Cortical areas abundant in extracellular matrix chondroitin sulphate proteoglycans are less affected by cytoskeletal changes in Alzheimer's disease

The necroptosis cell death pathway drives neurodegeneration in Alzheimer's disease

Although apoptosis, pyroptosis, and ferroptosis have been implicated in AD, none fully explains the extensive neuronal loss observed in AD brains. Recent evidence shows that necroptosis is abundant in AD, that necroptosis is closely linked to the appearance of Tau pathology, and that necroptosis markers accumulate in granulovacuolar neurodegeneration vesicles (GVD). We review here the neuron-specific activation of the granulovacuolar mediated neuronal-necroptosis pathway, the potential AD-relevant triggers upstream of this pathway, and the interaction of the necrosome with the endo-lysosomal pathway, possibly providing links to Tau pathology. In addition, we underscore the therapeutic potential of inhibiting necroptosis in neurodegenerative diseases such as AD, as this presents a novel avenue for drug development targeting neuronal loss to preserve cognitive abilities. Such an approach seems particularly relevant when combined with amyloid-lowering drugs.

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.

Perineuronal net deglycosylation associates with tauopathy-induced gliosis and neurodegeneration

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by clinical symptoms of memory and cognitive deficiencies. Postmortem evaluation of AD brain tissue shows proteinopathy that closely associate with the progression of this dementing disorder, including the accumulation of extracellular beta amyloid (Aβ) and intracellular hyperphosphorylated tau (pTau) with neurofibrillary tangles (NFTs). Current therapies targeting Aβ have limited clinical efficacy and life-threatening side effects and highlight the need for alternative treatments targeting pTau and other pathophysiologic mechanisms driving AD pathogenesis. The brain's extracellular matrices (ECM), particularly perineuronal nets (PNNs), play a crucial role in brain functioning and neurocircuit stability, and reorganization of these unique PNN matrices has been associated with the progression of AD and accumulation of pTau in humans. We hypothesize that AD-associated changes in PNNs may in part be driven by the accumulation of pTau within the brain. In this work, we investigated whether the presence of pTau influenced PNN structural integrity and PNN chondroitin sulfate-glycosaminoglycan (CS-GAG) compositional changes in two transgenic mouse models expressing tauopathy-related AD pathology, PS19 (P301S) and Tau4RTg2652 mice. We show that PS19 mice exhibit an age-dependent loss of hippocampal PNN CS-GAGs, but not the underlying aggrecan core protein structures, in association with pTau accumulation, gliosis, and neurodegeneration. The loss of PNN CS-GAGs were linked to shifts in CS-GAG sulfation patterns to favor the neuroregenerative isomer, 2S6S-CS. Conversely, Tau4RTg2652 mice exhibit stable PNN structures and normal CS-GAG isomer composition despite robust pTau accumulation, suggesting a critical interaction between neuronal PNN glycan integrity and neighboring glial cell activation. Overall, our findings provide insights into the complex relationship between PNN CS-GAGs, pTau pathology, gliosis, and neurodegeneration in mouse models of tauopathy, and offer new therapeutic insights and targets for AD treatment.

Multi-Scale Label-Free Human Brain Imaging with Integrated Serial Sectioning Polarization Sensitive Optical Coherence Tomography and Two-Photon Microscopy

The study of aging and neurodegenerative processes in the human brain requires a comprehensive understanding of cytoarchitectonic, myeloarchitectonic, and vascular structures. Recent computational advances have enabled volumetric reconstruction of the human brain using thousands of stained slices, however, tissue distortions and loss resulting from standard histological processing have hindered deformation-free reconstruction. Here, the authors describe an integrated serial sectioning polarization-sensitive optical coherence tomography (PSOCT) and two photon microscopy (2PM) system to provide label-free multi-contrast imaging of intact brain structures, including scattering, birefringence, and autofluorescence of human brain tissue. The authors demonstrate high-throughput reconstruction of 4 × 4 × 2cm3 sample blocks and simple registration between PSOCT and 2PM images that enable comprehensive analysis of myelin content, vascular structure, and cellular information. The high-resolution 2PM images provide microscopic validation and enrichment of the cellular information provided by the PSOCT optical properties on the same sample, revealing the densely packed fibers, capillaries, and lipofuscin-filled cell bodies in the cortex and white matter. It is  shown that the imaging system enables quantitative characterization of various pathological features in aging process, including myelin degradation, lipofuscin accumulation, and microvascular changes, which opens up numerous opportunities in the study of neurodegenerative diseases in the future.

Sensory processing deficits and related cortical pathological changes in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder primarily affecting cognitive functions. However, sensory deficits in AD start to draw attention due to their high prevalence and early onsets which suggest that they could potentially serve as diagnostic biomarkers and even contribute to the disease progression. This literature review examines the sensory deficits and cortical pathological changes observed in visual, auditory, olfactory, and somatosensory systems in AD patients, as well as in various AD animal models. Sensory deficits may emerge at the early stages of AD, or even precede the cognitive decline, which is accompanied by cortical pathological changes including amyloid-beta deposition, tauopathy, gliosis, and alterations in neuronal excitability, synaptic inputs, and functional plasticity. Notably, these changes are more pronounced in sensory association areas and superficial cortical layers, which may explain the relative preservation of basic sensory functions but early display of deficits of higher sensory functions. We propose that sensory impairment and the progression of AD may establish a cyclical relationship that mutually perpetuates each condition. This review highlights the significance of sensory deficits with or without cortical pathological changes in AD and emphasizes the need for further research to develop reliable early detection and intervention through sensory systems.

Matrix disequilibrium in Alzheimer's disease and conditions that increase Alzheimer's disease risk

Alzheimer's Disease (AD) and related dementias are a leading cause of death globally and are predicted to increase in prevalence. Despite this expected increase in the prevalence of AD, we have yet to elucidate the causality of the neurodegeneration observed in AD and we lack effective therapeutics to combat the progressive neuronal loss. Throughout the past 30 years, several non-mutually exclusive hypotheses have arisen to explain the causative pathologies in AD: amyloid cascade, hyper-phosphorylated tau accumulation, cholinergic loss, chronic neuroinflammation, oxidative stress, and mitochondrial and cerebrovascular dysfunction. Published studies in this field have also focused on changes in neuronal extracellular matrix (ECM), which is critical to synaptic formation, function, and stability. Two of the greatest non-modifiable risk factors for development of AD (aside from autosomal dominant familial AD gene mutations) are aging and APOE status, and two of the greatest modifiable risk factors for AD and related dementias are untreated major depressive disorder (MDD) and obesity. Indeed, the risk of developing AD doubles for every 5 years after ≥ 65, and the APOE4 allele increases AD risk with the greatest risk in homozygous APOE4 carriers. In this review, we will describe mechanisms by which excess ECM accumulation may contribute to AD pathology and discuss pathological ECM alterations that occur in AD as well as conditions that increase the AD risk. We will discuss the relationship of AD risk factors to chronic central nervous system and peripheral inflammation and detail ECM changes that may follow. In addition, we will discuss recent data our lab has obtained on ECM components and effectors in APOE4/4 and APOE3/3 expressing murine brain lysates, as well as human cerebrospinal fluid (CSF) samples from APOE3 and APOE4 expressing AD individuals. We will describe the principal molecules that function in ECM turnover as well as abnormalities in these molecular systems that have been observed in AD. Finally, we will communicate therapeutic interventions that have the potential to modulate ECM deposition and turnover in vivo.

The fate of interneurons, GABAA receptor sub-types and perineuronal nets in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurological disease, which is associated with gradual memory loss and correlated with synaptic hyperactivity and abnormal oscillatory rhythmic brain activity that precedes phenotypic alterations and is partly responsible for the spread of the disease pathology. Synaptic hyperactivity is thought to be because of alteration in the homeostasis of phasic and tonic synaptic inhibition, which is orchestrated by the GABA_A inhibitory system, encompassing subclasses of interneurons and GABA_A receptors, which play a vital role in cognitive functions, including learning and memory. Furthermore, the extracellular matrix, the perineuronal nets (PNNs) which often go unnoticed in considerations of AD pathology, encapsulate the inhibitory cells and neurites in critical brain regions and have recently come under the light for their crucial role in synaptic stabilisation and excitatory-inhibitory balance and when disrupted, serve as a potential trigger for AD-associated synaptic imbalance. Therefore, in this review, we summarise the current understanding of the selective vulnerability of distinct interneuron subtypes, their synaptic and extrasynaptic GABA_A R subtypes as well as the changes in PNNs in AD, detailing their contribution to the mechanisms of disease development. We aim to highlight how seemingly unique malfunction in each component of the interneuronal GABA inhibitory system can be tied together to result in critical circuit dysfunction, leading to the irreversible symptomatic damage observed in AD.

The Role of Extracellular Matrix in Human Neurodegenerative Diseases

The dense neuropil of the central nervous system leaves only limited space for extracellular substances free. The advent of immunohistochemistry, soon followed by advanced diagnostic tools, enabled us to explore the biochemical heterogeneity and compartmentalization of the brain extracellular matrix in exploratory and clinical research alike. The composition of the extracellular matrix is critical to shape neuronal function; changes in its assembly trigger or reflect brain/spinal cord malfunction. In this study, we focus on extracellular matrix changes in neurodegenerative disorders. We summarize its phenotypic appearance and biochemical characteristics, as well as the major enzymes which regulate and remodel matrix establishment in disease. The specifically built basement membrane of the central nervous system, perineuronal nets and perisynaptic axonal coats can protect neurons from toxic agents, and biochemical analysis revealed how the individual glycosaminoglycan and proteoglycan components interact with these molecules. Depending on the site, type and progress of the disease, select matrix components can either proactively trigger the formation of disease-specific harmful products, or reactively accumulate, likely to reduce tissue breakdown and neuronal loss. We review the diagnostic use and the increasing importance of medical screening of extracellular matrix components, especially enzymes, which informs us about disease status and, better yet, allows us to forecast illness.

Chondroitin Sulphate Proteoglycan Axonal Coats in the Human Mediodorsal Thalamic Nucleus

Mounting evidence supports a key involvement of the chondroitin sulfate proteoglycans (CSPGs) NG2 and brevican (BCAN) in the regulation of axonal functions, including axon guidance, fasciculation, conductance, and myelination. Prior work suggested the possibility that these functions may, at least in part, be carried out by specialized CSPG structures surrounding axons, termed axonal coats. However, their existence remains controversial. We tested the hypothesis that NG2 and BCAN, known to be associated with oligodendrocyte precursor cells, form axonal coats enveloping myelinated axons in the human brain. In tissue blocks containing the mediodorsal thalamic nucleus (MD) from healthy donors (n = 5), we used dual immunofluorescence, confocal microscopy, and unbiased stereology to characterize BCAN and NG2 immunoreactive (IR) axonal coats and measure the percentage of myelinated axons associated with them. In a subset of donors (n = 3), we used electron microscopy to analyze the spatial relationship between axons and NG2- and BCAN-IR axonal coats within the human MD. Our results show that a substantial percentage (∼64%) of large and medium myelinated axons in the human MD are surrounded by NG2- and BCAN-IR axonal coats. Electron microscopy studies show NG2- and BCAN-IR axonal coats are interleaved with myelin sheets, with larger axons displaying greater association with axonal coats. These findings represent the first characterization of NG2 and BCAN axonal coats in the human brain. The large percentage of axons surrounded by CSPG coats, and the role of CSPGs in axonal guidance, fasciculation, conductance, and myelination suggest that these structures may contribute to several key axonal properties.

The "Loss" of Perineuronal Nets in Alzheimer's Disease: Missing or Hiding in Plain Sight?

Perineuronal nets (PNNs) are chondroitin-sulfate glycosaminoglycan (CS-GAG) containing extracellular matrix structures that assemble around neurons involved in learning, memory, and cognition. Owing to the unique patterning of negative charges stemming from sulfate modifications to the attached CS-GAGs, these matrices play key roles in mediating glycan-protein binding, signaling interactions, and charged ion buffering of the underlying circuitry. Histochemical loss of PNN matrices has been reported for a range of neurocognitive and neurodegenerative diseases, implying that PNNs might be a key player in the pathogenesis of neurological disorders. In this hypothesis and theory article, we begin by highlighting PNN changes observed in human postmortem brain tissue associated with Alzheimer's disease (AD) and corresponding changes reported in rodent models of AD neuropathology. We then discuss the technical limitations surrounding traditional methods for PNN analyses and propose alternative explanations to these historical findings. Lastly, we embark on a global re-evaluation of the interpretations for PNN changes across brain regions, across species, and in relation to other neurocognitive disorders.

The Role of Extracellular Matrix Components in the Spreading of Pathological Protein Aggregates

Several neurodegenerative diseases are characterized by the accumulation of aggregated misfolded proteins. These pathological agents have been suggested to propagate in the brain via mechanisms similar to that observed for the prion protein, where a misfolded variant is transferred from an affected brain region to a healthy one, thereby inducing the misfolding and/or aggregation of correctly folded copies. This process has been characterized for several proteins, such as α-synuclein, tau, amyloid beta (Aβ) and less extensively for huntingtin and TDP-43. α-synuclein, tau, TDP-43 and huntingtin are intracellular proteins, and their aggregates are located in the cytosol or nucleus of neurons. They have been shown to spread between cells and this event occurs, at least partially, via secretion of these protein aggregates in the extracellular space followed by re-uptake. Conversely, Aβ aggregates are found mainly extracellularly, and their spreading occurs in the extracellular space between brain regions. Due to the inherent nature of their spreading modalities, these proteins are exposed to components of the extracellular matrix (ECM), including glycans, proteases and core matrix proteins. These ECM components can interact with or process pathological misfolded proteins, potentially changing their properties and thus regulating their spreading capabilities. Here, we present an overview of the documented roles of ECM components in the spreading of pathological protein aggregates in neurodegenerative diseases with the objective of identifying the current gaps in knowledge and stimulating further research in the field. This could potentially lead to the identification of druggable targets to slow down the spreading and/or progression of these pathologies.

Tau Protein Modulates Perineuronal Extracellular Matrix Expression in the TauP301L-acan Mouse Model

Tau mutations promote the formation of tau oligomers and filaments, which are neuropathological signs of several tau-associated dementias. Types of neurons in the CNS are spared of tau pathology and are surrounded by a specialized form of extracellular matrix; called perineuronal nets (PNs). Aggrecan, the major PN proteoglycans, is suggested to mediate PNs neuroprotective function by forming an external shield preventing the internalization of misfolded tau. We recently demonstrated a correlation between aggrecan amount and the expression and phosphorylation of tau in a TauP310L-acan mouse model, generated by crossbreeding heterozygous aggrecan mice with a significant reduction of aggrecan and homozygous TauP301L mice. Neurodegenerative processes have been associated with changes of PN structure and protein signature. In this study, we hypothesized that the structure and protein expression of PNs in this TauP310L-acan mouse is regulated by tau. Immunohistochemical and biochemical analyses demonstrate that protein levels of PN components differ between TauP301L^HET-acan^WT and TauP301L^HET-acan^HET mice, accompanied by changes in the expression of protein phosphatase 2 A. In addition, tau can modulate PN components such as brevican. Co-immunoprecipitation experiments revealed a physical connection between PN components and tau. These data demonstrate a complex, mutual interrelation of tau and the proteoglycans of the PN.

High Iron and Iron Household Protein Contents in Perineuronal Net-Ensheathed Neurons Ensure Energy Metabolism with Safe Iron Handling

A subpopulation of neurons is less vulnerable against iron-induced oxidative stress and neurodegeneration. A key feature of these neurons is a special extracellular matrix composition that forms a perineuronal net (PN). The PN has a high affinity to iron, which suggests an adapted iron sequestration and metabolism of the ensheathed neurons. Highly active, fast-firing neurons-which are often ensheathed by a PN-have a particular high metabolic demand, and therefore may have a higher need in iron. We hypothesize that PN-ensheathed neurons have a higher intracellular iron concentration and increased levels of iron proteins. Thus, analyses of cellular and regional iron and the iron proteins transferrin (Tf), Tf receptor 1 (TfR), ferritin H/L (FtH/FtL), metal transport protein 1 (MTP1 aka ferroportin), and divalent metal transporter 1 (DMT1) were performed on Wistar rats in the parietal cortex (PC), subiculum (SUB), red nucleus (RN), and substantia nigra (SNpr/SNpc). Neurons with a PN (PN⁺) have higher iron concentrations than neurons without a PN: PC 0.69 mM vs. 0.51 mM, SUB 0.84 mM vs. 0.69 mM, SN 0.71 mM vs. 0.63 mM (SNpr)/0.45 mM (SNpc). Intracellular Tf, TfR and MTP1 contents of PN⁺ neurons were consistently increased. The iron concentration of the PN itself is not increased. We also determined the percentage of PN⁺ neurons: PC 4%, SUB 5%, SNpr 45%, RN 86%. We conclude that PN⁺ neurons constitute a subpopulation of resilient pacemaker neurons characterized by a bustling iron metabolism and outstanding iron handling capabilities. These properties could contribute to the low vulnerability of PN⁺ neurons against iron-induced oxidative stress and degeneration.

Microbiological-Chemical Sourced Chondroitin Sulfates Protect Neuroblastoma SH-SY5Y Cells against Oxidative Stress and Are Suitable for Hydrogel-Based Controlled Release

Chondroitin sulfates (CS) are a class of sulfated glycosaminoglycans involved in many biological processes. Several studies reported their protective effect against neurodegenerative conditions like Alzheimer’s disease. CS are commonly derived from animal sources, but ethical concerns, the risk of contamination with animal proteins, and the difficulty in controlling the sulfation pattern have prompted research towards non-animal sources. Here we exploited two microbiological-chemical sourced CS (i.e., CS-A,C and CS-A,C,K,L) and Carbopol 974P NF/agarose semi-interpenetrating polymer networks (i.e., P.NaOH.0 and P.Ethanol.0) to set up a release system, and tested the neuroprotective role of released CS against H₂O₂-induced oxidative stress. After assessing that our CS (1–100 µM) require a 3 h pre-treatment for neuroprotection with SH-SY5Y cells, we evaluated whether the autoclave type (i.e., N- or B-type) affects hydrogel viscoelastic properties. We selected B-type autoclaves and repeated the study after loading CS (1 or 0.1 mg CS/0.5 mL gel). After loading 1 mg CS/0.5 mL gel, we evaluated CS release up to 7 days by 1,9-dimethylmethylene blue (DMMB) assay and verified the neuroprotective role of CS-A,C (1 µM) in the supernatants. We observed that CS-A,C exhibits a broader neuroprotective effect than CS-A,C,K,L. Moreover, sulfation pattern affects not only neuroprotection, but also drug release.

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.

Low molecular weight chondroitin sulfate ameliorates pathological changes in 5XFAD mice by improving various functions in the brain

Our previous study found that low molecular weight chondroitin sulfate (LMWCS) had neuroprotective effects against the toxicity of amyloid-β (Aβ) peptides both in vitro and in vivo, and we speculated that the effects might be related with its anti-oxidative activities. In this study, the anti-Alzheimer's disease (AD) activity of LMWCS was further studied in 5XFAD transgenic mice. After 4-month gavage, the levels of Aβ_1-42 level, amyloid precursor protein (APP) and presenilin 1 (PS1) were significantly decreased in the brains of 5XFAD mice, indicating the alteration of APP metabolism by LMWCS. Besides, LMWCS inhibited the secretions of pro-inflammatory cytokines, including interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α) and IL-6. Furthermore, the suppression of neuroinflammation by LMWCS was supported by the decreased expressions of glial fibrillary acidic protein (GFAP) and toll-like receptor 2 (TLR2) in the brains. LMWCS also reduced the production of reactive oxygen species (ROS) and the level of phospho-tau (Ser404) in the brains. Nevertheless, the changes in the behavior tests were moderate. In conclusion, LMWCS administration ameliorated APP metabolism, neuroinflammation, ROS production and tau protein abnormality in the brains of 5XFAD mice, displaying the potential to improve the pathological changes of AD mouse brain. LMWCS could be considered as a promising anti-AD drug candidate, nonetheless, the therapy regimen need to be optimized to improve its pharmacotherapy efficacy.

Surfactant protein C is associated with perineuronal nets and shows age-dependent changes of brain content and hippocampal deposits in wildtype and 3xTg mice

Surfactant protein C (SP-C) modulates cerebrospinal fluid (CSF) rheology. During ageing, its declining levels are accompanied by an increased burden of white matter lesions. Pulmonary SP-C intermediates harbouring the BRICHOS-domain prevent protein misfolding in the lungs. Thus, cerebral SP-C intermediates may counteract cerebral β-amyloidosis, a hallmark of Alzheimer's disease (AD). However, data on the molecular neuroanatomy of SP-C and its alterations in wildtype and triple transgenic (3xTg) mice, featuring essential elements of AD-neuropathology, are lacking. Therefore, this study investigated SP-C-containing structures in murine forebrains and their spatial relationships with vascular, glial and neuronal components of the neurovascular unit. Fluorescence labelling demonstrated neuronal SP-C in the medial habenula, the indusium griseum and the hippocampus. Glial counterstaining elucidated astrocytes in the corpus callosum co-expressing SP-C and S100β. Notably, perineuronal nets were associated with SP-C in the nucleus reticularis thalami, the lateral hypothalamus and the retrosplenial cortex. In the hippocampus of aged 3xTg mice, an increased number of dot-like depositions containing SP-C and Reelin, but devoid of BRICHOS-immunoreactivity were observed apart from AD-like lesions. Wildtype and 3xTg mice revealed an age-dependent increase of such deposits markedly pronounced in about 24-month-old 3xTg mice. SP-C levels of the intracellular and extracellular compartments in each group revealed an inverse correlation of SP-C and Reelin, with reduced SP-C and increased Reelin in an age-dependent fashion especially in 3xTg mice. Taken together, extracellular SP-C, as modulator of glymphatic clearance and potential ligand of PNs, declines in 3xTg mice, which show an accumulation of extracellular Reelin depositions during ageing.

Classification of Alzheimer's Disease Using Gaussian-Based Bayesian Parameter Optimization for Deep Convolutional LSTM Network

Alzheimer's disease (AD) is one of the most important causes of mortality in elderly people, and it is often challenging to use traditional manual procedures when diagnosing a disease in the early stages. The successful implementation of machine learning (ML) techniques has also shown their effectiveness and its reliability as one of the better options for an early diagnosis of AD. But the heterogeneous dimensions and composition of the disease data have undoubtedly made diagnostics more difficult, needing a sufficient model choice to overcome the difficulty. Therefore, in this paper, four different 2D and 3D convolutional neural network (CNN) frameworks based on Bayesian search optimization are proposed to develop an optimized deep learning model to predict the early onset of AD binary and ternary classification on magnetic resonance imaging (MRI) scans. Moreover, certain hyperparameters such as learning rate, optimizers, and hidden units are to be set and adjusted for the performance boosting of the deep learning model. Bayesian optimization enables to leverage advantage throughout the experiments: A persistent hyperparameter space testing provides not only the output but also about the nearest conclusions. In this way, the series of experiments needed to explore space can be substantially reduced. Finally, alongside the use of Bayesian approaches, long short-term memory (LSTM) through the process of augmentation has resulted in finding the better settings of the model that too in less iterations with an relative improvement (RI) of 7.03%, 12.19%, 10.80%, and 11.99% over the four systems optimized with manual hyperparameters tuning such that hyperparameters that look more appealing from past data as well as the conventional techniques of manual selection.

Role of the Extracellular Matrix in Alzheimer's Disease

Alzheimer’s disease (AD) is a neurodegenerative disease with complex pathological characteristics, whose etiology and pathogenesis are still unclear. Over the past few decades, the role of the extracellular matrix (ECM) has gained importance in neurodegenerative disease. In this review, we describe the role of the ECM in AD, focusing on the aspects of synaptic transmission, amyloid-β-plaque generation and degradation, Tau-protein production, oxidative-stress response, and inflammatory response. The function of ECM in the pathological process of AD will inform future research on the etiology and pathogenesis of AD.

Decoding perineuronal net glycan sulfation patterns in the Alzheimer's disease brain

The extracellular matrix (ECM) of the brain comprises unique glycan "sulfation codes" that influence neurological function. Perineuronal nets (PNNs) are chondroitin sulfate-glycosaminoglycan (CS-GAG) containing matrices that enmesh neural networks involved in memory and cognition, and loss of PNN matrices is reported in patients with neurocognitive and neuropsychiatric disorders including Alzheimer's disease (AD). Using liquid chromatography tandem mass spectrometry (LC-MS/MS), we show that patients with a clinical diagnosis of AD-related dementia undergo a re-coding of their PNN-associated CS-GAGs that correlates to Braak stage progression, hyperphosphorylated tau (p-tau) accumulation, and cognitive impairment. As these CS-GAG sulfation changes are detectable prior to the regional onset of classical AD pathology, they may contribute to the initiation and/or progression of the underlying degenerative processes and implicate the brain matrix sulfation code as a key player in the development of AD clinicopathology.

Neural Tissue Homeostasis and Repair Is Regulated via CS and DS Proteoglycan Motifs

Chondroitin sulfate (CS) is the most abundant and widely distributed glycosaminoglycan (GAG) in the human body. As a component of proteoglycans (PGs) it has numerous roles in matrix stabilization and cellular regulation. This chapter highlights the roles of CS and CS-PGs in the central and peripheral nervous systems (CNS/PNS). CS has specific cell regulatory roles that control tissue function and homeostasis. The CNS/PNS contains a diverse range of CS-PGs which direct the development of embryonic neural axonal networks, and the responses of neural cell populations in mature tissues to traumatic injury. Following brain trauma and spinal cord injury, a stabilizing CS-PG-rich scar tissue is laid down at the defect site to protect neural tissues, which are amongst the softest tissues of the human body. Unfortunately, the CS concentrated in gliotic scars also inhibits neural outgrowth and functional recovery. CS has well known inhibitory properties over neural behavior, and animal models of CNS/PNS injury have demonstrated that selective degradation of CS using chondroitinase improves neuronal functional recovery. CS-PGs are present diffusely in the CNS but also form denser regions of extracellular matrix termed perineuronal nets which surround neurons. Hyaluronan is immobilized in hyalectan CS-PG aggregates in these perineural structures, which provide neural protection, synapse, and neural plasticity, and have roles in memory and cognitive learning. Despite the generally inhibitory cues delivered by CS-A and CS-C, some CS-PGs containing highly charged CS disaccharides (CS-D, CS-E) or dermatan sulfate (DS) disaccharides that promote neural outgrowth and functional recovery. CS/DS thus has varied cell regulatory properties and structural ECM supportive roles in the CNS/PNS depending on the glycoform present and its location in tissue niches and specific cellular contexts. Studies on the fruit fly, Drosophila melanogaster and the nematode Caenorhabditis elegans have provided insightful information on neural interconnectivity and the role of the ECM and its PGs in neural development and in tissue morphogenesis in a whole organism environment.

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.

An Extracellular Perspective on CNS Maturation: Perineuronal Nets and the Control of Plasticity

During restricted time windows of postnatal life, called critical periods, neural circuits are highly plastic and are shaped by environmental stimuli. In several mammalian brain areas, from the cerebral cortex to the hippocampus and amygdala, the closure of the critical period is dependent on the formation of perineuronal nets. Perineuronal nets are a condensed form of an extracellular matrix, which surrounds the soma and proximal dendrites of subsets of neurons, enwrapping synaptic terminals. Experimentally disrupting perineuronal nets in adult animals induces the reactivation of critical period plasticity, pointing to a role of the perineuronal net as a molecular brake on plasticity as the critical period closes. Interestingly, in the adult brain, the expression of perineuronal nets is remarkably dynamic, changing its plasticity-associated conditions, including memory processes. In this review, we aimed to address how perineuronal nets contribute to the maturation of brain circuits and the regulation of adult brain plasticity and memory processes in physiological and pathological conditions.

The mechanistic link between selective vulnerability of the locus coeruleus and neurodegeneration in Alzheimer's disease

Alzheimer's disease (AD) is neuropathologically characterized by the intracellular accumulation of hyperphosphorylated tau and the extracellular deposition of amyloid-β plaques, which affect certain brain regions in a progressive manner. The locus coeruleus (LC), a small nucleus in the pons of the brainstem, is widely recognized as one of the earliest sites of neurofibrillary tangle formation in AD. Patients with AD exhibit significant neuronal loss in the LC, resulting in a marked reduction of its size and function. The LC, which vastly innervates several regions of the brain, is the primary source of the neurotransmitter norepinephrine (NE) in the central nervous system. Considering that NE is a major modulator of behavior, contributing to neuroprotection and suppression of neuroinflammation, degeneration of the LC in AD and the ultimate dysregulation of the LC-NE system has detrimental effects in the brain. In this review, we detail the neuroanatomy and function of the LC, its essential role in neuroprotection, and how this is dysregulated in AD. We discuss AD-related neuropathologic changes in the LC and mechanisms by which LC neurons are selectively vulnerable to insult. Further, we elucidate the neurotoxic effects of LC de-innervation both locally and at projection sites, and how this augments disease pathology, progression and severity. We summarize how preservation of the LC-NE system could be used in the treatment of AD and other neurodegenerative diseases affected by LC degeneration.

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.

Microglia facilitate loss of perineuronal nets in the Alzheimer's disease brain

BACKGROUND

Microglia, the brain's principal immune cell, are increasingly implicated in Alzheimer's disease (AD), but the molecular interfaces through which these cells contribute to amyloid beta (Aβ)-related neurodegeneration are unclear. We recently identified microglial contributions to the homeostatic and disease-associated modulation of perineuronal nets (PNNs), extracellular matrix structures that enwrap and stabilize neuronal synapses, but whether PNNs are altered in AD remains controversial.

METHODS

Extensive histological analysis was performed on male and female 5xFAD mice at 4, 8, 12, and 18 months of age to assess plaque burden, microgliosis, and PNNs. Findings were validated in postmortem AD tissue. The role of neuroinflammation in PNN loss was investigated via LPS treatment, and the ability to prevent or rescue disease-related reductions in PNNs was assessed by treating 5xFAD and 3xTg-AD model mice with colony-stimulating factor 1 receptor (CSF1R) inhibitor PLX5622 to deplete microglia.

FINDINGS

Utilizing the 5xFAD mouse model and human cortical tissue, we report that PNNs are extensively lost in AD in proportion to plaque burden. Activated microglia closely associate with and engulf damaged nets in the 5xFAD brain, and inclusions of PNN material are evident in mouse and human microglia, while aggrecan, a critical PNN component, deposits within human dense-core plaques. Disease-associated reductions in parvalbumin (PV)+ interneurons, frequently coated by PNNs, are preceded by PNN coverage and integrity impairments, and similar phenotypes are elicited in wild-type mice following microglial activation with LPS. Chronic pharmacological depletion of microglia prevents 5xFAD PNN loss, with similar results observed following depletion in aged 3xTg-AD mice, and this occurs despite plaque persistence.

INTERPRETATION

We conclude that phenotypically altered microglia facilitate plaque-dependent PNN loss in the AD brain.

FUNDING

The NIH (NIA, NINDS) and the Alzheimer's Association.

Chondroitin sulfate proteoglycans: key modulators of neuronal plasticity, long-term memory, neurodegenerative, and psychiatric disorders

The chondroitin sulfate proteoglycans (CSPGs) are large groups of heterogenous proteoglycans that are mainly expressed by reactive astrocytes in the central nervous system (CNS). They share similar core proteins and are post-transcriptionally modified by chondroitin sulfate glycosaminoglycans. CSPGs are the major components of the perineuronal nets (PNN) that regulate the opening and closure of the critical period. Mounting reports have documented the crucial roles of CSPGs in restricting neuronal plasticity, axonal growth, and pathfinding during development as well as axonal regeneration after CNS injury. Moreover, CSPGs and PNNs modulate long-term memory, which impairments frequently happened in several neurodegenerative and psychiatric disorders. This review will shortly introduce the expression patterns of CSPGs during development and after injury, the PNNs constitutions, the roles of CSPGs and PNNs in axonal regrowth, discuss the most recently identified roles of CSPGs and PNNs in mediating long-term memory and their correlation with brain disorders, and finally, propose a short perspective of future investigations. Hopefully, further explorations may validate the therapeutic potentials of PNNs and CSPGs.

Chondroitin sulfate proteoglycan-5 forms perisynaptic matrix assemblies in the adult rat cortex

Composition of the brain extracellular matrix changes in time as maturation proceeds. Chondroitin sulfate proteoglycan 5 (CSPG-5), also known as neuroglycan C, has been previously associated to differentiation since it shapes neurite growth and synapse forming. Here, we show that this proteoglycan persists in the postnatal rat brain, and its expression is higher in cortical regions with plastic properties, including hippocampus and the medial prefrontal cortex at the end of the second postnatal week. Progressively accumulating after birth, CSPG-5 typically concentrates around glutamatergic and GABAergic terminals in twelve-week old rat hippocampus. CSPG-5-containing perisynaptic matrix rings often appear at the peripheral margin of perineuronal nets. Electron microscopy and analysis of synaptosomal fraction showed that CSPG-5 accumulates around, and is associated to synapses, respectively. In vitro analyses suggest that neurons, but less so astrocytes, express CSPG-5 in rat primary neocortical cultures, and CSPG-5 produced by transfected neuroblastoma cells appear at endings and contact points of neurites. In human subjects, CSPG-5 expression shifts in brain areas of the default mode network of suicide victims, which may reflect an impact in the pathogenesis of psychiatric diseases or support diagnostic power.

Perineuronal net expression in the brain of a hibernating mammal

During hibernation, mammals like the 13-lined ground squirrel cycle between physiological extremes. Most of the hibernation season is spent in bouts of torpor, where body temperature, heart rate, and cerebral blood flow are all very low. However, the ground squirrels periodically enter into interbout arousals (IBAs), where physiological parameters return to non-hibernating levels. During torpor, neurons in many brain regions shrink and become electrically quiescent, but reconnect and regain activity during IBA. Previous work showed evidence of extracellular matrix (ECM) changes occurring in the hypothalamus during hibernation that could be associated with this plasticity. Here, we examined expression of a specialized ECM structure, the perineuronal net (PNN), in the forebrain of ground squirrels in torpor, IBA, and summer (non-hibernating). PNNs are known to restrict plasticity, and could be important for retaining essential connections in the brain during hibernation. We found PNNs in three regions of the hypothalamus: ventrolateral hypothalamus, paraventricular nucleus (PVN), and anterior hypothalamic area. We also found PNNs throughout the cerebral cortex, amygdala, and lateral septum. The total area covered by PNNs within the PVN was significantly higher during IBA compared to non-hibernating and torpor (P < 0.01). Additionally, the amount of PNN coverage area per Nissl-stained neuron in the PVN was significantly higher in hibernation compared to non-hibernating (P < 0.05). No other significant differences were found across seasons. The PVN is involved in food intake and homeostasis, and PNNs found here could be essential for retaining vital life functions during hibernation.

The Effect of Hapln4 Link Protein Deficiency on Extracellular Space Diffusion Parameters and Perineuronal Nets in the Auditory System During Aging

Hapln4 is a link protein which stabilizes the binding between lecticans and hyaluronan in perineuronal nets (PNNs) in specific brain regions, including the medial nucleus of the trapezoid body (MNTB). The aim of this study was: (1) to reveal possible age-related alterations in the extracellular matrix composition in the MNTB and inferior colliculus, which was devoid of Hapln4 and served as a negative control, (2) to determine the impact of the Hapln4 deletion on the values of the ECS diffusion parameters in young and aged animals and (3) to verify that PNNs moderate age-related changes in the ECS diffusion, and that Hapln4-brevican complex is indispensable for the correct protective function of the PNNs. To achieve this, we evaluated the ECS diffusion parameters using the real-time iontophoretic method in the selected region in young adult (3 to 6-months-old) and aged (12 to 18-months-old) wild type and Hapln4 knock-out (KO) mice. The results were correlated with an immunohistochemical analysis of the ECM composition and astrocyte morphology. We report that the ECM composition is altered in the aged MNTB and aging is a critical point, revealing the effect of Hapln4 deficiency on the ECS diffusion. All of our findings support the hypothesis that the ECM changes in the MNTB of aged KO animals affect the ECS parameters indirectly, via morphological changes of astrocytes, which are in direct contact with synapses and can be influenced by the ongoing synaptic transmission altered by shifts in the ECM composition.

The roles of perineuronal nets and the perinodal extracellular matrix in neuronal function

Perineuronal nets (PNNs) are extracellular matrix (ECM) chondroitin sulfate proteoglycan (CSPG)-containing structures that surround the soma and dendrites of various mammalian neuronal cell types. PNNs appear during development around the time that the critical periods for developmental plasticity end and are important for both their onset and closure. A similar structure - the perinodal ECM - surrounds the axonal nodes of Ranvier and appears as myelination is completed, acting as an ion-diffusion barrier that affects axonal conduction speed. Recent work has revealed the importance of PNNs in controlling plasticity in the CNS. Digestion, blocking or removal of PNNs influences functional recovery after a variety of CNS lesions. PNNs have further been shown to be involved in the regulation of memory and have been implicated in a number of psychiatric disorders.

Glycosaminoglycans from Alzheimer's disease hippocampus have altered capacities to bind and regulate growth factors activities and to bind tau

Glycosaminoglycans (GAGs), including heparan sulfates and chondroitin sulfates, are major components of the extracellular matrix. Upon interacting with heparin binding growth factors (HBGF), GAGs participate to the maintaintenance of tissue homeostasis and contribute to self-healing. Although several processes regulated by HBGF are altered in Alzheimer’s disease, it is unknown whether the brain GAG capacities to bind and regulate the function of HBGF or of other heparin binding proteins, as tau, are modified in this disease. Here, we show that total sulfated GAGs from hippocampus of Alzheimer’s disease have altered capacities to bind and potentiate the activities of growth factors including FGF-2, VEGF, and BDNF while their capacity to bind to tau is remarkable increased. Alterations of GAG structures and capacities to interact with and regulate the activity of heparin binding proteins might contribute to impaired tissue homeostasis in the Alzheimer’s disease brain.

Distribution of the Extracellular Matrix in the Pararubral Area of the Rat

Previously we described similarities and differences in the organization and molecular composition of an aggrecan based extracellular matrix (ECM) in three precerebellar nuclei, the inferior olive, the prepositus hypoglossi nucleus and the red nucleus of the rat associated with their specific cytoarchitecture, connection and function in the vestibular system. The aim of present study is to map the ECM pattern in a mesencephalic precerebellar nucleus, the pararubral area, which has a unique function among the precerebellar nuclei with its retinal connection and involvement in the circadian rhythm regulation. Using histochemistry and immunohistochemistry we have described for the first time the presence of major ECM components, the hyaluronan, aggrecan, versican, neurocan, brevican, tenascin-R (TN-R), and the HAPLN1 link protein in the pararubral area. The most common form of the aggrecan based ECM was the diffuse network in the neuropil, but each type of the condensed forms was also recognizable. Characteristic perineuronal nets (PNNs) were only recognizable with Wisteria floribunda agglutinin (WFA) and aggrecan staining around some of the medium-sized neurons, whereas the small cells were rarely surrounded by a weakly stained PNNs. The moderate expression of key molecules of PNN, the hyaluronan (HA) and HAPLN1 suggests that the lesser stability of ECM assembly around the pararubral neurons may allow quicker response to the modified neuronal activity and contributes to the high level of plasticity in the vestibular system.

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.

Impaired Cognitive Function after Perineuronal Net Degradation in the Medial Prefrontal Cortex

Perineuronal nets (PNNs) are highly organized components of the extracellular matrix that surround a subset of mature neurons in the CNS. These structures play a critical role in regulating neuronal plasticity, particularly during neurodevelopment. Consistent with this role, their presence is associated with functional and structural stability of the neurons they ensheath. A loss of PNNs in the prefrontal cortex (PFC) has been suggested to contribute to cognitive impairment in disorders such as schizophrenia. However, the direct consequences of PNN loss in medial PFC (mPFC) on cognition has not been demonstrated. Here, we examined behavior after disruption of PNNs in mPFC of Long-Evans rats following injection of the enzyme chondroitinase ABC (ChABC). Our data show that ChABC-treated animals were impaired on tests of object oddity perception. Performance in the cross-modal object recognition (CMOR) task was not significantly different for ChABC-treated rats, although ChABC-treated rats were not able to perform above chance levels whereas control rats were. ChABC-treated animals were not significantly different from controls on tests of prepulse inhibition (PPI), set-shifting (SS), reversal learning, or tactile and visual object recognition memory. Posthumous immunohistochemistry confirmed significantly reduced PNNs in mPFC due to ChABC treatment. Moreover, PNN density in the mPFC predicted performance on the oddity task, where higher PNN density was associated with better performance. These findings suggest that PNN loss within the mPFC impairs some aspects of object oddity perception and recognition and that PNNs contribute to cognitive function in young adulthood.

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.

The Perineuronal 'Safety' Net? Perineuronal Net Abnormalities in Neurological Disorders

Perineuronal nets (PNN) are extracellular matrix (ECM) assemblies that preferentially ensheath parvalbumin (PV) expressing interneurons. Converging evidence indicates that PV cells and PNN are impaired in a variety of neurological disorders. PNN development and maintenance is necessary for a number of processes within the CNS, including regulation of GABAergic cell function, protection of neurons from oxidative stress, and closure of developmental critical period plasticity windows. Understanding PNN functions may be essential for characterizing the mechanisms of altered cortical excitability observed in neurodegenerative and neurodevelopmental disorders. Indeed, PNN abnormalities have been observed in post-mortem brain tissues of patients with schizophrenia and Alzheimer’s disease. There is impaired development of PNNs and enhanced activity of its key regulator matrix metalloproteinase-9 (MMP-9) in Fragile X Syndrome, a common genetic cause of autism. MMP-9, a protease that cleaves ECM, is differentially regulated in a number of these disorders. Despite this, few studies have addressed the interactions between PNN expression, MMP-9 activity and neuronal excitability. In this review, we highlight the current evidence for PNN abnormalities in CNS disorders associated with altered network function and MMP-9 levels, emphasizing the need for future work targeting PNNs in pathophysiology and therapeutic treatment of neurological disorders.

Proteolytic Remodeling of Perineuronal Nets: Effects on Synaptic Plasticity and Neuronal Population Dynamics

The perineuronal net (PNN) represents a lattice-like structure that is prominently expressed along the soma and proximal dendrites of parvalbumin- (PV-) positive interneurons in varied brain regions including the cortex and hippocampus. It is thus apposed to sites at which PV neurons receive synaptic input. Emerging evidence suggests that changes in PNN integrity may affect glutamatergic input to PV interneurons, a population that is critical for the expression of synchronous neuronal population discharges that occur with gamma oscillations and sharp-wave ripples. The present review is focused on the composition of PNNs, posttranslation modulation of PNN components by sulfation and proteolysis, PNN alterations in disease, and potential effects of PNN remodeling on neuronal plasticity at the single-cell and population level.

A Sweet Talk: The Molecular Systems of Perineuronal Nets in Controlling Neuronal Communication

Perineuronal nets (PNNs) are mesh-like structures, composed of a hierarchical assembly of extracellular matrix molecules in the central nervous system (CNS), ensheathing neurons and regulating plasticity. The mechanism of interactions between PNNs and neurons remain uncharacterized. In this review, we pose the question: how do PNNs regulate communication to and from neurons? We provide an overview of the current knowledge on PNNs with a focus on the cellular interactions. PNNs ensheath a subset of the neuronal population with distinct molecular aspects in different areas of the CNS. PNNs control neuronal communication through molecular interactions involving specific components of the PNNs. This review proposes that the PNNs are an integral part of neurons, crucial for the regulation of plasticity in the CNS.

Postnatal development of GABAergic interneurons and perineuronal nets in mouse temporal cortex subregions

In human neuropsychiatric disorders, there are functional and anatomical abnormalities of GABAergic interneurons in each temporal cortex subregion. Furthermore, accumulation of amyloid-β is observed in the temporal cortex in the early stages of Alzheimer's disease. Each subregion of the temporal cortex has an important role in coordinating the input and output of the hippocampus. When subregions of the temporal cortex are impaired, memory and learning ability decrease. GABAergic interneurons control excitatory neurons, forming the cortico-cortical and cortico-hippocampal networks. However, in temporal cortex subregions, details of the distribution and developmental processes of GABAergic interneurons and perineuronal nets (PNNs) have not been elucidated. Here we examined the development of GABAergic interneurons and PNNs in mouse temporal cortex subregions. Results indicate that temporal cortex GABAergic interneurons have developmental stages different to those of the primary sensory cortex. In addition, the density of PNNs in the temporal cortex is lower than that in the sensory cortex. Furthermore, we found that the Wisteria floribunda agglutinin-reactive extracellular matrix molecule is present in the upper level of layer 1 of the temporal cortex. These results support the idea that mouse temporal cortex subregions develop differently from other cortical regions and have region-specific characteristics after maturation. The present study results suggested that the structure of the temporal cortex is significantly different from the sensory cortex and that temporal cortex may be highly vulnerable to neuropsychiatric and neurodegenerative disorders.

A deficiency of the link protein Bral2 affects the size of the extracellular space in the thalamus of aged mice

Bral2 is a link protein stabilizing the binding between lecticans and hyaluronan in perineuronal nets and axonal coats (ACs) in specific brain regions. Using the real-time iontophoretic method and diffusion-weighted magnetic resonance, we determined the extracellular space (ECS) volume fraction (α), tortuosity (λ), and apparent diffusion coefficient of water (ADC_W ) in the thalamic ventral posteromedial nucleus (VPM) and sensorimotor cortex of young adult (3-6 months) and aged (14-20 months) Bral2-deficient (Bral2^-/- ) mice and age-matched wild-type (wt) controls. The results were correlated with an analysis of extracellular matrix composition. In the cortex, no changes between wt and Bral2^-/- were detected, either in the young or aged mice. In the VPM of aged but not in young Bral2^-/- mice, we observed a significant decrease in α and ADC_W in comparison with age-matched controls. Bral2 deficiency led to a reduction of both aggrecan- and brevican-associated perineuronal nets and a complete disruption of brevican-based ACs in young as well as aged VPM. Our data suggest that aging is a critical point that reveals the effect of Bral2 deficiency on VPM diffusion. This effect is probably mediated through the enhanced age-related damage of neurons lacking protective ACs, or the exhausting of compensatory mechanisms maintaining unchanged diffusion parameters in young Bral2^-/- animals. A decreased ECS volume in aged Bral2^-/- mice may influence the diffusion of neuroactive substances, and thus extrasynaptic and also indirectly synaptic transmission in this important nucleus of the somatosensory pathway.

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.

Antibody recognizing 4-sulfated chondroitin sulfate proteoglycans restores memory in tauopathy-induced neurodegeneration

Chondroitin sulfate proteoglycans (CSPGs) are the main active component of perineuronal nets (PNNs). Digestion of the glycosaminoglycan chains of CSPGs with chondroitinase ABC or transgenic attenuation of PNNs leads to prolongation of object recognition memory and activation of various forms of plasticity in the adult central nervous system. The inhibitory properties of the CSPGs depend on the pattern of sulfation of their glycosaminoglycans, with chondroitin 4-sulfate (C4S) being the most inhibitory form. In this study, we tested a number of candidates for functional blocking of C4S, leading to selection of an antibody, Cat316, which specifically recognizes C4S and blocks its inhibitory effects on axon growth. It also partly blocks binding of semaphorin 3A to PNNs and attenuates PNN formation. We asked whether injection of Cat316 into the perirhinal cortex would have the same effects on memory as chondroitinase ABC treatment. We found that masking C4S with the Cat316 antibody extended long-term object recognition memory in normal wild-type mice to 24 hours, similarly to chondroitinase or transgenic PNN attenuation. We then tested Cat316 for restoration of memory in a neurodegeneration model. Mice expressing tau with the P301S mutation showed profound loss of object recognition memory at 4 months of age. Injection of Cat316 into the perirhinal cortex normalized object recognition at 3 hours in P301S mice. These data indicate that Cat316 binding to C4S in the extracellular matrix can restore plasticity and memory in the same way as chondroitinase ABC digestion. Our results suggest that antibodies to C4S could be a useful therapeutic to restore memory function in neurodegenerative disorders.

Perineuronal nets and vocal plasticity in songbirds: A proposed mechanism to explain the difference between closed-ended and open-ended learning

Perineuronal nets (PNN) are aggregations of chondroitin sulfate proteoglycans surrounding the soma and proximal processes of neurons, mostly GABAergic interneurons expressing parvalbumin. They limit the plasticity of their afferent synaptic connections. In zebra finches PNN develop in an experience-dependent manner in the song control nuclei HVC and RA (nucleus robustus arcopallialis) when young birds crystallize their song. Because songbird species that are open-ended learners tend to recapitulate each year the different phases of song learning until their song crystallizes at the beginning of the breeding season, we tested whether seasonal changes in PNN expression would be found in the song control nuclei of a seasonally breeding species such as the European starling. Only minimal changes in PNN densities and total number of cells surrounded by PNN were detected. However, comparison of the density of PNN and of PNN surrounding parvalbumin-positive cells revealed that these structures are far less numerous in starlings that show extensive adult vocal plasticity, including learning of new songs throughout the year, than in the closed-ended learner zebra finches. Canaries that also display some vocal plasticity across season but were never formally shown to learn new songs in adulthood were intermediate in this respect. Together these data suggest that establishment of PNN around parvalbumin-positive neurons in song control nuclei has diverged during evolution to control the different learning capacities observed in songbird species. This differential expression of PNN in different songbird species could represent a key cellular mechanism mediating species variation between closed-ended and open-ended learning strategies. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 975-994, 2017.

Extracellular matrix proteomics in schizophrenia and Alzheimer's disease

Brain extracellular matrix (ECM) is a highly organized system that consists of collagens, noncollagenous proteins, glycoproteins, hyaluronan, and proteoglycans. Recognized physiological roles of ECM include developmental regulation, tissue homeostasis, cell migration, cell proliferation, cell differentiation, neuronal plasticity, and neurite outgrowth. Aberrant ECM structure is associated with brain neurodegenerative conditions. This review focuses on two neurodegenerative conditions, schizophrenia and Alzheimer's disease, and summarizes recent findings of altered ECM components, including proteoglycans, glycosaminoglycans, proteins, and glycoproteins, and proteins and genes related to other brain components. The scope includes immunohistochemical, genomics, transcriptomics, proteomics, and glycomics studies, and a critical assessment of current state of proteomic studies for neurodegenerative disorders. The intent is to summarize the ECM molecular alterations associated with neurodegenerative pathophysiology. Graphical Abstract Brain extracellular matrix showing HSPGs, CSPGs, HA, collagens, and other glycoproteins.

Parvalbumin neurons and perineuronal nets in the mouse prefrontal cortex

The prefrontal cortex (PFC) plays a key role in cognitive functions, memory, and attention. Alterations in parvalbumin interneurons (PV neurons) and perineuronal nets (PNNs) within the PFC have been implicated in schizophrenia and autism spectrum disorder pathology. However, it remains unclear why PV neurons and PNNs in the PFC are selectively impaired. Here we aimed to clarify if PV neurons and PNNs in the PFC have region-specific features. We found that PV neurons and PNNs were increased in a region-specific manner in the PFC during postnatal development. In the mature PFC, the expression of PV protein is lower than in other parts of the cortex. Furthermore, PNNs in the mature PFC are not typical lattice-like structures and do not have the major components of PNNs and tenascin-R. The present study indicates that PV neurons and PNNs have region-specific features, and our results suggest that PV neurons and PNNs have structural vulnerability within the PFC.

Chondroitin 4-O-Sulfotransferase Is Indispensable for Sulfation of Chondroitin and Plays an Important Role in Maintaining Normal Life Span and Oxidative Stress Responses in Nematodes

Chondroitin sulfate (CS)/chondroitin (Chn) chains are indispensable for embryonic cell division and cytokinesis in the early developmental stages in Caenorhabditis elegans and mice, whereas heparan sulfate (HS) is essential for axon guidance during nervous system development. These data indicate that the fundamental functions of CS and HS are conserved from worms to mammals and that the function of CS/Chn differs from that of HS. Although previous studies have shown that C. elegans produces HS and non-sulfated Chn, whether the organism produces CS remains unclear. Here, we demonstrate that C. elegans produces a small amount of 4-O-sulfated Chn and report the identification of C41C4.1, an orthologue of the human chondroitin 4-O-sulfotransferase gene. Loss of C41C4.1 in C. elegans resulted in a decline in 4-O-sulfation of CS and an increase in the number of sulfated units in HS. C41C4.1 deletion mutants exhibited reduced survival rates after synchronization with sodium hypochlorite. Collectively, these results show for the first time that CS glycans are present in C. elegans and that the Chn 4-O-sulfotransferase responsible for the sulfation plays an important role in protecting nematodes from oxidative stress.

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.

Heterogeneous expression of extracellular matrix molecules in the red nucleus of the rat

Previous studies in our laboratory showed that the organization and heterogeneous molecular composition of extracellular matrix is associated with the variable cytoarchitecture, connections and specific functions of the vestibular nuclei and two related areas of the vestibular neural circuits, the inferior olive and prepositus hypoglossi nucleus. The aim of the present study is to reveal the organization and distribution of various molecular components of extracellular matrix in the red nucleus, a midbrain premotor center. Morphologically and functionally the red nucleus is comprised of the magno- and parvocellular parts, with overlapping neuronal population. By using histochemical and immunohistochemical methods, the extracellular matrix appeared as perineuronal net, axonal coat, perisynaptic matrix or diffuse network in the neuropil. In both parts of the red nucleus we have observed positive hyaluronan, tenascin-R, link protein, and lectican (aggrecan, brevican, versican, neurocan) reactions. Perineuronal nets were detected with each of the reactions and the aggrecan showed the most intense staining in the pericellular area. The two parts were clearly distinguished on the basis of neurocan and HAPLN1 expression as they have lower intensity in the perineuronal nets of large cells and in the neuropil of the magnocellular part. Additionally, in contrast to this pattern, the aggrecan was heavily labeled in the magnocellular region sharply delineating from the faintly stained parvocellular area. The most characteristic finding was that the appearance of perineuronal nets was related with the neuronal size independently from its position within the two subdivisions of red nucleus. In line with these statements none of the extracellular matrix molecules were restricted exclusively to the magno- or parvocellular division. The chemical heterogeneity of the perineuronal nets may support the recently accepted view that the red nucleus comprises more different populations of neurons than previously reported.