TMX2 Is a Crucial Regulator of Cellular Redox State, and Its Dysfunction Causes Severe Brain Developmental Abnormalities.

The redox state of the neural progenitors regulates physiological processes such as neuronal differentiation and dendritic and axonal growth. The relevance of endoplasmic reticulum (ER)-associated oxidoreductases in these processes is largely unexplored. We describe a severe neurological disorder caused by bi-allelic loss-of-function variants in thioredoxin (TRX)-related transmembrane-2 (TMX2); these variants were detected by exome sequencing in 14 affected individuals from ten unrelated families presenting with congenital microcephaly, cortical polymicrogyria, and other migration disorders. TMX2 encodes one of the five TMX proteins of the protein disulfide isomerase family, hitherto not linked to human developmental brain disease. Our mechanistic studies on protein function show that TMX2 localizes to the ER mitochondria-associated membranes (MAMs), is involved in posttranslational modification and protein folding, and undergoes physical interaction with the MAM-associated and ER folding chaperone calnexin and ER calcium pump SERCA2. These interactions are functionally relevant because TMX2-deficient fibroblasts show decreased mitochondrial respiratory reserve capacity and compensatory increased glycolytic activity. Intriguingly, under basal conditions TMX2 occurs in both reduced and oxidized monomeric form, while it forms a stable dimer under treatment with hydrogen peroxide, recently recognized as a signaling molecule in neural morphogenesis and axonal pathfinding. Exogenous expression of the pathogenic TMX2 variants or of variants with an in vitro mutagenized TRX domain induces a constitutive TMX2 polymerization, mimicking an increased oxidative state. Altogether these data uncover TMX2 as a sensor in the MAM-regulated redox signaling pathway and identify it as a key adaptive regulator of neuronal proliferation, migration, and organization in the developing brain.

Callosal septa express guidance cues and are paramedian guideposts for human corpus callosum development.

The early development and growth of the corpus callosum are supported by several midline transient structures in mammals that include callosal septa (CS), which are present only in the second half of gestation in humans. Here we provide new data that support the guidance role of CS in corpus callosum development, derived from the analysis of 46 postmortem fetal brains, ranging in age from 16 to 40 post conception weeks (PCW). Using immunohistochemical methods, we show the expression pattern of guidance cues ephrinA4 and neogenin, extracellular protein fibronectin, as well as non-activated microglia in the CS. We found that the dynamic changes in expression of guidance cues, cellular and extracellular matrix constituents in the CS correlate well with the growth course of the corpus callosum at midsagittal level. The CS reach and maintain their developmental maximum between 20 and 26 PCW and can be visualized as hypointense structures in the ventral callosal portion with ex vivo (in vitro) T2-weighted 3T magnetic resonance imaging (MRI). The maximum of septal development overlaps with an increase in the callosal midsagittal area, whereas the slow, gradual resolution of CS coincides with a plateau of midsagittal callosal growth. The recognition of CS existence in human fetal brain and the ability to visualize them by ex vivoMRI attributes a potential diagnostic value to these transient structures, as advancement in imaging technologies will likely also enable in vivoMRI visualization of the CS in the near future.

Neuroplastin deletion in glutamatergic neurons impairs selective brain functions and calcium regulation: implication for cognitive deterioration.

The cell adhesion molecule neuroplastin (Np) is a novel candidate to influence human intelligence. Np-deficient mice display complex cognitive deficits and reduced levels of Plasma Membrane Ca2+ ATPases (PMCAs), an essential regulator of the intracellular Ca2+ concentration ([iCa2+]) and neuronal activity. We show abundant expression and conserved cellular and molecular features of Np in glutamatergic neurons in human hippocampal-cortical pathways as characterized for the rodent brain. In Nptn lox/loxEmx1Cre mice, glutamatergic neuron-selective Np ablation resulted in behavioral deficits indicating hippocampal, striatal, and sensorimotor dysfunction paralleled by highly altered activities in hippocampal CA1 area, sensorimotor cortex layers I-III/IV, and the striatal sensorimotor domain detected by single-photon emission computed tomography. Altered hippocampal and cortical activities correlated with reduction of distinct PMCA paralogs in Nptn lox/loxEmx1Cre mice and increased [iCa2+] in cultured mutant neurons. Human and rodent Np enhanced the post-transcriptional expression of and co-localized with PMCA paralogs in the plasma membrane of transfected cells. Our results indicate Np as essential for PMCA expression in glutamatergic neurons allowing proper [iCa2+] regulation and normal circuit activity. Neuron-type-specific Np ablation empowers the investigation of circuit-coded learning and memory and identification of causal mechanisms leading to cognitive deterioration.