be potential novel therapeutic targets.

Surfactants influence polymer nanoparticle fate within the brain

Drug delivery to the brain is limited by poor penetration of pharmaceutical agents across the blood-brain barrier (BBB), within the brain parenchyma, and into specific cells of interest. Nanotechnology can overcome these barriers, but its ability to do so is dependent on nanoparticle physicochemical properties including surface chemistry. Surface chemistry can be determined by a number of factors, including by the presence of stabilizing surfactant molecules introduced during the formulation process. Nanoparticles coated with poloxamer 188 (F68), poloxamer 407 (F127), and polysorbate 80 (P80) have demonstrated uptake in BBB endothelial cells and enhanced accumulation within the brain. However, the impact of surfactants on nanoparticle fate, and specifically on brain extracellular diffusion or intracellular targeting, must be better understood to design nanotherapeutics to efficiently overcome drug delivery barriers in the brain. Here, we evaluated the effect of the biocompatible and commonly used surfactants cholic acid (CHA), F68, F127, P80, and poly (vinyl alcohol) (PVA) on poly (lactic-co-glycolic acid)-poly (ethylene glycol) (PLGA-PEG) nanoparticle transport to and within the brain. The inclusion of these surfactant molecules decreases diffusive ability through brain tissue, reflecting the surfactant's role in encouraging cellular interaction at short length and time scales. After in vivo administration, PLGA-PEG/P80 nanoparticles demonstrated enhanced penetration across the BBB and subsequent internalization within neurons and microglia. Surfactants incorporated into the formulation of PLGA-PEG nanoparticles therefore represent an important design parameter for controlling nanoparticle fate within the brain.

Modulatory Effects of Monoamines and Perineuronal Nets on Output of Cerebellar Purkinje Cells

Classically, the cerebellum has been thought to play a significant role in motor coordination. However, a growing body of evidence for novel neural connections between the cerebellum and various brain regions indicates that the cerebellum also contributes to other brain functions implicated in reward, language, and social behavior. Cerebellar Purkinje cells (PCs) make inhibitory GABAergic synapses with their target neurons: other PCs and Lugaro/globular cells via PC axon collaterals, and neurons in the deep cerebellar nuclei (DCN) via PC primary axons. PC-Lugaro/globular cell connections form a cerebellar cortical microcircuit, which is driven by serotonin and noradrenaline. PCs' primary outputs control not only firing but also synaptic plasticity of DCN neurons following the integration of excitatory and inhibitory inputs in the cerebellar cortex. Thus, strong PC-mediated inhibition is involved in cerebellar functions as a key regulator of cerebellar neural networks. In this review, we focus on physiological characteristics of GABAergic transmission from PCs. First, we introduce monoaminergic modulation of GABAergic transmission at synapses of PC-Lugaro/globular cell as well as PC-large glutamatergic DCN neuron, and a Lugaro/globular cell-incorporated microcircuit. Second, we review the physiological roles of perineuronal nets (PNNs), which are organized components of the extracellular matrix and enwrap the cell bodies and proximal processes, in GABA release from PCs to large glutamatergic DCN neurons and in cerebellar motor learning. Recent evidence suggests that alterations in PNN density in the DCN can regulate cerebellar functions.

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.

Novel Influences of Sex and APOE Genotype on Spinal Plasticity and Recovery of Function after Spinal Cord Injury

Spinal cord injuries can abolish both motor and sensory function throughout the body. Spontaneous recovery after injury is limited and can vary substantially between individuals. Despite an abundance of therapeutic approaches that have shown promise in preclinical models, there is currently a lack of effective treatment strategies that have been translated to restore function after spinal cord injury (SCI) in the human population. We hypothesized that sex and genetic background of injured individuals could impact how they respond to treatment strategies, presenting a barrier to translating therapies that are not tailored to the individual. One gene of particular interest is APOE, which has been extensively studied in the brain because of its allele-specific influences on synaptic plasticity, metabolism, inflammation, and neurodegeneration. Despite its prominence as a therapeutic target in brain injury and disease, little is known about how it influences neural plasticity and repair processes in the spinal cord. Using humanized mice, we examined how the ε3 and ε4 alleles of APOE influence the efficacy of therapeutic intermittent hypoxia (IH) in inducing spinally-mediated plasticity after cervical SCI (cSCI). IH is sufficient to enhance plasticity and restore motor function after experimental SCI in genetically similar rodent populations, but its effect in human subjects is more variable (Golder and Mitchell, 2005; Hayes et al., 2014). Our results demonstrate that both sex and APOE genotype determine the extent of respiratory motor plasticity that is elicited by IH, highlighting the importance of considering these clinically relevant variables when translating therapeutic approaches for the SCI community.

Diurnal changes in perineuronal nets and parvalbumin neurons in the rat medial prefrontal cortex

Perineuronal nets (PNNs) surrounding fast-spiking, parvalbumin (PV) interneurons provide excitatory:inhibitory balance, which is impaired in several disorders associated with altered diurnal rhythms, yet few studies have examined diurnal rhythms of PNNs or PV cells. We measured the intensity and number of PV cells and PNNs labeled with Wisteria floribunda agglutinin (WFA) and also the oxidative stress marker 8-oxo-deoxyguanosine (8-oxo-dG) in rat prelimbic medial prefrontal cortex (mPFC) at Zeitgeber times (ZT) ZT0 (lights-on, inactive phase), ZT6 (mid-inactive phase), ZT12 (lights-off, active phase), and ZT18 (mid-active phase). Relative to ZT0, the intensities of PNN and PV labeling were increased in the dark (active) phase compared with the light (inactive) phase. The intensity of 8-oxo-dG was decreased from ZT0 at all times (ZT6,12,18). We also measured GAD 65/67 and vGLUT1 puncta apposed to PV cells with and without PNNs. There were more excitatory puncta on PV cells with PNNs at ZT18 vs. ZT6, but no changes in PV cells without PNNs and no changes in inhibitory puncta. Whole-cell slice recordings in fast-spiking (PV) cells with PNNs showed an increased ratio of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor:N-methyl-D-aspartate receptor (AMPA: NMDA) at ZT18 vs. ZT6. The number of PV cells and PV/PNN cells containing orthodenticle homeobox 2 (OTX2), which maintains PNNs, showed a strong trend toward an increase from ZT6 to ZT18. Diurnal fluctuations in PNNs and PV cells are expected to alter cortical excitatory:inhibitory balance and provide new insights into treatments for diseases impacted by disturbances in sleep and circadian rhythms.

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.

Hypothalamic perineuronal net assembly is required for sustained diabetes remission induced by fibroblast growth factor 1 in rats

We recently showed that perineuronal nets (PNNs) enmesh glucoregulatory neurons in the arcuate nucleus (Arc) of the mediobasal hypothalamus (MBH)¹, but whether these PNNs play a role in either the pathogenesis of type 2 diabetes (T2D) or its treatment remains unclear. Here we show that PNN abundance within the Arc is markedly reduced in the Zucker diabetic fatty (ZDF) rat model of T2D, compared with normoglycaemic rats, correlating with altered PNN-associated sulfation patterns of chondroitin sulfate glycosaminoglycans in the MBH. Each of these PNN-associated changes is reversed following a single intracerebroventricular (icv) injection of fibroblast growth factor 1 (FGF1) at a dose that induces sustained diabetes remission in male ZDF rats. Combined with previous work localizing this FGF1 effect to the Arc area^2-4, our finding that enzymatic digestion of Arc PNNs markedly shortens the duration of diabetes remission following icv FGF1 injection in these animals identifies these extracellular matrix structures as previously unrecognized participants in the mechanism underlying diabetes remission induced by the central action of FGF1.

Aggrecan modulates the expression and phosphorylation of tau in a novel bigenic TauP301L - Acan mouse model

Selected types of neurons in the central nervous system are associated with a specialized form of extracellular matrix. These so-called perineuronal nets (PNs) are supramolecular structures surrounding neuronal somata, proximal dendrites and axon initial segments. PNs are involved in the regulation of plasticity and synaptic physiology. In addition, PNs were proposed to carry neuroprotective functions as PN-ensheathed neurons are mostly spared of tau pathology in brains of Alzheimer patients. Recently, the neuroprotective action of PNs was confirmed experimentally, demonstrating (i) that mainly aggrecan mediates the neuroprotective function of PNs and (ii) that aggrecan seems to generate an external shielding preventing the internalization of pathological forms of tau. In the present study, we aimed at extending these findings and hypothesized that aggrecan further provides an intracellular protection by preventing mutation-triggered formation of pathological forms of tau. We used crossbreds of TauP301L mice and heterozygous aggrecan mice which are characterized by spontaneous deletion of the aggrecan allele. We analysed the extent of tau pathology in dependence of aggrecan protein amount by applying immunohistochemistry, Western blotting and ELISA. The results clearly indicate that aggrecan has no significant impact on tau aggregation in the brainstem of our mouse model. Still, reduced aggrecan levels were accompanied by increased levels of tau protein and reduced number of Tau-1-positive neurons, which indicate an increase in phosphorylation of tau. In conclusion, these data demonstrate a correlation between aggrecan and P301L mutation-triggered tau expression and phosphorylation in our bigenic mouse model.

Advances in the Signaling Pathways Downstream of Glial-Scar Axon Growth Inhibitors

Axon growth inhibitors generated by reactive glial scars play an important role in failure of axon regeneration after CNS injury in mature mammals. Among the inhibitory factors, chondroitin sulfate proteoglycans (CSPGs) are potent suppressors of axon regeneration and are important molecular targets for designing effective therapies for traumatic brain injury or spinal cord injury (SCI). CSPGs bind with high affinity to several transmembrane receptors, including two members of the leukocyte common antigen related (LAR) subfamily of receptor protein tyrosine phosphatases (RPTPs). Recent studies demonstrate that multiple intracellular signaling pathways downstream of these two RPTPs mediate the growth-inhibitory actions of CSPGs. A better understanding of these signaling pathways may facilitate development of new and effective therapies for CNS disorders characterized by axonal disconnections. This review will focus on recent advances in the downstream signaling pathways of scar-mediated inhibition and their potential as the molecular targets for CNS repair.

Magnetic Resonance Imaging and Modeling of the Glymphatic System

The glymphatic system is a newly discovered waste drainage pathway in the brain; it plays an important role in many neurological diseases. Ongoing research utilizing various cerebrospinal fluid tracer infusions, either directly or indirectly into the brain parenchyma, is investigating clearance pathways by using distinct imaging techniques. In the present review, we discuss the role of the glymphatic system in various neurological diseases and efflux pathways of brain waste clearance based on current evidence and controversies. We mainly focus on new magnetic resonance imaging (MRI) modeling techniques, along with traditional computational modeling, for a better understanding of the glymphatic system function. Future sophisticated modeling techniques hold the potential to generate quantitative maps for glymphatic system parameters that could contribute to the diagnosis, monitoring, and prognosis of neurological diseases. The non-invasive nature of MRI may provide a safe and effective way to translate glymphatic system measurements from bench-to-bedside.

Reduced Sulfation Enhanced Oxytosis and Ferroptosis in Mouse Hippocampal HT22 Cells

Sulfation is a common modification of extracellular glycans, tyrosine residues on proteins, and steroid hormones, and is important in a wide variety of signaling pathways. We investigated the role of sulfation on endogenous oxidative stress, such as glutamate-induced oxytosis and erastin-induced ferroptosis, using mouse hippocampal HT22 cells. Sodium chlorate competitively inhibits the formation of 3′-phosphoadenosine 5′-phosphosulfate, the high energy sulfate donor in cellular sulfation reactions. The treatment of HT22 cells with sodium chlorate decreased sulfation of heparan sulfate proteoglycans and chondroitin sulfate proteoglycans. Sodium chlorate and β-d-xyloside, which prevents proteoglycan glycosaminoglycan chain attachment, exacerbated both glutamate- and erastin-induced cell death, suggesting that extracellular matrix influenced oxytosis and ferroptosis. Moreover, sodium chlorate enhanced the generation of reactive oxygen species and influx of extracellular Ca²⁺ in the process of oxytosis and ferroptosis. Interestingly, sodium chlorate did not affect antioxidant glutathione levels. Western blot analysis revealed that sodium chlorate enhanced erastin-induced c-Jun N-terminal kinase phosphorylation, which is preferentially activated by cell stress-inducing signals. Collectively, our findings indicate that sulfation is an important modification for neuroprotection against oxytosis and ferroptosis in neuronal hippocampal cells.

Ionized calcium in human cerebrospinal fluid and its influence on intrinsic and synaptic excitability of hippocampal pyramidal neurons in the rat

It is well-known that the extracellular concentration of calcium affects neuronal excitability and synaptic transmission. Less is known about the physiological concentration of extracellular calcium in the brain. In electrophysiological brain slice experiments, the artificial cerebrospinal fluid traditionally contains relatively high concentrations of calcium (2-4 mM) to support synaptic transmission and suppress neuronal excitability. Using an ion-selective electrode, we determined the fraction of ionized calcium in healthy human cerebrospinal fluid to 1.0 mM of a total concentration of 1.2 mM (86%). Using patch-clamp and extracellular recordings in the CA1 region in acute slices of rat hippocampus, we then compared the effects of this physiological concentration of calcium with the commonly used 2 mM on neuronal excitability, synaptic transmission, and long-term potentiation (LTP) to examine the magnitude of changes in this range of extracellular calcium. Increasing the total extracellular calcium concentration from 1.2 to 2 mM decreased spontaneous action potential firing, induced a depolarization of the threshold, and increased the rate of both de- and repolarization of the action potential. Evoked synaptic transmission was approximately doubled, with a balanced effect between inhibition and excitation. In 1.2 mM calcium high-frequency stimulation did not result in any LTP, whereas a prominent LTP was observed at 2 or 4 mM calcium. Surprisingly, this inability to induce LTP persisted during blockade of GABAergic inhibition. In conclusion, an increase from the physiological 1.2 mM to 2 mM calcium in the artificial cerebrospinal fluid has striking effects on neuronal excitability, synaptic transmission, and the induction of LTP. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/. Read the Editorial Highlight for this article on page 435.

Layer-specific expression of extracellular matrix molecules in the mouse somatosensory and piriform cortices

In the developing central nervous system (CNS), extracellular matrix (ECM) molecules have regulating roles such as in brain development, neural-circuit maturation, and synaptic-function control. However, excluding the perineuronal net (PNN) area, the distribution, constituent elements, and expression level of granular ECM molecules (diffuse ECM) present in the mature CNS remain unclear. Diffuse ECM molecules in the CNS share the components of PNNs and are likely functional. As cortical functions are greatly region-dependent, we hypothesized that ECM molecules would differ in distribution, expression level, and components in a region- and layer-dependent manner. We examined the layer-specific expression of several chondroitin sulfate proteoglycans (aggrecan, neurocan, and brevican), tenascin-R, Wisteria floribunda agglutinin (WFA)-positive molecules, hyaluronic acid, and link protein in the somatosensory and piriform cortices of mature mice. Furthermore, we investigated expression changes in WFA-positive molecules due to aging. In the somatosensory cortex, PNN density was particularly high at layer 4 (L4), but not all diffuse ECM molecules were highly expressed at L4 compared to the other layers. There was almost no change in tenascin-R and hyaluronic acid in any somatosensory-cortex layer. Neurocan showed high expression in L1 of the somatosensory cortex. In the piriform cortex, many ECM molecules showed higher expression in L1 than in the other layers. However, hyaluronic acid showed high expression in deep layers. Here, we clarified that ECM molecules differ in constituent elements and expression in a region- and layer-dependent manner. Region-specific expression of ECM molecules is possibly related to functions such as region-specific plasticity and vulnerability.

Perineuronal nets decrease membrane capacitance of peritumoral fast spiking interneurons in a model of epilepsy

Brain tumor patients commonly present with epileptic seizures. We show that tumor-associated seizures are the consequence of impaired GABAergic inhibition due to an overall loss of peritumoral fast spiking interneurons (FSNs) concomitant with a significantly reduced firing rate of those that remain. The reduced firing is due to the degradation of perineuronal nets (PNNs) that surround FSNs. We show that PNNs decrease specific membrane capacitance of FSNs permitting them to fire action potentials at supra-physiological frequencies. Tumor-released proteolytic enzymes degrade PNNs, resulting in increased membrane capacitance, reduced firing, and hence decreased GABA release. These studies uncovered a hitherto unknown role of PNNs as an electrostatic insulator that reduces specific membrane capacitance, functionally akin to myelin sheaths around axons, thereby permitting FSNs to exceed physiological firing rates. Disruption of PNNs may similarly account for excitation-inhibition imbalances in other forms of epilepsy and PNN protection through proteolytic inhibition may provide therapeutic benefits.

Unveiling the Extracellular Space of the Brain: From Super-resolved Microstructure to In Vivo Function

The extracellular space occupies approximately one-fifth of brain volume, molding a spider web of gaps filled with interstitial fluid and extracellular matrix where neurons and glial cells perform in concert. Yet, very little is known about the spatial organization and dynamics of the extracellular space, let alone its influence on brain function, owing to a lack of appropriate techniques (and a traditional bias toward the inside of cells, not the spaces in between). At the same time, it is clear that understanding fundamental brain functions, such as synaptic transmission, memory, sleep, and recovery from disease, calls for more focused research on the extracellular space of the brain. This review article highlights several key research areas, covering recent methodological and conceptual progress that illuminates this understudied, yet critically important, brain compartment, providing insights into the opportunities and challenges of this nascent field.

Perineuronal nets in brain physiology and disease

Perineuronal nets (PNNs) in the brain are condensed glycosaminoglycan-rich extracellular matrix structures with heterogeneous composition yet specific organization. They typically assemble around a subset of fast-spiking interneurons that are implicated in learning and memory. Owing to their unique structural organization, PNNs have neuroprotective capacities but also participate in signal transduction and in controlling neuronal activity and plasticity. In this review, we define PNN structure in detail and describe its various biochemical and physiological functions. We further discuss the role of PNNs in brain disorders such as schizophrenia, bipolar disorder, Alzheimer disease and addictions. Lastly, we describe therapeutic approaches that target PNNs to alter brain physiology and counter brain dysfunction.

Influence of the extracellular matrix on water mobility in subcortical gray matter

PURPOSE

Water mobility in tissues is related to the microstructure that modulates diffusion and spin relaxation. Previous work has shown that the extracellular matrix (ECM) impacts water diffusion in cartilage. To investigate if similar contributions to image contrast exist for brain, which is characterized by a substantially lower ECM content, diffusion and relaxation were studied in fixed samples from goat and human thalamus before and after enzymatic digestion of ECM compounds. Selected experiments in human corpus callosum were included for comparing subcortical gray matter and white matter.

METHODS

Digestion of matrix components was achieved by treatment with hyaluronidase. Nonlocalized pulsed field gradient measurements were performed with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>b</mml:mi></mml:math> values between 0.6 and 18,000 s/mm² at 3T and temperatures between 0°C and 20°C, in addition to T₁ and T₂ relaxation measurements. The data were fitted to multiexponential models to account for different water compartments. After the measurements, the samples were sliced and stained for ECM-sensitive markers to verify efficient digestion.

RESULTS

Microstructural alterations associated with hyaluronan digestion did not lead to measurable effects on water diffusion or <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>T</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:math> . However, T₁ of the main relaxographic component, attributed to intra-/extracellular water, decreased by 7%.

CONCLUSION

Investigations with very strong gradients did not reveal a detectable effect on water diffusion or <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>T</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:math> after hyaluronan removal, indicating that the brain ECM content is too low to produce a detectable effect. The subtle alteration of T₁ upon hyaluronidase treatment might reflect a modulation of intercompartmental water exchange properties.

Regulation of synaptic plasticity in hippocampal area CA2

Synaptic plasticity in the hippocampus is thought to play a vital role in both the refinement of neuronal circuits during development and in learning in the mature brain. Synapses in hippocampal area CA1 are known for a robust capacity for long-term potentiation (LTP), whereas synapses in the stratum radiatum of hippocampal area CA2 are particularly resistant to such changes. Although we have yet to fully understand the mechanisms behind this resistance to plasticity, a number of genes and extracellular matrix components highly expressed in CA2 appear to function as molecular brakes on plasticity and develop postnatally in the rodent brain. Curiously, the developmental profile of several CA2-enriched molecules is suggestive of a still undefined critical window of plasticity in the hippocampus.

The Impact of Perineuronal Net Digestion Using Chondroitinase ABC on the Intrinsic Physiology of Cortical Neurons

Perineuronal nets (PNNs) are a form of aggregate Extracellular Matrix (ECM) in the brain. Recent evidence suggests that the postnatal deposition of PNNs may play an active role in regulating neuroplasticity and, potentially, neurological disorders. Observations of high levels of PNN expression around somas, proximal dendrites, and axon initial segments of a subtype of neurons have also led to proposals that PNNs may modulate the intrinsic properties of the neurons they ensheathe. While high levels of PNNs are postnatally expressed throughout the neocortex, it is still unclear how they impact the neuronal physiology of the many classes and subtypes of neurons that exist. In this study, we demonstrate that Chondroitinase ABC digestion of PNNs from acute cortical slices from juvenile mice (P28-35) resulted in neuron-specific impacts on intrinsic physiology. Fast spiking (FS) interneurons showed decreased input resistance, resting membrane potential (RMP), reduced action potential (AP) peaks and altered spontaneous synaptic inputs. Low-Threshold Spiking interneurons showed altered rebound depolarizations and decreased frequency of spontaneous synaptic inputs. Putative excitatory neurons; regular spiking, bursting, and doublet phenotypes did not demonstrate any alterations. Our data indicate that chABC-sensitive PNNs may specifically regulate the intrinsic and synaptic physiology of inhibitory interneurons.

Perineuronal Nets in the Deep Cerebellar Nuclei Regulate GABAergic Transmission and Delay Eyeblink Conditioning

Perineuronal nets (PNNs), composed mainly of chondroitin sulfate proteoglycans, are the extracellular matrix that surrounds cell bodies, proximal dendrites, and axon initial segments of adult CNS neurons. PNNs are known to regulate neuronal plasticity, although their physiological roles in cerebellar functions have yet to be elucidated. Here, we investigated the contribution of PNNs to GABAergic transmission from cerebellar Purkinje cells (PCs) to large glutamatergic neurons in the deep cerebellar nuclei (DCN) in male mice by recording IPSCs from cerebellar slices, in which PNNs were depleted with chondroitinase ABC (ChABC). We found that PNN depletion increased the amplitude of evoked IPSCs and enhanced the paired-pulse depression. ChABC treatment also facilitated spontaneous IPSCs and increased the miniature IPSC frequency without changing not only the amplitude but also the density of PC terminals, suggesting that PNN depletion enhances presynaptic GABA release. We also demonstrated that the enhanced GABAergic transmission facilitated rebound firing in large glutamatergic DCN neurons, which is expected to result in the efficient induction of synaptic plasticity at synapses onto DCN neurons. Furthermore, we tested whether PNN depletion affects cerebellar motor learning. Mice having received the enzyme into the interpositus nuclei, which are responsible for delay eyeblink conditioning, exhibited the conditioned response at a significantly higher rate than control mice. Therefore, our results suggest that PNNs of the DCN suppress GABAergic transmission between PCs and large glutamatergic DCN neurons and restrict synaptic plasticity associated with motor learning in the adult cerebellum.SIGNIFICANCE STATEMENT Perineuronal nets (PNNs) are one of the extracellular matrices of adult CNS neurons and implicated in regulating various brain functions. Here we found that enzymatic PNN depletion in the mouse deep cerebellar nuclei (DCN) reduced the paired-pulse ratio of IPSCs and increased the miniature IPSC frequency without changing the amplitude, suggesting that PNN depletion enhances GABA release from the presynaptic Purkinje cell (PC) terminals. Mice having received the enzyme in the interpositus nuclei exhibited a higher conditioned response rate in delay eyeblink conditioning than control mice. These results suggest that PNNs regulate presynaptic functions of PC terminals in the DCN and functional plasticity of synapses on DCN neurons, which influences the flexibility of adult cerebellar functions.

The role of brain barriers in fluid movement in the CNS: is there a 'glymphatic' system?

Brain fluids are rigidly regulated to provide stable environments for neuronal function, e.g., low K⁺, Ca²⁺, and protein to optimise signalling and minimise neurotoxicity. At the same time, neuronal and astroglial waste must be promptly removed. The interstitial fluid (ISF) of the brain tissue and the cerebrospinal fluid (CSF) bathing the CNS are integral to this homeostasis and the idea of a glia-lymph or 'glymphatic' system for waste clearance from brain has developed over the last 5 years. This links bulk (convective) flow of CSF into brain along the outside of penetrating arteries, glia-mediated convective transport of fluid and solutes through the brain extracellular space (ECS) involving the aquaporin-4 (AQP4) water channel, and finally delivery of fluid to venules for clearance along peri-venous spaces. However, recent evidence favours important amendments to the 'glymphatic' hypothesis, particularly concerning the role of glia and transfer of solutes within the ECS. This review discusses studies which question the role of AQP4 in ISF flow and the lack of evidence for its ability to transport solutes; summarizes attributes of brain ECS that strongly favour the diffusion of small and large molecules without ISF flow; discusses work on hydraulic conductivity and the nature of the extracellular matrix which may impede fluid movement; and reconsiders the roles of the perivascular space (PVS) in CSF-ISF exchange and drainage. We also consider the extent to which CSF-ISF exchange is possible and desirable, the impact of neuropathology on fluid drainage, and why using CSF as a proxy measure of brain components or drug delivery is problematic. We propose that new work and key historical studies both support the concept of a perivascular fluid system, whereby CSF enters the brain via PVS convective flow or dispersion along larger caliber arteries/arterioles, diffusion predominantly regulates CSF/ISF exchange at the level of the neurovascular unit associated with CNS microvessels, and, finally, a mixture of CSF/ISF/waste products is normally cleared along the PVS of venules/veins as well as other pathways; such a system may or may not constitute a true 'circulation', but, at the least, suggests a comprehensive re-evaluation of the previously proposed 'glymphatic' concepts in favour of a new system better taking into account basic cerebrovascular physiology and fluid transport considerations.

The absence of brain-specific link protein Bral2 in perineuronal nets hampers auditory temporal resolution and neural adaptation in mice

Brain-specific link protein Bral2 represents a substantial component of perineuronal nets (PNNs) enwrapping neurons in the central nervous system. To elucidate the role of Bral2 in auditory signal processing, the hearing function in knockout Bral2(-/-) (KO) mice was investigated using behavioral and electrophysiological methods and compared with wild type Bral2(+/+) (WT) mice. The amplitudes of the acoustic startle reflex (ASR) and the efficiency of the prepulse inhibition of ASR (PPI of ASR), produced by prepulse noise stimulus or gap in continuous noise, was similar in 2-week-old WT and KO mice. Over the 2-month postnatal period the increase of ASR amplitudes was significantly more evident in WT mice than in KO mice. The efficiency of the PPI of ASR significantly increased in the 2-month postnatal period in WT mice, whereas in KO mice the PPI efficiency did not change. Hearing thresholds in 2-month-old WT mice, based on the auditory brainstem response (ABR) recordings, were significantly lower at high frequencies than in KO mice. However, amplitudes and peak latencies of individual waves of click-evoked ABR did not differ significantly between WT and KO mice. Temporal resolution and neural adaptation were significantly better in 2-month-old WT mice than in age-matched KO mice. These results support a hypothesis that the absence of perineuronal net formation at the end of the developmental period in the KO mice results in higher hearing threshold at high frequencies and weaker temporal resolution ability in adult KO animals compared to WT mice.

Electrodiffusion phenomena in neuroscience: a neglected companion

The emerging technological revolution in genetically encoded molecular sensors and super-resolution imaging provides neuroscientists with a pass to the real-time nano-world. On this small scale, however, classical principles of electrophysiology do not always apply. This is in large part because the nanoscopic heterogeneities in ionic concentrations and the local electric fields associated with individual ions and their movement can no longer be ignored. Here, we review basic principles of molecular electrodiffusion in the cellular environment of organized brain tissue. We argue that accurate interpretation of physiological observations on the nanoscale requires a better understanding of the underlying electrodiffusion phenomena.

Ex Vivo Measurement of Electrically Evoked Dopamine Release in Zebrafish Whole Brain

Zebrafish (Danio rerio) have recently emerged as useful model organism for the study of neuronal function. Here, fast-scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes was used to measure locally evoked dopamine release and uptake in zebrafish whole brain preparations and results were compared with those obtained from brain slices. Evoked dopamine release ([DA]_max) was similar in whole brain and sagittal brain slice preparations (0.49 ± 0.13 μM in whole brain and 0.59 ± 0.28 μM in brain slices). Treatment with α-methyl-p-tyrosine methyl ester (αMPT), an inhibitor of tyrosine hydroxylase, diminished release and the electrochemical signal reappeared after subsequent drug washout. No observed change in stimulated release current occurred after treatment with desipramine or fluoxetine in the whole brain. Treatment with the uptake inhibitors, nomifensine or GBR 12909 increased [DA]_max, while treatment with sulpiride, a D2 dopamine autoreceptor antagonist, resulted in increased stimulated dopamine release in whole brain, but had no effect on release in slices. Dopamine release in whole brains increased progressively up to an electrical stimulation frequency of 25 Hz, while release in slices increased up to a frequency of only 10 Hz and then plateaued, highlighting another key difference between these preparations. We observed a lag in peak dopamine release following stimulation, which we address using diffusion models and pharmacological treatments. Collectively, these results demonstrate the electrochemical determination of dopamine release in the whole, intact brain of a vertebrate species ex vivo and are an important step for carrying out further experiments in zebrafish.

Damaged Neocortical Perineuronal Nets Due to Experimental Focal Cerebral Ischemia in Mice, Rats and Sheep

As part of the extracellular matrix (ECM), perineuronal nets (PNs) are polyanionic, chondroitin sulfate proteoglycan (CSPG)-rich coatings of certain neurons, known to be affected in various neural diseases. Although these structures are considered as important parts of the neurovascular unit (NVU), their role during evolution of acute ischemic stroke and subsequent tissue damage is poorly understood and only a few preclinical studies analyzed PNs after acute ischemic stroke. By employing three models of experimental focal cerebral ischemia, this study was focused on histopathological alterations of PNs and concomitant vascular, glial and neuronal changes according to the NVU concept. We analyzed brain tissues obtained 1 day after ischemia onset from: (a) mice after filament-based permanent middle cerebral artery occlusion (pMCAO); (b) rats subjected to thromboembolic MACO; and (c) sheep at 14 days after electrosurgically induced focal cerebral ischemia. Multiple fluorescence labeling was applied to explore simultaneous alterations of NVU and ECM. Serial mouse sections labeled with the net marker Wisteria floribunda agglutinin (WFA) displayed largely decomposed and nearly erased PNs in infarcted neocortical areas that were demarcated by up-regulated immunoreactivity for vascular collagen IV (Coll IV). Subsequent semi-quantitative analyses in mice confirmed significantly decreased WFA-staining along the ischemic border zone and a relative decrease in the directly ischemia-affected neocortex. Triple fluorescence labeling throughout the three animal models revealed up-regulated Coll IV and decomposed PNs accompanied by activated astroglia and altered immunoreactivity for parvalbumin, a calcium-binding protein in fast-firing GABAergic neurons which are predominantly surrounded by neocortical PNs. Furthermore, ischemic neocortical areas in rodents simultaneously displayed less intense staining of WFA, aggrecan, the net components neurocan, versican and the cartilage link protein (CRTL) as well as markers in net-bearing neurons such as the potassium channel subunit Kv3.1b and neuronal nuclei (NeuN). In summary, theconsistent observations based on three different stroke models confirmed that PNs are highly sensitive constituents of the NVU along with impaired associated GABAergic neurons. These results suggest that PNs could be promising targets of future stroke treatment, and further studies should address their reorganization and plasticity in both stabilizing the acute stroke as well as supportive effects during the chronic phase of stroke.

Brain Extracellular Space: The Final Frontier of Neuroscience

Brain extracellular space is the narrow microenvironment that surrounds every cell of the central nervous system. It contains a solution that closely resembles cerebrospinal fluid with the addition of extracellular matrix molecules. The space provides a reservoir for ions essential to the electrical activity of neurons and forms an intercellular chemical communication channel. Attempts to reveal the size and structure of the extracellular space using electron microscopy have had limited success; however, a biophysical approach based on diffusion of selected probe molecules has proved useful. A point-source paradigm, realized in the real-time iontophoresis method using tetramethylammonium, as well as earlier radiotracer methods, have shown that the extracellular space occupies ∼20% of brain tissue and small molecules have an effective diffusion coefficient that is two-fifths that in a free solution. Monte Carlo modeling indicates that geometrical constraints, including dead-space microdomains, contribute to the hindrance to diffusion. Imaging the spread of macromolecules shows them increasingly hindered as a function of size and suggests that the gaps between cells are predominantly ∼40 nm with wider local expansions that may represent dead-spaces. Diffusion measurements also characterize interactions of ions and proteins with the chondroitin and heparan sulfate components of the extracellular matrix; however, the many roles of the matrix are only starting to become apparent. The existence and magnitude of bulk flow and the so-called glymphatic system are topics of current interest and controversy. The extracellular space is an exciting area for research that will be propelled by emerging technologies.

Nanoscale diffusion in the synaptic cleft and beyond measured with time-resolved fluorescence anisotropy imaging

Neural activity relies on molecular diffusion within nanoscopic spaces outside and inside nerve cells, such as synaptic clefts or dendritic spines. Measuring diffusion on this small scale in situ has not hitherto been possible, yet this knowledge is critical for understanding the dynamics of molecular events and electric currents that shape physiological signals throughout the brain. Here we advance time-resolved fluorescence anisotropy imaging combined with two-photon excitation microscopy to map nanoscale diffusivity in ex vivo brain slices. We find that in the brain interstitial gaps small molecules move on average ~30% slower than in a free medium whereas inside neuronal dendrites this retardation is ~70%. In the synaptic cleft free nanodiffusion is decelerated by ~46%. These quantities provide previously unattainable basic constrains for the receptor actions of released neurotransmitters, the electrical conductance of the brain interstitial space and the limiting rate of molecular interactions or conformational changes in the synaptic microenvironment.

Abolished perineuronal nets and altered parvalbumin-immunoreactivity in the nucleus reticularis thalami of wildtype and 3xTg mice after experimental stroke

Treatment strategies for ischemic stroke are still limited, since numerous attempts were successful only in preclinical research but failed under clinical condition. To overcome this translational roadblock, clinical relevant stroke models should consider co-morbidities, age-related effects and the complex neurovascular unit (NVU) concept. The NVU includes neurons, vessels and glial cells with astrocytic endfeet in close relation to the extracellular matrix (ECM). However, the role of the ECM after stroke-related tissue damage is poorly understood and mostly neglected for treatment strategies. This study is focused on alterations of perineuronal nets (PNs) as ECM constituents and parvalbumin-containing GABAergic neurons in mice with emphasis on the nucleus reticularis thalami (NRT) in close proximity to the ischemic lesion as induced by a filament-based stroke model. One day after ischemia onset, immunofluorescence-based quantitative analyses revealed drastically declined PNs in the ischemia-affected NRT from 3- and 12-month-old wildtype and co-morbid triple-transgenic (3xTg) mice with Alzheimer-like alterations. Parvalbumin-positive cells decreased numerically in the ischemia-affected NRT, while staining intensity did not differ between the affected and non-affected hemisphere. Additional qualitative analyses demonstrated ischemia-induced loss of PNs and allocated neuropil ECM immunoreactive for aggrecan and neurocan, and impaired immunoreactivity for calbindin, the potassium channel subunit Kv3.1b and the glutamate decarboxylase isoforms GAD65 and GAD67 in the NRT. In conclusion, these data confirm PNs as highly sensitive constituents of the ECM along with impaired neuronal integrity of GABAergic neurons. Therefore, specific targeting of ECM components might appear as a promising strategy for future treatment strategies in stroke.

Perineuronal Nets Enhance the Excitability of Fast-Spiking Neurons

Perineuronal nets (PNNs) are specialized complexes of extracellular matrix molecules that surround the somata of fast-spiking neurons throughout the vertebrate brain. PNNs are particularly prevalent throughout the auditory brainstem, which transmits signals with high speed and precision. It is unknown whether PNNs contribute to the fast-spiking ability of the neurons they surround. Whole-cell recordings were made from medial nucleus of the trapezoid body (MNTB) principal neurons in acute brain slices from postnatal day 21 (P21) to P27 mice. PNNs were degraded by incubating slices in chondroitinase ABC (ChABC) and were compared to slices that were treated with a control enzyme (penicillinase). ChABC treatment did not affect the ability of MNTB neurons to fire at up to 1000 Hz when driven by current pulses. However, f–I (frequency–intensity) curves constructed by injecting Gaussian white noise currents superimposed on DC current steps showed that ChABC treatment reduced the gain of spike output. An increase in spike threshold may have contributed to this effect, which is consistent with the observation that spikes in ChABC-treated cells were delayed relative to control-treated cells. In addition, parvalbumin-expressing fast-spiking cortical neurons in >P70 slices that were treated with ChABC also had reduced excitability and gain. The development of PNNs around somata of fast-spiking neurons may be essential for fast and precise sensory transmission and synaptic inhibition in the brain.

Ion exchanger in the brain: Quantitative analysis of perineuronally fixed anionic binding sites suggests diffusion barriers with ion sorting properties

Perineuronal nets (PNs) are a specialized form of brain extracellular matrix, consisting of negatively charged glycosaminoglycans, glycoproteins and proteoglycans in the direct microenvironment of neurons. Still, locally immobilized charges in the tissue have not been accessible so far to direct observations and quantifications. Here, we present a new approach to visualize and quantify fixed charge-densities on brain slices using a focused proton-beam microprobe in combination with ionic metallic probes. For the first time, we can provide quantitative data on the distribution and net amount of pericellularly fixed charge-densities, which, determined at 0.4–0.5 M, is much higher than previously assumed. PNs, thus, represent an immobilized ion exchanger with ion sorting properties high enough to partition mobile ions in accord with Donnan-equilibrium. We propose that fixed charge-densities in the brain are involved in regulating ion mobility, the volume fraction of extracellular space and the viscosity of matrix components.

The neuronal extracellular matrix restricts distribution and internalization of aggregated Tau-protein

Alzheimer's disease (AD) is a chronic degenerative disorder characterized by fibrillary aggregates of Aß and Tau-protein. Formation and progression of these pathological hallmarks throughout the brain follow a specific spatio-temporal pattern which provides the basis for neuropathological staging. Previously, we could demonstrate that cortical and subcortical neurons are less frequently affected by neurofibrillary degeneration if they are enwrapped by a specialized form of the hyaluronan-based extracellular matrix (ECM), the so called 'perineuronal net' (PN). PNs are composed of large aggregating chondroitin sulfate proteoglycans connected to a hyaluronan backbone, stabilized by link proteins and cross-linked via tenascin-R. Recently, PN-associated neurons were shown to be better protected against iron-induced neurodegeneration compared to neurons without PN, indicating a neuroprotective function. Here, we investigated the role of PNs in distribution and internalization of exogenous Tau-protein by using organotypic slice cultures of wildtype mice as well as mice lacking the ECM-components aggrecan, HAPLN1 or tenascin-R. We could demonstrate that PNs restrict both distribution and internalization of Tau. Accordingly, PN-ensheathed neurons were less frequently affected by Tau-internalization, than neurons without PN. Finally, the PNs as well as their three investigated components were shown to modulate the processes of distribution as well as internalization of Tau.

Astrocytes and extracellular matrix in extrasynaptic volume transmission

Volume transmission is a form of intercellular communication that does not require synapses; it is based on the diffusion of neuroactive substances across the brain extracellular space (ECS) and their binding to extrasynaptic high-affinity receptors on neurons or glia. Extracellular diffusion is restricted by the limited volume of the ECS, which is described by the ECS volume fraction α, and the presence of diffusion barriers, reflected by tortuosity λ, that are created, for example, by fine astrocytic processes or extracellular matrix (ECM) molecules. Organized astrocytic processes, ECM scaffolds or myelin sheets channel the extracellular diffusion so that it is facilitated in a certain direction, i.e. anisotropic. The diffusion properties of the ECS are profoundly influenced by various processes such as the swelling and morphological rebuilding of astrocytes during either transient or persisting physiological or pathological states, or the remodelling of the ECM in tumorous or epileptogenic tissue, during Alzheimer's disease, after enzymatic treatment or in transgenic animals. The changing diffusion properties of the ECM influence neuron-glia interaction, learning abilities, the extent of neuronal damage and even cell migration. From a clinical point of view, diffusion parameter changes occurring during pathological states could be important for diagnosis, drug delivery and treatment.

T-type calcium channels contribute to calcium disturbances in brain during hyponatremia

Disturbance of calcium homeostasis is implicated in the normal process of aging and brain pathology prevalent in the elderly such as Alzheimer's, Parkinson's, and amyotrophic lateral sclerosis. Previous studies demonstrated that applying a hyponatremic iso-osmotic (low-NaCl) artificial cerebrospinal fluid (ACSF) to rodent hippocampus causes extracellular calcium to rapidly decrease. Restoring normonatremia after low-NaCl treatment causes a rapid increase in extracellular calcium that overshoots baseline. This study examined the amplitude, timing, and mechanism of these surprising calcium changes. We also tested whether hyponatremia increased calcium entry into brain cells or calcium binding to chondroitin sulfate (CS), a negatively charged constituent of the extracellular matrix (ECM) that may be occupied by sodium during normonatremia. We report three major findings. First we show that CS does not contribute to extracellular calcium changes during low-NaCl treatments. Second, we show that the time to minimum extracellular calcium during low-NaCl treatment is significantly shorter than the time to maximum extracellular calcium in recovery from low-NaCl treatment. Third, we show that the decrease in extracellular calcium observed during hyponatremia is attenuated by ML 218, a highly selective T-type calcium channel blocker. Together these data suggest that calcium rapidly enters cells at the onset of low-NaCl treatment and is extruded from cells when normonatremia is restored. Calcium binding to CS does not significantly contribute to calcium changes in brain during hyponatremia. Differences in timing suggest that extracellular calcium changes during and in recovery from hyponatremia occur by distinct mechanisms or by a multistep process. Finally, partial block of extracellular calcium influx by ML 218 suggests that T-type channels are involved in calcium entering cells during hyponatremia. Given the high prevalence of hyponatremia among elderly patients and the growing understanding of calcium's role in multiple neurologic pathologies, this study promotes a novel approach for studying and potentially preventing the effects of hyponatremia on calcium dysregulation in brain tissue.

The extracellular matrix molecule brevican is an integral component of the machinery mediating fast synaptic transmission at the calyx of Held

KEY POINTS

The proteoglycan brevican is a major component of the extracellular matrix of perineuronal nets and is highly enriched in the perisynaptic space suggesting a role for synaptic transmission. We have introduced the calyx of Held in the auditory brainstem as a model system to study the impact of brevican on dynamics and reliability of synaptic transmission. In vivo extracellular single-unit recordings at the calyx of Held in brevican-deficient mice yielded a significant increase in the action potential (AP) transmission delay and a prolongation of pre- and postsynaptic APs. The changes in dynamics of signal transmission were accompanied by the reduction of presynaptic vGlut1 and ultrastructural changes in the perisynaptic space. These data show that brevican is an important mediator of fast synaptic transmission at the calyx of Held.

ABSTRACT

The extracellular matrix is an integral part of the neural tissue. Its most conspicuous manifestation in the brain are the perineuronal nets (PNs) which surround somata and proximal dendrites of distinct neuron types. The chondroitin sulfate proteoglycan brevican is a major component of PNs. In contrast to other PN-comprising proteoglycans (e.g. aggrecan and neurocan), brevican is mainly expressed in the perisynaptic space closely associated with both the pre- and postsynaptic membrane. This specific localization prompted the hypothesis that brevican might play a role in synaptic transmission. In the present study we specifically investigated the role of brevican in synaptic transmission at a central synapse, the calyx of Held in the medial nucleus of the trapezoid body, by the use of in vivo electrophysiology, immunohistochemistry, biochemistry and electron microscopy. In vivo extracellular single-unit recordings were acquired in brevican-deficient mice and the dynamics and reliability of synaptic transmission were compared to wild-type littermates. In knockout mice, the speed of pre-to-postsynaptic action potential (AP) transmission was reduced and the duration of the respective pre- and postsynaptic APs increased. The reliability of signal transmission, however, was not affected by the lack of brevican. The changes in dynamics of signal transmission were accompanied by the reduction of (i) presynaptic vGlut1 and (ii) the size of subsynaptic cavities. The present results suggest an essential role of brevican for the functionality of high-speed synaptic transmission at the calyx of Held.