The protein tyrosine phosphatase RPTPζ/phosphacan is critical for perineuronal net structure

Glycosaminoglycans' for brain health: Harnessing glycosaminoglycan based biomaterials for treating central nervous system diseases and in-vitro modeling

Dysfunction of the central nervous system (CNS) following traumatic brain injuries (TBI), spinal cord injuries (SCI), or strokes remains challenging to address using existing medications and cell-based therapies. Although therapeutic cell administration, such as stem cells and neuronal progenitor cells (NPCs), have shown promise in regenerative properties, they have failed to provide substantial benefits. However, the development of living cortical tissue engineered grafts, created by encapsulating these cells within an extracellular matrix (ECM) mimetic hydrogel scaffold, presents a promising functional replacement for damaged cortex in cases of stroke, SCI, and TBI. These grafts facilitate neural network repair and regeneration following CNS injuries. Given that natural glycosaminoglycans (GAGs) are a major constituent of the CNS, GAG-based hydrogels hold potential for the next generation of CNS healing therapies and in vitro modeling of CNS diseases. Brain-specific GAGs not only offer structural and biochemical signaling support to encapsulated neural cells but also modulate the inflammatory response in lesioned brain tissue, facilitating host integration and regeneration. This review briefly discusses different roles of GAGs and their related proteoglycan counterparts in healthy and diseases brain and explores current trends and advancements in GAG-based biomaterials for treating CNS injuries and modeling diseases. Additionally, it examines injectable, 3D bioprintable, and conductive GAG-based scaffolds, highlighting their clinical potential for in vitro modeling of patient-specific neural dysfunction and their ability to enhance CNS regeneration and repair following CNS injury in vivo.

The Role of Tenascin-C on the Structural Plasticity of Perineuronal Nets and Synaptic Expression in the Hippocampus of Male Mice

Neuronal plasticity is a crucial mechanism for an adapting nervous system to change. It is shown to be regulated by perineuronal nets (PNNs), the condensed forms of the extracellular matrix (ECM) around neuronal bodies. By assessing the changes in the number, intensity, and structure of PNNs, the ultrastructure of the PNN mesh, and the expression of inhibitory and excitatory synaptic inputs on these neurons, we aimed to clarify the role of an ECM glycoprotein, tenascin-C (TnC), in the dorsal hippocampus. To enhance neuronal plasticity, TnC-deficient (TnC-/-) and wild-type (TnC+/+) young adult male mice were reared in an enriched environment (EE) for 8 weeks. Deletion of TnC in TnC-/- mice showed an ultrastructural reduction of the PNN mesh and an increased inhibitory input in the dentate gyrus (DG), and an increase in the number of PNNs with a rise in the inhibitory input in the CA2 region. EE induced an increased inhibitory input in the CA2, CA3, and DG regions; in DG, the change was also followed by an increased intensity of PNNs. No changes in PNNs or synaptic expression were found in the CA1 region. We conclude that the DG and CA2 regions emerged as focal points of alterations in PNNs and synaptogenesis with EE as mediated by TnC.

Perineuronal Net Microscopy: From Brain Pathology to Artificial Intelligence

Perineuronal nets (PNN) are a special highly structured type of extracellular matrix encapsulating synapses on large populations of CNS neurons. PNN undergo structural changes in schizophrenia, epilepsy, Alzheimer's disease, stroke, post-traumatic conditions, and some other brain disorders. The functional role of the PNN microstructure in brain pathologies has remained largely unstudied until recently. Here, we review recent research implicating PNN microstructural changes in schizophrenia and other disorders. We further concentrate on high-resolution studies of the PNN mesh units surrounding synaptic boutons to elucidate fine structural details behind the mutual functional regulation between the ECM and the synaptic terminal. We also review some updates regarding PNN as a potential pharmacological target. Artificial intelligence (AI)-based methods are now arriving as a new tool that may have the potential to grasp the brain's complexity through a wide range of organization levels-from synaptic molecular events to large scale tissue rearrangements and the whole-brain connectome function. This scope matches exactly the complex role of PNN in brain physiology and pathology processes, and the first AI-assisted PNN microscopy studies have been reported. To that end, we report here on a machine learning-assisted tool for PNN mesh contour tracing.

Adolescent intermittent ethanol exposure decreases perineuronal nets in the hippocampus in a sex dependent manner: Modulation through pharmacological inhibition of RPTPβ/ζ

Adolescence is a critical period for brain maturation in which this organ undergoes critical plasticity mechanisms that increase its vulnerability to the effects of alcohol. Significantly, ethanol-induced disruption of hippocampal neurogenesis has been related to cognitive decline in adulthood. During adolescence, the maturation of perineuronal nets (PNNs), extracellular matrix structures highly affected by ethanol consumption, plays a fundamental role in neurogenesis and plasticity in the hippocampus. Receptor Protein Tyrosine Phosphatase (RPTP) β/ζ is a critical anchor point for PNNs on the cell surface. Using the adolescent intermittent access to ethanol (IAE) model, we previously showed that MY10, a small-molecule inhibitor of RPTPβ/ζ, reduces chronic ethanol consumption in adolescent male mice but not in females and prevents IAE-induced neurogenic loss in the male hippocampus. We have now tested if these effects of MY10 are related to sex-dependent modulatory actions on ethanol-induced effects in PNNs. Our findings suggest a complex interplay between alcohol exposure, neural structures, and sex-related differences in the modulation of PNNs and parvalbumin (PV)-positive cells in the hippocampus. In general, IAE increased the number of PV + cells in the female hippocampus and reduced PNNs intensity in different hippocampal regions, particularly in male mice. Notably, we found that pharmacological inhibition of RPTPβ/ζ with MY10 regulates ethanol-induced alterations of PNNs intensity, which correlates with the protection of hippocampal neurogenesis from ethanol neurotoxic effects and may be related to the capacity of MY10 to increase the gene expression of key components of PNNs.

Keratan sulfate, an electrosensory neurosentient bioresponsive cell instructive glycosaminoglycan

The roles of keratan sulfate (KS) as a proton detection glycosaminoglycan in neurosensory processes in the central and peripheral nervous systems is reviewed. The functional properties of the KS-proteoglycans aggrecan, phosphacan, podocalyxcin as components of perineuronal nets in neurosensory processes in neuronal plasticity, cognitive learning and memory are also discussed. KS-glycoconjugate neurosensory gels used in electrolocation in elasmobranch fish species and KS substituted mucin like conjugates in some tissue contexts in mammals need to be considered in sensory signalling. Parallels are drawn between KS's roles in elasmobranch fish neurosensory processes and its roles in mammalian electro mechanical transduction of acoustic liquid displacement signals in the cochlea by the tectorial membrane and stereocilia of sensory inner and outer hair cells into neural signals for sound interpretation. The sophisticated structural and functional proteins which maintain the unique high precision physical properties of stereocilia in the detection, transmittance and interpretation of acoustic signals in the hearing process are important. The maintenance of the material properties of stereocilia are essential in sound transmission processes. Specific, emerging roles for low sulfation KS in sensory bioregulation are contrasted with the properties of high charge density KS isoforms. Some speculations are made on how the molecular and electrical properties of KS may be of potential application in futuristic nanoelectronic, memristor technology in advanced ultrafast computing devices with low energy requirements in nanomachines, nanobots or molecular switches which could be potentially useful in artificial synapse development. Application of KS in such innovative areas in bioregulation are eagerly awaited.

Protein O-mannosylation: one sugar, several pathways, many functions

Recent research has unveiled numerous important functions of protein glycosylation in development, homeostasis, and diseases. A type of glycosylation taking the center stage is protein O-mannosylation, a posttranslational modification conserved in a wide range of organisms, from yeast to humans. In animals, protein O-mannosylation plays a crucial role in the nervous system, whereas protein O-mannosylation defects cause severe neurological abnormalities and congenital muscular dystrophies. However, the molecular and cellular mechanisms underlying protein O-mannosylation functions and biosynthesis remain not well understood. This review outlines recent studies on protein O-mannosylation while focusing on the functions in the nervous system, summarizes the current knowledge about protein O-mannosylation biosynthesis, and discusses the pathologies associated with protein O-mannosylation defects. The evolutionary perspective revealed by studies in the Drosophila model system are also highlighted. Finally, the review touches upon important knowledge gaps in the field and discusses critical questions for future research on the molecular and cellular mechanisms associated with protein O-mannosylation functions.

The mechanisms of perineuronal net abnormalities in contributing aging and neurological diseases

The perineuronal net (PNN) is a highly latticed extracellular matrix in the central nervous system, which is composed of hyaluronic acid, proteoglycan, hyaluronan and proteoglycan link protein (Hapln), and tenascin. PNN is predominantly distributed in GABAergic interneurons expressing Parvalbumin (PV) and plays a critical role in synaptic function, learning and memory, oxidative stress, and inflammation. In addition, PNN's structure and function are also modulated by a variety of factors, including protein tyrosine phosphatase σ (PTPσ), orthodenticle homeo-box 2 (Otx2), and erb-b2 receptor tyrosine kinase 4 (ErbB4). Glycosaminoglycan (GAG), a component of proteoglycan, also influences PNN through its sulfate mode. PNN undergoes abnormal changes during aging and in various neurological diseases, such as Alzheimer's disease, Parkinson's disease, schizophrenia, autism spectrum disorder, and multiple sclerosis. Nevertheless, there is limited report on the relationship between PNN and aging or age-related neurological diseases. This review elaborates on the mechanisms governing PNN regulation and summarizes how PNN abnormalities contribute to aging and neurological diseases, offering insights for potential treatments.

Role of IL-34 and its receptors in inflammatory diseases

In recent years, IL-34 has been widely discussed as a novel cytokine. IL-34 is a pro-inflammatory cytokine binding four distinct receptors, namely CSF-1R, syndecan-1, PTP-ζ and TREM2. Previous studies have shown that IL-34 and its receptors play important roles in the development and progression of various inflammatory diseases. Therefore, IL-34 has the potential to be a biomarker and therapeutic target for inflammatory diseases. However, further study is still needed to identify the specific mechanism through which IL-34 contributes to illness. In this article, we review the recent advances in the biological roles of IL-34 and its receptors as well as their roles in the development and therapeutic application of inflammatory diseases.

Perineuronal Nets: Subtle Structures with Large Implications

Perineuronal nets (PNNs) are specialized structures of the extracellular matrix that surround the soma and proximal dendrites of certain neurons in the central nervous system, particularly parvalbumin-expressing interneurons. Their appearance overlaps the maturation of neuronal circuits and the closure of critical periods in different regions of the brain, setting their connectivity and abruptly reducing their plasticity. As a consequence, the digestion of PNNs, as well as the removal or manipulation of their components, leads to a boost in this plasticity and can play a key role in the functional recovery from different insults and in the etiopathology of certain neurologic and psychiatric disorders. Here we review the structure, composition, and distribution of PNNs and their variation throughout the evolutive scale. We also discuss methodological approaches to study these structures. The function of PNNs during neurodevelopment and adulthood is discussed, as well as the influence of intrinsic and extrinsic factors on these specialized regions of the extracellular matrix. Finally, we review current data on alterations in PNNs described in diseases of the central nervous system (CNS), focusing on psychiatric disorders. Together, all the data available point to the PNNs as a promising target to understand the physiology and pathologic conditions of the CNS.

Brain-specific glycosylation enzyme GnT-IX maintains levels of protein tyrosine phosphatase receptor PTPRZ, thereby mediating glioma growth

Gliomas are the most prevalent primary tumor of the central nervous system. Despite advances in imaging technologies, neurosurgical techniques, and radiotherapy, a cure for high-grade glioma remains elusive. Several groups have reported that protein tyrosine phosphatase receptor type Z (PTPRZ) is highly expressed in glioblastoma, and that targeting PTPRZ attenuates tumor growth in mice. PTPRZ is modified with diverse glycan, including the PTPRZ-unique human natural killer-1 capped O-mannosyl core M2 glycans. However, the regulation and function of these unique glycans are unclear. Using CRISPR genome-editing technology, we first demonstrated that disruption of the PTPRZ gene in human glioma LN-229 cells resulted in profoundly reduced tumor growth in xenografted mice, confirming the potential of PTPRZ as a therapeutic target for glioma. Furthermore, multiple glycan analyses revealed that PTPRZ derived from glioma patients and from xenografted glioma expressed abundant levels of human natural killer-1-capped O-Man glycans via extrinsic signals. Finally, since deficiency of O-Man core M2 branching enzyme N-acetylglucosaminyltransferase IX (GnT-IX) was reported to reduce PTPRZ protein levels, we disrupted the GnT-IX gene in LN-229 cells and found a significant reduction of glioma growth both in vitro and in the xenograft model. These results suggest that the PTPR glycosylation enzyme GnT-IX may represent a promising therapeutic target for glioma.

Protein-protein interactions between tenascin-R and RPTPζ/phosphacan are critical to maintain the architecture of perineuronal nets

Neural plasticity, the ability to alter the structure and function of neural circuits, varies throughout the age of an individual. The end of the hyperplastic period in the central nervous system coincides with the appearance of honeycomb-like structures called perineuronal nets (PNNs) that surround a subset of neurons. PNNs are a condensed form of neural extracellular matrix that include the glycosaminoglycan hyaluronan and extracellular matrix proteins such as aggrecan and tenascin-R (TNR). PNNs are key regulators of developmental neural plasticity and cognitive functions, yet our current understanding of the molecular interactions that help assemble them remains limited. Disruption of Ptprz1, the gene encoding the receptor protein tyrosine phosphatase RPTPζ, altered the appearance of nets from a reticulated structure to puncta on the surface of cortical neuron bodies in adult mice. The structural alterations mirror those found in Tnr-/- mice, and TNR is absent from the net structures that form in dissociated cultures of Ptprz1-/- cortical neurons. These findings raised the possibility that TNR and RPTPζ cooperate to promote the assembly of PNNs. Here, we show that TNR associates with the RPTPζ ectodomain and provide a structural basis for these interactions. Furthermore, we show that RPTPζ forms an identical complex with tenascin-C, a homolog of TNR that also regulates neural plasticity. Finally, we demonstrate that mutating residues at the RPTPζ-TNR interface impairs the formation of PNNs in dissociated neuronal cultures. Overall, this work sets the stage for analyzing the roles of protein-protein interactions that underpin the formation of nets.

Inhibition of RPTPβ/ζ reduces chronic ethanol intake in adolescent mice and modulates ethanol effects on hippocampal neurogenesis and glial responses in a sex-dependent manner

Pleiotrophin (PTN) is a cytokine that modulates ethanol drinking and reward and regulates glial responses in different contexts. PTN is an inhibitor of Receptor Protein Tyrosine Phosphatase (RPTP) β/ζ. Inhibition of RPTPβ/ζ reduces binge-like drinking in adult male mice. Whether inhibition of RPTPβ/ζ is effective in reducing ethanol consumption during adolescence and in both sexes remained to be studied. In this work, male and female adolescent mice underwent an intermittent access to ethanol (IAE) 2-bottle choice protocol. Treatment with MY10 (60 mg/kg, i.g.), a small-molecule RPTPβ/ζ inhibitor, reduced chronic 3-week ethanol consumption only in male mice. We detected an ethanol-induced overall decrease in hippocampal GFAPir and Iba1ir, independently of the treatment received, suggesting that RPTPβ/ζ is not key in the regulation of IAE-induced glial responses. However, we found a significant negative correlation between the size of microglial cells and the number of hippocampal neuronal progenitors only in male mice after IAE. This correlation was disrupted by treatment with MY10 before each drinking session, which may be related to the ability of MY10 to regulate the intensity of the perineuronal nets (PNNs) in the hippocampus in a sex-dependent manner. The data show for the first time that inhibition of RPTPβ/ζ reduces chronic voluntary ethanol consumption in adolescent mice in a sex-dependent manner. In addition, we show evidence for sex-specific differences in the effects of IAE on glial responses and hippocampal neurogenesis, which may be related to different actions of the RPTPβ/ζ signalling pathway in the brains of male and female mice.

Protein tyrosine phosphatase 69D is a substrate of protein O-mannosyltransferases 1-2 that is required for the wiring of sensory axons in Drosophila

Mutations in protein O-mannosyltransferases (POMTs) result in severe brain defects and congenital muscular dystrophies characterized by abnormal glycosylation of α-dystroglycan (α-Dg). However, neurological phenotypes of POMT mutants are not well understood, and the functional substrates of POMTs other than α-Dg remain unknown. Using a Drosophila model, here we reveal that Dg alone cannot account for the phenotypes of POMT mutants, and identify Protein tyrosine phosphatase 69D (PTP69D) as a gene interacting with POMTs in producing the abdomen rotation phenotype. Using RNAi-mediated knockdown, mutant alleles, and a dominant-negative form of PTP69D, we reveal that PTP69D is required for the wiring of larval sensory axons. We also found that PTP69D and POMT genes interact in this process, and that their interactions lead to complex synergistic or antagonistic effects on axon wiring phenotypes, depending on the mode of genetic manipulation. Using glycoproteomic approaches, we further characterized the glycosylation of the PTP69D transgenic construct expressed in genetic strains with different levels of POMT activity. We found that the PTP69D construct carries many O-linked mannose modifications when expressed in Drosophila with wild-type or ectopically upregulated expression of POMTs. These modifications were absent in POMT mutants, suggesting that PTP69D is a substrate of POMT-mediated O-mannosylation. Taken together, our results indicate that PTP69D is a novel functional substrate of POMTs that is required for axon connectivity. This mechanism of POMT-mediated regulation of receptor-type protein tyrosine phosphatase functions could potentially be conserved in mammals and may shed new light on the etiology of neurological defects in muscular dystrophies.

The extracellular matrix and perineuronal nets in memory

All components of the CNS are surrounded by a diffuse extracellular matrix (ECM) containing chondroitin sulphate proteoglycans (CSPGs), heparan sulphate proteoglycans (HSPGs), hyaluronan, various glycoproteins including tenascins and thrombospondin, and many other molecules that are secreted into the ECM and bind to ECM components. In addition, some neurons, particularly inhibitory GABAergic parvalbumin-positive (PV) interneurons, are surrounded by a more condensed cartilage-like ECM called perineuronal nets (PNNs). PNNs surround the soma and proximal dendrites as net-like structures that surround the synapses. Attention has focused on the role of PNNs in the control of plasticity, but it is now clear that PNNs also play an important part in the modulation of memory. In this review we summarize the role of the ECM, particularly the PNNs, in the control of various types of memory and their participation in memory pathology. PNNs are now being considered as a target for the treatment of impaired memory. There are many potential treatment targets in PNNs, mainly through modulation of the sulphation, binding, and production of the various CSPGs that they contain or through digestion of their sulphated glycosaminoglycans.

Human brain sialoglycan ligand for CD33, a microglial inhibitory Siglec implicated in Alzheimer's disease

Alzheimer's disease (AD) is characterized by accumulation of misfolded proteins. Genetic studies implicate microglia, brain-resident phagocytic immune cells, in AD pathogenesis. As positive effectors, microglia clear toxic proteins, whereas as negative effectors, they release proinflammatory mediators. An imbalance of these functions contributes to AD progression. Polymorphisms of human CD33, an inhibitory microglial receptor, are linked to AD susceptibility; higher CD33 expression correlates with increased AD risk. CD33, also called Siglec-3, is a member of the sialic acid-binding immunoglobulin-type lectin (Siglec) family of immune regulatory receptors. Siglec-mediated inhibition is initiated by binding to complementary sialoglycan ligands in the tissue environment. Here, we identify a single sialoglycoprotein in human cerebral cortex that binds CD33 as well as Siglec-8, the most abundant Siglec on human microglia. The ligand, which we term receptor protein tyrosine phosphatase zeta (RPTPζ)S3L, is composed of sialylated keratan sulfate chains carried on a minor isoform/glycoform of RPTPζ (phosphacan) and is found in the extracellular milieu of the human brain parenchyma. Brains from human AD donors had twofold higher levels of RPTPζS3L than age-matched control donors, raising the possibility that RPTPζS3L overexpression limits misfolded protein clearance contributing to AD pathology. Mice express the same structure, a sialylated keratan sulfate RPTPζ isoform, that binds mouse Siglec-F and crossreacts with human CD33 and Siglec-8. Brains from mice engineered to lack RPTPζ, the sialyltransferase St3gal4, or the keratan sulfate sulfotransferase Chst1 lacked Siglec binding, establishing the ligand structure. The unique CD33 and Siglec-8 ligand, RPTPζS3L, may contribute to AD progression.

Structural and Functional Modulation of Perineuronal Nets: In Search of Important Players with Highlight on Tenascins

The extracellular matrix (ECM) of the brain plays a crucial role in providing optimal conditions for neuronal function. Interactions between neurons and a specialized form of ECM, perineuronal nets (PNN), are considered a key mechanism for the regulation of brain plasticity. Such an assembly of interconnected structural and regulatory molecules has a prominent role in the control of synaptic plasticity. In this review, we discuss novel ways of studying the interplay between PNN and its regulatory components, particularly tenascins, in the processes of synaptic plasticity, mechanotransduction, and neurogenesis. Since enhanced neuronal activity promotes PNN degradation, it is possible to study PNN remodeling as a dynamical change in the expression and organization of its constituents that is reflected in its ultrastructure. The discovery of these subtle modifications is enabled by the development of super-resolution microscopy and advanced methods of image analysis.