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

Heterogeneity of brain extracellular matrix and astrocyte activation

From the blood brain barrier to the synaptic space, astrocytes provide structural, metabolic, ionic, and extracellular matrix (ECM) support across the brain. Astrocytes include a vast array of subtypes, their phenotypes and functions varying both regionally and temporally. Astrocytes' metabolic and regulatory functions poise them to be quick and sensitive responders to injury and disease in the brain as revealed by single cell sequencing. Far less is known about the influence of the local healthy and aging microenvironments on these astrocyte activation states. In this forward-looking review, we describe the known relationship between astrocytes and their local microenvironment, the remodeling of the microenvironment during disease and injury, and postulate how they may drive astrocyte activation. We suggest technology development to better understand the dynamic diversity of astrocyte activation states, and how basal and activation states depend on the ECM microenvironment. A deeper understanding of astrocyte response to stimuli in ECM-specific contexts (brain region, age, and sex of individual), paves the way to revolutionize how the field considers astrocyte-ECM interactions in brain injury and disease and opens routes to return astrocytes to a healthy quiescent state.

How the forebrain transitions to adulthood: developmental plasticity markers in a long-lived rodent reveal region diversity and the uniqueness of adolescence

Maturation of the forebrain involves transitions from higher to lower levels of synaptic plasticity. The timecourse of these changes likely differs between regions, with the stabilization of some networks scaffolding the development of others. To gain better insight into neuroplasticity changes associated with maturation to adulthood, we examined the distribution of two molecular markers for developmental plasticity. We conducted the examination on male and female degus (Octodon degus), a rodent species with a relatively long developmental timecourse that offers a promising model for studying both development and age-related neuropathology. Immunofluorescent staining was used to measure perineuronal nets (PNNs), an extracellular matrix structure that emerges during the closure of critical plasticity periods, as well as microglia, resident immune cells that play a crucial role in synapse remodeling during development. PNNs (putatively restricting plasticity) were found to be higher in non-juvenile (>3 month) degus, while levels of microglia (putatively mediating plasticity) decreased across ages more gradually, and with varying timecourses between regions. Degus also showed notable variation in PNN levels between cortical layers and hippocampal subdivisions that have not been previously reported in other species. These results offer a glimpse into neuroplasticity changes occurring during degu maturation and highlight adolescence as a unique phase of neuroplasticity, in which PNNs have been established but microglia remain relatively high.

From molecules to behavior: Implications for perineuronal net remodeling in learning and memory

Perineuronal nets (PNNs) are condensed extracellular matrix (ECM) structures found throughout the central nervous system that regulate plasticity. They consist of a heterogeneous mix of ECM components that form lattice-like structures enwrapping the cell body and proximal dendrites of particular neurons. During development, accumulating research has shown that the closure of various critical periods of plasticity is strongly linked to experience-driven PNN formation and maturation. PNNs provide an interface for synaptic contacts within the holes of the structure, generally promoting synaptic stabilization and restricting the formation of new synaptic connections in the adult brain. In this way, they impact both synaptic structure and function, ultimately influencing higher cognitive processes. PNNs are highly plastic structures, changing their composition and distribution throughout life and in response to various experiences and memory disorders, thus serving as a substrate for experience- and disease-dependent cognitive function. In this review, we delve into the proposed mechanisms by which PNNs shape plasticity and memory function, highlighting the potential impact of their structural components, overall architecture, and dynamic remodeling on functional outcomes in health and disease.

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.

Dynamic regulation of the extracellular matrix in reward memory processes: a question of time

Substance use disorders are a global health problem with increasing prevalence resulting in significant socioeconomic burden and increased mortality. Converging lines of evidence point to a critical role of brain extracellular matrix (ECM) molecules in the pathophysiology of substance use disorders. An increasing number of preclinical studies highlight the ECM as a promising target for development of novel cessation pharmacotherapies. The brain ECM is dynamically regulated during learning and memory processes, thus the time course of ECM alterations in substance use disorders is a critical factor that may impact interpretation of the current studies and development of pharmacological therapies. This review highlights the evidence for the involvement of ECM molecules in reward learning, including drug reward and natural reward such as food, as well as evidence regarding the pathophysiological state of the brain's ECM in substance use disorders and metabolic disorders. We focus on the information regarding time-course and substance specific changes in ECM molecules and how this information can be leveraged for the development of therapeutic strategies.

Reduced expression of perineuronal nets in the normotopic somatosensory cortex of the tish rat

The tish (telencephalic internal structural heterotopia) rat is a naturally occurring and unique model of a malformation of cortical development (MCD) arising from a sponeantous mutation in the Eml1 gene. Tish rats are characterized by a macroscopic bilateral heterotopic dysplastic cortex (HDCx) and an overlaying, intact normotopic neocortex (NNCx). These two cortices are functional and have been reported to innervate and establish connections with subcortical regions including the thalamus, resulting in a dual-cortical representation. Additionally, impaired GABAergic neurotransmission and early-onset spike wave discharge bursts have been reported in developing tish rats. Perineuronal nets (PNNs) are specialized extraceullar matrix structures that predominately surround and stabilize parvalbumin-positive (PV⁺) GABAergic interneurons and are essential components of the neural landscape. Here, we report a significant reduction in the average number of WFA⁺-PNNs in the normotopic somatosensory cortex (NSSCx) of the tish rat at two developmental time points, P16 and P35, corresponding to a decrease in the number of PV⁺ interneurons ensheathed by a PNN in the NSSCx. Compared with control animals, PNN expression was partially, but significantly restored following treatment with insulin-like growth factor 1 (IGF-1). These data suggest that the 'dual cortical representation' in the setting of an MCD reduces the cortical activation necessary for proper PNN expression likely contributing to the impairments in GABAergic neurotransmission and network excitability previously identified in the tish rat.

Perineuronal nets affect memory and learning after synapse withdrawal

Perineuronal nets (PNNs) enwrap mature neurons, playing a role in the control of plasticity and synapse dynamics. PNNs have been shown to have effects on memory formation, retention and extinction in a variety of animal models. It has been proposed that the cavities in PNNs, which contain synapses, can act as a memory store and that they remain stable after events that cause synaptic withdrawal such as anoxia or hibernation. We examine this idea by monitoring place memory before and after synaptic withdrawal caused by acute hibernation-like state (HLS). Animals lacking hippocampal PNNs due to enzymatic digestion by chondroitinase ABC or knockout of the PNN component aggrecan were compared with wild type controls. HLS-induced synapse withdrawal caused a memory deficit, but not to the level of untreated naïve animals and not worsened by PNN attenuation. After HLS, only animals lacking PNNs showed memory restoration or relearning. Absence of PNNs affected the restoration of excitatory synapses on PNN-bearing neurons. The results support a role for hippocampal PNNs in learning, but not in long-term memory storage for correction of deficits.

Microglia enable mature perineuronal nets disassembly upon anesthetic ketamine exposure or 60-Hz light entrainment in the healthy brain

Perineuronal nets (PNNs), components of the extracellular matrix, preferentially coat parvalbumin-positive interneurons and constrain critical-period plasticity in the adult cerebral cortex. Current strategies to remove PNN are long-lasting, invasive, and trigger neuropsychiatric symptoms. Here, we apply repeated anesthetic ketamine as a method with minimal behavioral effect. We find that this paradigm strongly reduces PNN coating in the healthy adult brain and promotes juvenile-like plasticity. Microglia are critically involved in PNN loss because they engage with parvalbumin-positive neurons in their defined cortical layer. We identify external 60-Hz light-flickering entrainment to recapitulate microglia-mediated PNN removal. Importantly, 40-Hz frequency, which is known to remove amyloid plaques, does not induce PNN loss, suggesting microglia might functionally tune to distinct brain frequencies. Thus, our 60-Hz light-entrainment strategy provides an alternative form of PNN intervention in the healthy adult brain.

Neuron and astrocyte aggregation and sorting in three-dimensional neuronal constructs

Aggregation and self-sorting of cells in three dimensional cultures have been described for non-neuronal cells. Despite increased interest in engineered neural tissues for treating brain injury or for modeling neurological disorders in vitro, little data is available on collective cell movements in neuronal aggregates. Migration and sorting of cells may alter these constructs' morphology and, therefore, the function of their neural circuitry. In this work, linear, adhered rat and human 3D neuronal-astrocyte cultures were developed to enable the study of aggregation and sorting of these cells. An in silico model of the contraction, clustering, and cell sorting in the 3D cultures was also developed. Experiments and computational modeling showed that aggregation was mainly a neuron mediated process, and formation of astrocyte-rich sheaths in 3D cultures depended on differential attraction between neurons and astrocytes. In silico model predicted formation of self-assembled neuronal layers in disk-shaped 3D cultures. Neuronal activity patterns were found to correlate with local morphological differences. This model of neuronal and astrocyte aggregation and sorting may benefit future design of neuronal constructs.

Molecular signature of extracellular matrix pathology in schizophrenia

Growing evidence points to a critical involvement of the extracellular matrix (ECM) in the pathophysiology of schizophrenia (SZ). Decreases of perineuronal nets (PNNs) and altered expression of chondroitin sulphate proteoglycans (CSPGs) in glial cells have been identified in several brain regions. GWAS data have identified several SZ vulnerability variants of genes encoding for ECM molecules. Given the potential relevance of ECM functions to the pathophysiology of this disorder, it is necessary to understand the extent of ECM changes across brain regions, their region‐ and sex‐specificity and which ECM components contribute to these changes. We tested the hypothesis that the expression of genes encoding for ECM molecules may be broadly disrupted in SZ across several cortical and subcortical brain regions and include key ECM components as well as factors such as ECM posttranslational modifications and regulator factors. Gene expression profiling of 14 neocortical brain regions, caudate, putamen and hippocampus from control subjects (n = 14/region) and subjects with SZ (n = 16/region) was conducted using Affymetrix microarray analysis. Analysis across brain regions revealed widespread dysregulation of ECM gene expression in cortical and subcortical brain regions in SZ, impacting several ECM functional key components. SRGN, CD44, ADAMTS1, ADAM10, BCAN, NCAN and SEMA4G showed some of the most robust changes. Region‐, sex‐ and age‐specific gene expression patterns and correlation with cognitive scores were also detected. Taken together, these findings contribute to emerging evidence for large‐scale ECM dysregulation in SZ and point to molecular pathways involved in PNN decreases, glial cell dysfunction and cognitive impairment in SZ.

BDNF and JNK Signaling Modulate Cortical Interneuron and Perineuronal Net Development: Implications for Schizophrenia-Linked 16p11.2 Duplication Syndrome

Schizophrenia (SZ) is a neurodevelopmental disorder caused by the interaction of genetic and environmental risk factors. One of the strongest genetic risk variants is duplication (DUP) of chr.16p11.2. SZ is characterized by cortical gamma-amino-butyric acid (GABA)ergic interneuron dysfunction and disruption to surrounding extracellular matrix structures, perineuronal nets (PNNs). Developmental maturation of GABAergic interneurons, and also the resulting closure of the critical period of cortical plasticity, is regulated by brain-derived neurotrophic factor (BDNF), although the mechanisms involved are unknown. Here, we show that BDNF promotes GABAergic interneuron and PNN maturation through JNK signaling. In mice reproducing the 16p11.2 DUP, where the JNK upstream activator Taok2 is overexpressed, we find that JNK is overactive and there are developmental abnormalities in PNNs, which persist into adulthood. Prefrontal cortex parvalbumin (PVB) expression is reduced, while PNN intensity is increased. Additionally, we report a unique role for TAOK2 signaling in the regulation of PVB interneurons. Our work implicates TAOK2-JNK signaling in cortical interneuron and PNN development, and in the responses to BDNF. It also demonstrates that over-activation of this pathway in conditions associated with SZ risk causes long-lasting disruption in cortical interneurons.

Comparative Analysis of Brain Stiffness Among Amniotes Using Glyoxal Fixation and Atomic Force Microscopy

Brain structures are diverse among species despite the essential molecular machinery of neurogenesis being common. Recent studies have indicated that differences in the mechanical properties of tissue may result in the dynamic deformation of brain structure, such as folding. However, little is known about the correlation between mechanical properties and species-specific brain structures. To address this point, a comparative analysis of mechanical properties using several animals is required. For a systematic measurement of the brain stiffness of remotely maintained animals, we developed a novel strategy of tissue-stiffness measurement using glyoxal as a fixative combined with atomic force microscopy. A comparison of embryonic and juvenile mouse and songbird brain tissue revealed that glyoxal fixation can maintain brain structure as well as paraformaldehyde (PFA) fixation. Notably, brain tissue fixed by glyoxal remained much softer than PFA-fixed brains, and it can maintain the relative stiffness profiles of various brain regions. Based on this method, we found that the homologous brain regions between mice and songbirds exhibited different stiffness patterns. We also measured brain stiffness in other amniotes (chick, turtle, and ferret) following glyoxal fixation. We found stage-dependent and species-specific stiffness in pallia among amniotes. The embryonic chick and matured turtle pallia showed gradually increasing stiffness along the apico-basal tissue axis, the lowest region at the most apical region, while the ferret pallium exhibited a catenary pattern, that is, higher in the ventricular zone, the inner subventricular zone, and the cortical plate and the lowest in the outer subventricular zone. These results indicate that species-specific microenvironments with distinct mechanical properties emerging during development might contribute to the formation of brain structures with unique morphology.

Diverse Roles for Hyaluronan and Hyaluronan Receptors in the Developing and Adult Nervous System

Hyaluronic acid (HA) plays a vital role in the extracellular matrix of neural tissues. Originally thought to hydrate tissues and provide mechanical support, it is now clear that HA is also a complex signaling molecule that can regulate cell processes in the developing and adult nervous systems. Signaling properties are determined by molecular weight, bound proteins, and signal transduction through specific receptors. HA signaling regulates processes such as proliferation, differentiation, migration, and process extension in a variety of cell types including neural stem cells, neurons, astrocytes, microglia, and oligodendrocyte progenitors. The synthesis and catabolism of HA and the expression of HA receptors are altered in disease and influence neuroinflammation and disease pathogenesis. This review discusses the roles of HA, its synthesis and breakdown, as well as receptor expression in neurodevelopment, nervous system function and disease.

Lateralized Expression of Cortical Perineuronal Nets during Maternal Experience is Dependent on MECP2

Cortical neuronal circuits along the sensorimotor pathways are shaped by experience during critical periods of heightened plasticity in early postnatal development. After closure of critical periods, measured histologically by the formation and maintenance of extracellular matrix structures called perineuronal nets (PNNs), the adult mouse brain exhibits restricted plasticity and maturity. Mature PNNs are typically considered to be stable structures that restrict synaptic plasticity on cortical parvalbumin+ (PV+) GABAergic neurons. Changes in environment (i.e., novel behavioral training) or social contexts (i.e., motherhood) are known to elicit synaptic plasticity in relevant neural circuitry. However, little is known about concomitant changes in the PNNs surrounding the cortical PV+ GABAergic neurons. Here, we show novel changes in PNN density in the primary somatosensory cortex (SS1) of adult female mice after maternal experience [called surrogate (Sur)], using systematic microscopy analysis of a whole brain region. On average, PNNs were increased in the right barrel field and decreased in the left forelimb regions. Individual mice had left hemisphere dominance in PNN density. Using adult female mice deficient in methyl-CpG-binding protein 2 (MECP2), an epigenetic regulator involved in regulating experience-dependent plasticity, we found that MECP2 is critical for this precise and dynamic expression of PNN. Adult naive Mecp2-heterozygous (Het) females had increased PNN density in specific subregions in both hemispheres before maternal experience, compared with wild-type (WT) littermate controls. The laterality in PNN expression seen in naive Het (NH) was lost after maternal experience in Sur Het (SH) mice, suggesting possible intact mechanisms for plasticity. Together, our results identify subregion and hemisphere-specific alterations in PNN expression in adult females, suggesting extracellular matrix plasticity as a possible neurobiological mechanism for adult behaviors in rodents.

Neuronal Pentraxin 2 Binds PNNs and Enhances PNN Formation

The perineuronal net (PNN) is a mesh-like proteoglycan structure on the neuronal surface which is involved in regulating plasticity. The PNN regulates plasticity via multiple pathways, one of which is direct regulation of synapses through the control of AMPA receptor mobility. Since neuronal pentraxin 2 (Nptx2) is a known regulator of AMPA receptor mobility and Nptx2 can be removed from the neuronal surface by PNN removal, we investigated whether Nptx2 has a function in the PNN. We found that Nptx2 binds to the glycosaminoglycans hyaluronan and chondroitin sulphate E in the PNN. Furthermore, in primary cortical neuron cultures, the addition of NPTX2 to the culture medium enhances PNN formation during PNN development. These findings suggest Nptx2 as a novel PNN binding protein with a role in the mechanism of PNN formation.

Enzymatic Digestion of Hyaluronan-Based Brain Extracellular Matrix in vivo Can Induce Seizures in Neonatal Mice

It is well-known that hyaluronic acid (HA) as a component of brain extracellular matrix (ECM) plays a pivotal role in the nervous system and is involved in synaptic plasticity changes in vascular cognitive impairment and dementia. HA breakdown is a feature of the acute stage of stroke injury and may be detrimental through enhancement of the inflammatory response. Recent studies have shown that knockout mice lacking hyaluronic acid synthetase demonstrates epileptic phenotype in vivo and removal of HA leads to delayed development of epileptiform activity in cultured hippocampal neurons in vitro. Here, we studied whether digestion of hyaluronic acid in the hippocampus in early postnatal period can trigger seizures. Hyaluronidase (Hyal) (5 U/μl) was bilaterally injected into C57BL/6j mice (P17) CA1 field of hippocampus using the stereotaxic method to remove hyaluronan-based ECM. Transcriptome analysis of hippocampal tissue 2 h after enzymatic digestion of hyaluronan-based brain ECM revealed increased gene expression of proteins involved in inflammation reactions (TLR2, CCL2,3,5), neuroinflammation, axonal guidance and ephrin receptor signaling, versus the vehicle group. Mice injected with hyaluronidase exhibited delayed audiogenic seizures and improvement in working memory 72 h after injection, while there were no changes in locomotor activity, anxious level and exploratory behavior due to the open field test. The obtained results point to a link between the activation of neuroinflammation by enzymatic digestion of hyaluronan-based brain ECM during the neonatal period and their subsequent reactivity to seizures, which may play an important role in the functional features of the developing brain, including its seizure propensity.