Proteoglycan (Keratan Sulfate) Barrier in Developing Human Forebrain Isolates Cortical Epileptic Networks From Deep Heterotopia, Insulates Axonal Fascicles, and Explains Why Axosomatic Synapses Are Inhibitory

Age-dependent increase of perineuronal nets in the human hippocampus and precocious aging in epilepsy

OBJECTIVE

Perineuronal nets (PNN) are specialized extracellular matrix (ECM) components of the central nervous system, frequently accumulating at the surface of inhibitory GABAergic interneurons. While an altered distribution of PNN has been observed in neurological disorders including Alzheimer's disease, schizophrenia and epilepsy, their anatomical distribution also changes during physiological brain maturation and aging. Such an age-dependent shift was experimentally associated also with hippocampal engram formation during brain maturation. Our aim was to histopathologically assess PNN in the hippocampus of adult and pediatric patients with temporal lobe epilepsy (TLE) compared to age-matched post-mortem control subjects and to compare PNN-related changes with memory impairment observed in our patient cohort.

METHODS

Sixty-six formalin-fixed and paraffin-embedded tissue specimens of the human hippocampus were retrieved from the European Epilepsy Brain Bank. Twenty-nine patients had histopathologically confirmed hippocampal sclerosis (HS), and eleven patients suffered from TLE without HS. PNN were immunohistochemically visualized using an antibody directed against aggrecan and manually counted from hippocampus subfields and the subiculum.

RESULTS

PNN density increased with age in both human controls and TLE patients. However, their density was significantly higher in all HS patients compared to age-matched controls. Intriguingly, TLE patients presented presurgically with better memory when their hippocampal PNN density was higher (p < 0.05).

SIGNIFICANCE

Our results were compatible with age-dependent ECM specialization in the human hippocampus and its precocious aging in the epileptic condition. These observations confirm recent experimental animal models and also support the notion that PNN play a role in memory formation in the human brain.

PLAIN LANGUAGE SUMMARY

"Perineuronal nets" (PNN) are a specialized compartment of the extracellular matrix (ECM), especially surrounding highly active neurons of the mammalian brain. There is evidence that PNN play a role in memory formation, brain maturation, and in some pathologies like Alzheimer's disease, schizophrenia or epilepsy. In this study, we investigated the role of PNN in patients suffering from drug-resistant focal epilepsy compared to controls. We found that with increasing age, more neurons are surrounded by PNN. Similarly, all epilepsy patients but especially patients with better memory performance also had more PNN. This study raises further interest in studying ECM molecules in the human brain under physiological and pathophysiological conditions.

Keratan sulfate proteoglycan: putative template for neuroblast migratory and axonal fascicular pathways and fetal expression in globus pallidus, thalamus, and olfactory bulb

Keratan sulfate (KS) is a proteoglycan secreted in the fetal brain astrocytes and radial glia into extracellular parenchyma as granulofilamentous deposits. KS surrounds neurons except dendritic spines, repelling glutamatergic and facilitating GABAergic axons. The same genes are expressed in both neuroblast migration and axonal growth. This study examines timing of KS during morphogenesis of some normally developing human fetal forebrain structures. Twenty normal human fetal brains from 9-41 weeks gestational age were studied at autopsy. KS was examined by immunoreactivity in formalin-fixed paraffin sections, plus other markers including synaptophysin, S-100β protein, vimentin and nestin. Radial and tangential neuroblast migratory pathways from subventricular zone to cortical plate were marked by KS deposits as early as 9wk GA, shortly after neuroblast migration initiated. During later gestation this reactivity gradually diminished and disappeared by term. Long axonal fascicles of the internal capsule and short fascicles of intrinsic bundles of globus pallidus and corpus striatum also appeared as early as 9-12wk, as fascicular sleeves before axons even entered. Intense KS occurs in astrocytic cytoplasm and extracellular parenchyma at 9wk in globus pallidus, 15wk thalamus, 18wk corpus striatum, 22wk cortical plate, and hippocampus postnatally. Corpus callosum and anterior commissure do not exhibit KS at any age. Optic chiasm shows reactivity at the periphery but not around intrinsic subfasciculi. We postulate that KS forms a chemical template for many long and short axonal fascicles before axons enter and neuroblast migratory pathways at initiation of migration. Cross-immunoreactivity with aggrecan may render difficult molecular distinction.

Gene Expression Studies in Down Syndrome: What Do They Tell Us about Disease Phenotypes?

Down syndrome is a well-studied aneuploidy condition in humans, which is associated with various disease phenotypes including cardiovascular, neurological, haematological and immunological disease processes. This review paper aims to discuss the research conducted on gene expression studies during fetal development. A descriptive review was conducted, encompassing all papers published on the PubMed database between September 1960 and September 2022. We found that in amniotic fluid, certain genes such as COL6A1 and DSCR1 were found to be affected, resulting in phenotypical craniofacial changes. Additionally, other genes such as GSTT1, CLIC6, ITGB2, C21orf67, C21orf86 and RUNX1 were also identified to be affected in the amniotic fluid. In the placenta, dysregulation of genes like MEST, SNF1LK and LOX was observed, which in turn affected nervous system development. In the brain, dysregulation of genes DYRK1A, DNMT3L, DNMT3B, TBX1, olig2 and AQP4 has been shown to contribute to intellectual disability. In the cardiac tissues, dysregulated expression of genes GART, ETS2 and ERG was found to cause abnormalities. Furthermore, dysregulation of XIST, RUNX1, SON, ERG and STAT1 was observed, contributing to myeloproliferative disorders. Understanding the differential expression of genes provides insights into the genetic consequences of DS. A better understanding of these processes could potentially pave the way for the development of genetic and pharmacological therapies.

Neuroembryonic and fetal brain development: Relevance to fetal/neonatal neurological training

Insight into neuroembryology, developmental neuroanatomy and neurophysiology distinguish the diagnostic approaches of paediatric from adult neurologists and general paediatricians. These fundamental disciplines of basic neuroscience could be more effectively taught during paediatric neurology and most residency programmes, that will strengthen career-long learning. Interdisciplinary training of fetal-neonatal neurology within these programs requires working knowledge of neuroembryology applied to maternal reproductive health influencing the maternal-placental-fetal triad, neonate, and young child. Systematic didactic teaching of development in terms of basic neuroscience with neuropathological context would better address needed clinical skill sets to be incorporated into paediatric neurology and neonatology residencies to address brain health and diseases across childhood. Trainees need to recognize the continuity of development, established by maternal reproductive health before conception with gene -environment influences over the first 1000 days. Considerations of neuroembryology that explain earlier brain development during the first half of pregnancy enhances an understanding of effects throughout gestation through parturition and into neonatal life. Neonatal EEG training enhances these clinical descriptions by applying serial EEG-state analyses of premature neonates through early childhood to recognize evolving patterns associated with neuronal maturation and synaptogenesis. Neuroimaging studies offer comparisons of normal structural images with malformations and destructive lesions to correlate with clinical and neurophysiological findings. This analysis better assesses aberrant developmental processes in the context of neuroembryology. Time-specific developmental events and semantic precision are important for accurate phenotypic descriptions for a better understanding of etiopathogenesis with maturation. Certification of paediatric neurology training programme curricula should apply practical knowledge of basic neuroscience in the context of nervous system development and maturation from conception through postnatal time periods. Interdisciplinary fetal-neonatal neurology training constitutes an important educational component for career-long learning.

Maturation of metastases in peripheral neuroblastic tumors (neuroblastoma) of children

Peripheral neuroblastic tumors of childhood exhibit 3 principal neural crest lineages: primitive neuroblastoma, ganglioneuroblastoma, and ganglioneuroma. They are unique in undergoing maturation of neurons (ganglion cells) and Schwann cells, thereby recapitulating normal fetal neuronal development in the brain. Precision in estimating neurogenesis is enhanced by immunoreactivities of markers of neuronal maturation. Whether organ tissue factors in different sites of metastases influence rates of maturation and whether metastases are similar to their primary neuroblastic tumor are incompletely documented. Four young children, 1 with a mixed primary adrenal tumor and 3 with metastases were studied at surgery or autopsy. Immunocytochemical reactivities included microtubule-associated protein-2, synaptophysin, chromogranin-A, somatostatin, keratan sulfate, vimentin, S-100β protein, and PHOX2B. Primary tumors were non-uniform with regions of either poor or enhanced maturation. Both neuronal and Schwannian lineages were represented in each tumor type but differed in proportions. Bi- or multi-nucleated ganglion cells matured equal to mononuclear forms. Ganglion cell maturation was similar in metastases regardless of the target organ. Metastases resembled primary tumors. Immunocytochemical markers of neuronal and of Schwann cell maturation provide greater diagnostic precision to supplement histological criteria. Interval between diagnosis of primary tumor and metastases, metastatic target tissues, and chemotherapy over an interval of time do not appear to influence neuroblastic or Schwann cell differentiation.

Sequences of synaptogenesis in the human fetal and neonatal brain by synaptophysin immunocytochemistry

Synaptogenesis is the final phase of axonal pathfinding. Its sequences of spatial and temporal development in the immature nervous system are precisely timed and consistent. Synaptophysin, a principal structural glycoprotein of synaptic vesicle membranes regardless of the chemical transmitter substance within, is a reliable means of demonstrating sequences of synaptogenesis in human fetal brain tissue at autopsy and is resistant to postmortem autolysis. Furthermore, synaptophysin molecules are demonstrated during axoplasmic flow before being assembled into membranes in immature axons and also mature axons of neurons with a high metabolic rate. In brain malformations these sequences often are altered both in distribution of synapses and in timing, often delayed but sometimes precocious, with postnatal clinical manifestations such as epilepsy and cognitive development.

Beta3Gn-T7 Is a Keratan Sulfate β1,3 N-Acetylglucosaminyltransferase in the Adult Brain

Keratan sulfate (KS) glycan is covalently attached to a core protein of proteoglycans. KS is abundant in neuropils and presents densely in close proximity to the perineuronal region of the perineuronal net-positive neurons in the adult brain under physiological conditions. We previously showed that the synthesis of KS positive for the R-10G antibody in the adult brain is mediated by GlcNAc-6-sulfotransferase 3 (GlcNAc6ST3; encoded by Chst5). Deficiency in both GlcNAc6ST3 and GlcNAc6ST1, encoded by Chst2, completely abolished KS. Protein-tyrosine phosphatase receptor type z1 (Ptprz1)/phosphacan was identified as a KS scaffold. KS requires the extension of GlcNAc by β1,3 N-acetylglucosaminyltransferase (Beta3Gn-T). Members of the Beta3Gn-T family involved in the synthesis of adult brain KS have not been identified. In this study, we show by a method of gene targeting that Beta3Gn-T7, encoded by B3gnt7, is a major Beta3Gn-T for the synthesis of KS in neuropils and the perineuronal region in the adult brain. Intriguingly, the B3gnt7 gene is selectively expressed in oligodendrocyte precursor cells (OPCs) and oligodendrocytes similar to that of GlcNAc6ST3. These results indicate that Beta3Gn-T7 in oligodendrocyte lineage cells may play a role in the formation of neuropils and perineuronal nets in the adult brain through the synthesis of R-10G-positive KS-modified proteoglycan.

Semaphorin-Plexin Signaling: From Axonal Guidance to a New X-Linked Intellectual Disability Syndrome

BACKGROUND

Semaphorins and plexins are ligands and cell surface receptors that regulate multiple neurodevelopmental processes such as axonal growth and guidance. PLXNA3 is a plexin gene located on the X chromosome that encodes the most widely expressed plexin receptor in fetal brain, plexin-A3. Plexin-A3 knockout mice demonstrate its role in semaphorin signaling in vivo. The clinical manifestations of semaphorin/plexin neurodevelopmental disorders have been less widely explored. This study describes the neurological and neurodevelopmental phenotypes of boys with maternally inherited hemizygous PLXNA3 variants.

METHODS

Data-sharing through GeneDx and GeneMatcher allowed identification of individuals with autism or intellectual disabilities (autism/ID) and hemizygous PLXNA3 variants in collaboration with their physicians and genetic counselors, who completed questionnaires about their patients. In silico analyses predicted pathogenicity for each PLXNA3 variant.

RESULTS

We assessed 14 boys (mean age, 10.7 [range 2 to 25] years) with maternally inherited hemizygous PLXNA3 variants and autism/ID ranging from mild to severe. Other findings included fine motor dyspraxia (92%), attention-deficit/hyperactivity traits, and aggressive behaviors (63%). Six patients (43%) had seizures. Thirteen boys (93%) with PLXNA3 variants showed novel or very low allele frequencies and probable damaging/disease-causing pathogenicity in one or more predictors. We found a genotype-phenotype correlation between PLXNA3 cytoplasmic domain variants (exons 22 to 32) and more severe neurodevelopmental disorder phenotypes (P < 0.05).

CONCLUSIONS

We report 14 boys with maternally inherited, hemizygous PLXNA3 variants and a range of neurodevelopmental disorders suggesting a novel X-linked intellectual disability syndrome. Greater understanding of PLXNA3 variant pathogenicity in humans will require additional clinical, computational, and experimental validation.

Focal cortical dysplasia type 1

The ILAE classification of Focal Cortical Dysplasia (FCD) from 2011 has quickly gained acceptance in clinical practice and research and is now widely used around the world. This histopathology‐based classification scheme proposed three subtypes, that is, FCD Type 1 (with architectural abnormalities of the neocortex), FCD Type 2 (with cytoarchitectural abnormalities of the neocortex) and FCD Type 3 (architectural abnormalities of the neocortex associated with another principle lesion acquired during early life). Valuable knowledge was gathered during the last decade validating the clinical, pathological and genetic classification of FCD Type 2. This is in contrast to FCD subtype 1 and 3 with only few robust or new insights. Herein, we provide an overview about current knowledge about FCD Type 1 and its three subtypes. Available data strengthened, however, FCD Type 1A in particular, whereas a comprehensive clinico‐pathological specification for FCD Type 1B and 1C subtypes remain to be shown. The lack of a valid animal model for FCD Type 1 further supports our call and the ongoing need for systematic research studies based on a careful clinico‐pathological and genetic stratification of patients and human brain tissues.

Excitatory/Inhibitory Synaptic Ratios in Polymicrogyria and Down Syndrome Help Explain Epileptogenesis in Malformations

BACKGROUND

The ratio between excitatory (glutamatergic) and inhibitory (GABAergic) inputs into maturing individual cortical neurons influences their epileptic potential. Structural factors during development that alter synaptic inputs can be demonstrated neuropathologically. Increased mitochondrial activity identifies neurons with excessive discharge rates.

METHODS

This study focuses on the neuropathological examinaion of surgical resections for epilepsy and at autopsy, in fetuses, infants, and children, using immunocytochemical markers, and electron microscopy in selected cases. Polymicrogyria and Down syndrome are highlighted.

RESULTS

Factors influencing afferent synaptic ratios include the following: (1) synaptic short-circuitry in fused molecular zones of adjacent gyri (polymicrogyria); (2) impaired development of dendritic spines decreasing excitation (Down syndrome); (3) extracellular keratan sulfate proteoglycan binding to somatic membranes but not dendritic spines may be focally diminished (cerebral atrophy, schizencephaly, lissencephaly, polymicrogyria) or augmented, ensheathing individual axons (holoprosencephaly), or acting as a barrier to axonal passage in the U-fiber layer. If keratan is diminished, glutamate receptors on the neuronal soma enable ectopic axosomatic excitatory synapses to form; (4) dysplastic, megalocytic neurons and balloon cells in mammalian target of rapamycin disorders; (5) satellitosis of glial cells displacing axosomatic synapses; (6) peri-neuronal inflammation (tuberous sclerosis) and heat-shock proteins.

CONCLUSIONS

Synaptic ratio of excitatory/inhibitory afferents is a major fundamental basis of epileptogenesis at the neuronal level. Neuropathology can demonstrate subcellular changes that help explain either epilepsy or lack of seizures in immature brains. Synaptic ratios in malformations influence postnatal epileptogenesis. Single neurons can be hypermetabolic and potentially epileptogenic.