Identification of DCX gene mutation in lissencephaly spectrum with subcortical band heterotopia using whole exome sequencing

Experimental models of human cortical malformations: from mammals to 'acortical' zebrafish

Human neocortex controls and integrates cognition, emotions, perception and complex behaviors. Aberrant cortical development can be triggered by multiple genetic and environmental factors, causing cortical malformations. Animal models, especially rodents, are a valuable tool to probe molecular and physiological mechanisms of cortical malformations. Complementing rodent studies, the zebrafish (Danio rerio) is an important model organism in biomedicine. Although the zebrafish (like other fishes) lacks neocortex, here we argue that this species can still be used to model various aspects and brain phenomena related to human cortical malformations. We also discuss novel perspectives in this field, covering both advantages and limitations of using mammalian and zebrafish models in cortical malformation research. Summarizing mounting evidence, we also highlight the importance of translationally-relevant insights into the pathogenesis of cortical malformations from animal models, and discuss future strategies of research in the field.

X-linked neuronal migration disorders: Gender differences and insights for genetic screening

Cortical development depends on neuronal migration of both excitatory and inhibitory interneurons. Neuronal migration disorders (NMDs) are conditions characterised by anatomical cortical defects leading to varying degrees of neurocognitive impairment, developmental delay and seizures. Refractory epilepsy affects 15 million people worldwide, and it is thought that cortical developmental disorders are responsible for 25% of childhood cases. However, little is known about the epidemiology of these disorders, nor are their aetiologies fully understood, though many are associated with sporadic genetic mutations. In this review, we aim to highlight X-linked NMDs including lissencephaly, periventricular nodular heterotopia and polymicrogyria because of their mostly familial inheritance pattern. We focus on the most prominent genes responsible: including DCX, ARX, FLNA, FMR1, L1CAM, SRPX2, DDX3X, NSHDL, CUL4B and OFD1, outlining what is known about their prevalence among NMDs, and the underlying pathophysiology. X-linked disorders are important to recognise clinically, as females often have milder phenotypes. Consequently, there is a greater chance they survive to reproductive age and risk passing the mutations down. Effective genetic screening is important to prevent and treat these conditions, and for this, we need to know gene mutations and have a clear understanding of the function of the genes involved. This review summarises the knowledge base and provides clear direction for future work by both scientists and clinicians alike.

NFIX-Mediated Inhibition of Neuroblast Branching Regulates Migration Within the Adult Mouse Ventricular–Subventricular Zone

Understanding the migration of newborn neurons within the brain presents a major challenge in contemporary biology. Neuronal migration is widespread within the developing brain but is also important within the adult brain. For instance, stem cells within the ventricular–subventricular zone (V-SVZ) and the subgranular zone of dentate gyrus of the adult rodent brain produce neuroblasts that migrate to the olfactory bulb and granule cell layer of the dentate gyrus, respectively, where they regulate key brain functions including innate olfactory responses, learning, and memory. Critically, our understanding of the factors mediating neuroblast migration remains limited. The transcription factor nuclear factor I X (NFIX) has previously been implicated in embryonic cortical development. Here, we employed conditional ablation of Nfix from the adult mouse brain and demonstrated that the removal of this gene from either neural stem and progenitor cells, or neuroblasts, within the V-SVZ culminated in neuroblast migration defects. Mechanistically, we identified aberrant neuroblast branching, due in part to increased expression of the guanylyl cyclase natriuretic peptide receptor 2 (Npr2), as a factor contributing to abnormal migration in Nfix-deficient adult mice. Collectively, these data provide new insights into how neuroblast migration is regulated at a transcriptional level within the adult brain.

The function of sperm-associated antigen 6 in neuronal proliferation and differentiation

Sperm-associated antigen 6 (SPAG6) is initially found in human testis and is essential for sperm motility and male fertility. Later studies indicate that it also express in the chick Central Nervous System and human embryonic stem cells. However, the function of Spag6 in cortical development is still largely unclear. Using in utero electroporation, we showed that overexpression of Spag6 induced the transfected cells excluded from the proliferation zone of the mouse cortex. Ki67 Co-labeling and BrdU incorporation experiment suggested that overexpression of Spag6 inhibited proliferation of neural progenitor cells. Furthermore, we demonstrated that Spag6-overexpressing cells preferred to differentiated into neurons, which could be labeled by Brn2, rather than GFAP positive astrocytes. Taken together, our data indicate that Spag6 plays an essential role in the process of neuronal proliferation and differentiation.

A Perspective on the Role of microRNA-128 Regulation in Mental and Behavioral Disorders

MiRNAs are short, non-coding RNA molecules that regulate gene expression post-transcriptionally. Over the past decade, misregulated miRNA pathways have been associated with various diseases such as cancer, neurodegenerative diseases, and neurodevelopmental disorders. In this article, we aim to discuss the role played by miR-128 in neuropsychiatric disorders, and highlight potential target genes from an in silico analysis of predicted miR-128 targets. We also discuss the differences of target gene determination based on a bioinformatics or empirical approach. Using data from TargetScan and published reports, we narrowed the miR-128 target gene list to those that are known to be associated with neuropsychiatric disorders, and found that these genes can be classified into 29 gene clusters and are mostly enriched in cancer and MAPK signaling pathways. We also highlight some recent studies on several of the miR-128 targets which should be investigated further as potential candidate genes for therapeutic interventions.

Genes and brain malformations associated with abnormal neuron positioning

Neuronal positioning is a fundamental process during brain development. Abnormalities in this process cause several types of brain malformations and are linked to neurodevelopmental disorders such as autism, intellectual disability, epilepsy, and schizophrenia. Little is known about the pathogenesis of developmental brain malformations associated with abnormal neuron positioning, which has hindered research into potential treatments. However, recent advances in neurogenetics provide clues to the pathogenesis of aberrant neuronal positioning by identifying causative genes. This may help us form a foundation upon which therapeutic tools can be developed. In this review, we first provide a brief overview of neural development and migration, as they relate to defects in neuronal positioning. We then discuss recent progress in identifying genes and brain malformations associated with aberrant neuronal positioning during human brain development.