Somatic mosaicism of the PI3K-AKT-MTOR pathway is associated with hemimegalencephaly in fetal brains.

It is known that somatic activation of PI3K-AKT-MTOR signaling causes malformations of cortical development varying from hemimegalencephaly to focal cortical dysplasia. However, there have been few reports of fetal cases. Here we report two fetal cases of hemimegalencephaly, one associated with mosaic mutations in PIK3CA and another in AKT1. Both brains showed polymicrogyria, multiple subarachnoidal, subcortical, and subventricular heterotopia resulting from abnormal proliferation of neural stem/progenitor cells, cell differentiation, and migration of neuroblasts. Scattered cell nests immunoreactive for phosphorylated-S6 ribosomal protein (P-RPS6) (Ser240/244) were observed in the polymicrogyria-like cortical plate, intermediate zone, and arachnoid space, suggesting that the PI3K-AKT-MTOR pathway was actually activated in these cells. Pathological analyses could shed light on the mechanisms involved in disrupted brain development in the somatic mosaicism of the PI3K-AKT-MTOR pathway.

Impaired catabolism of free oligosaccharides due to MAN2C1 variants causes a neurodevelopmental disorder.

Free oligosaccharides (fOSs) are soluble oligosaccharide species generated during N-glycosylation of proteins. Although little is known about fOS metabolism, the recent identification of NGLY1 deficiency, a congenital disorder of deglycosylation (CDDG) caused by loss of function of an enzyme involved in fOS metabolism, has elicited increased interest in fOS processing. The catabolism of fOSs has been linked to the activity of a specific cytosolic mannosidase, MAN2C1, which cleaves α1,2-, α1,3-, and α1,6-mannose residues. In this study, we report the clinical, biochemical, and molecular features of six individuals, including two fetuses, with bi-allelic pathogenic variants in MAN2C1; the individuals are from four different families. These individuals exhibit dysmorphic facial features, congenital anomalies such as tongue hamartoma, variable degrees of intellectual disability, and brain anomalies including polymicrogyria, interhemispheric cysts, hypothalamic hamartoma, callosal anomalies, and hypoplasia of brainstem and cerebellar vermis. Complementation experiments with isogenic MAN2C1-KO HAP1 cells confirm the pathogenicity of three of the identified MAN2C1 variants. We further demonstrate that MAN2C1 variants lead to accumulation and delay in the processing of fOSs in proband-derived cells. These results emphasize the involvement of MAN2C1 in human neurodevelopmental disease and the importance of fOS catabolism.

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.

The polymicrogyria-associated GPR56 promoter preferentially drives gene expression in developing GABAergic neurons in common marmosets.

GPR56, a member of the adhesion G protein-coupled receptor family, is abundantly expressed in cells of the developing cerebral cortex, including neural progenitor cells and developing neurons. The human GPR56 gene has multiple presumptive promoters that drive the expression of the GPR56 protein in distinct patterns. Similar to coding mutations of the human GPR56 gene that may cause GPR56 dysfunction, a 15-bp homozygous deletion in the cis-regulatory element upstream of the noncoding exon 1 of GPR56 (e1m) leads to the cerebral cortex malformation and epilepsy. To clarify the expression profile of the e1m promoter-driven GPR56 in primate brain, we generated a transgenic marmoset line in which EGFP is expressed under the control of the human minimal e1m promoter. In contrast to the endogenous GPR56 protein, which is highly enriched in the ventricular zone of the cerebral cortex, EGFP is mostly expressed in developing neurons in the transgenic fetal brain. Furthermore, EGFP is predominantly expressed in GABAergic neurons, whereas the total GPR56 protein is evenly expressed in both GABAergic and glutamatergic neurons, suggesting the GABAergic neuron-preferential activity of the minimal e1m promoter. These results indicate a possible pathogenic role for GABAergic neuron in the cerebral cortex of patients with GPR56 mutations.

The pathology of incipient polymicrogyria.

OBJECTIVE: To characterise the early tissue changes of post encephaloclastic polymicrogyria in the human fetus. METHODS: We identified and reviewed the clinical histories and autopsy pathology of post ischemic fetal cerebral cortical injury at less than 30weeks gestational age (GA). The histology of local cortical abnormalities was examined with neuronal, glial, microglial and vascular immunohistochemical markers. RESULTS: We identified eight cases ranging from 18 to 29weeks GA: 5 cases show full thickness cortical infarcts and 3 show periSylvian post-ischemic necrosis of the cerebral cortex. The maximal age is less than 10weeks after injury. There are abnormalities in gross fissuration as early as one month after injury. Disruption of the pia limitans was associated with a microglial and glial response and full thickness cortical injury. Macrophages were often seen accumulating deep to abnormal cortex. Hyperplasia of the subpial granular cell layer was universal in perilesional cortex. Cajal Retzius neuron hyperplasia, aggregation, and both superficial and deep displacement were noted. Where there was loss and dispersal of early cortical pyramidal neurons there was usually no pseudolaminar necrosis. Radial glia by 18weeks GA showed altered growth patterns and lateral branching. Altered migration of primitive elements was often prominent. Particularly prior to 20weeks GA subadjacent subplate neurons showed striking hypertrophy. CONCLUSIONS: The array of histological changes encompasses all tissue elements of the affected brains, early in the evolution polymicrogyria. Although subpial alterations were ubiquitous, not all changes are referable to alterations in the pia limitans. The role of the necroinflammatory response in the genesis of abnormal cytoarchitecture deserves further study.

Rescue of neurodegeneration in the Fig4 null mouse by a catalytically inactive FIG4 transgene.

The lipid phosphatase FIG4 is a subunit of the protein complex that regulates biosynthesis of the signaling lipid PI(3,5)P2. Mutations of FIG4 result in juvenile lethality and spongiform neurodegeneration in the mouse, and are responsible for the human disorders Charcot-Marie-Tooth disease, Yunis-Varon syndrome and polymicrogyria with seizures. We previously demonstrated that conditional expression of a wild-type FIG4 transgene in neurons is sufficient to rescue most of the abnormalities of Fig4 null mice, including juvenile lethality and extensive neurodegeneration. To evaluate the contribution of the phosphatase activity to the in vivo function of Fig4, we introduced the mutation p.Cys486Ser into the Sac phosphatase active-site motif CX5RT. Transfection of the Fig4(Cys486Ser) cDNA into cultured Fig4(-/-) fibroblasts was effective in preventing vacuolization. The neuronal expression of an NSE-Fig4(Cys486Ser) transgene in vivo prevented the neonatal neurodegeneration and juvenile lethality seen in Fig4 null mice. These observations demonstrate that the catalytically inactive FIG4 protein provides significant function, possibly by stabilization of the PI(3,5)P2 biosynthetic complex and/or localization of the complex to endolysosomal vesicles. Despite this partial rescue, later in life the NSE-Fig4(Cys486Ser) transgenic mice display significant abnormalities that include hydrocephalus, defective myelination and reduced lifespan. The late onset phenotype of the NSE-Fig4(Cys486Ser) transgenic mice demonstrates that the phosphatase activity of FIG4 has an essential role in vivo.

Increased expression of GAP43 in interneurons in a rat model of experimental polymicrogyria.

To investigate seizure susceptibility in polymicrogyria, the seizure threshold and growth-associated protein GAP43 expression were analyzed in a rat experimental model of polymicrogyria induced by intracerebral injection of ibotenate. A total of 72 neonates from 9 pregnant rats were used. Intraperitoneal pentylenetetrazole injection did not induce any seizure activity in the control rats, although it elicited seizures of variable severity in the polymicrogyria rats. Fluoro-Jade B-positive degenerating interneurons were found in the polymicrogyria brains; however, no such neurons were detected in the control brains. In the polymicrogyria rats, the GAP43 expression was significantly and widely distributed in the brain, and the percentage of parvalbumin-positive interneurons in the GAP43-positive cells was significantly higher than that observed in the nonphosphorylated neurofilament-positive pyramidal cells. We conclude that the relatively selective vulnerability of inhibitory interneurons constitutes the basis for the decreased seizure threshold observed in this model of polymicrogyria.