Novel missense variants in brain morphogenic genes associated with depression and schizophrenia

Introduction

Impaired function of brain morphogenic genes is considered one of the predisposing factors for the manifestation of psychiatric and cognitive disorders, such as paranoid schizophrenia (SCZ) and major depressive disorder (MDD). Identification of such genes (genes of neurotrophic factors and guidance molecules among them) and their deleterious genetic variants serves as a key to diagnosis, prevention, and possibly treatment of such disorders. In this study, we have examined the prevalence of genomic variants in brain morphogenic genes in individuals with SCZ and MDD within a Russian population.

Methods

We have performed whole-exome sequencing of 21 DNA samples: 11 from individuals with SCZ and 10 with MDD, followed by ARMS (Amplification-Refractory Mutation System) based screening of detected single nucleotide variants (SNVs) in larger groups: 102 for individuals with SCZ, 79 for those with MDD and 103 for healthy donors.

Results

Whole-exome sequencing has revealed 226 missense mutations in 79 genes (out of 140 studied), some of which occur in patients with psychiatric disorders significantly more frequently than in healthy donors. We have identified previously undescribed genomic variants in brain morphogenic genes: CDH2 (rs1944294-T and rs17445840-T), DCHS2 (rs11935573-G and rs12500437-G/T) and CDH23 (rs1227051-G/A), significantly associated with the incidence of SCZ and MDD in the Russian population. For some SNVs (rs6265-T, rs1944294-T, rs11935573-G, rs4760-G) sex-biased differences in their prevalence between SCZ/MDD patients and healthy donors was detected.

Discussion

However, the functional significance of the SNVs identified has still to be confirmed in cellular and animal models. Once it is fulfilled, these SNVs have the potential to complement the diagnostic toolbox for assessing susceptibility to mental disorders. The data obtained indirectly confirm the importance of adequate brain structure formation for its correct functioning and preservation of mental health.

Morphogenetic theory of mental and cognitive disorders: the role of neurotrophic and guidance molecules

Mental illness and cognitive disorders represent a serious problem for the modern society. Many studies indicate that mental disorders are polygenic and that impaired brain development may lay the ground for their manifestation. Neural tissue development is a complex and multistage process that involves a large number of distant and contact molecules. In this review, we have considered the key steps of brain morphogenesis, and the major molecule families involved in these process. The review provides many indications of the important contribution of the brain development process and correct functioning of certain genes to human mental health. To our knowledge, this comprehensive review is one of the first in this field. We suppose that this review may be useful to novice researchers and clinicians wishing to navigate the field.

Prenatal onset of the neuroradiologic phenotype of pyruvate carboxylase deficiency due to homozygous PC c.1828G > A mutations

Pyruvate carboxylase (PC) deficiency (MIM# 266150) is an autosomal recessive disorder with three subtypes. Patients homozygous for the c.1828G > A mutation in the PC gene belong to type A, which typically has infantile onset, severe to profound developmental delay, hypotonia, and lactic acidemia. We report the neuroimaging abnormalities in a 33-week gestation infant homozygous for the c.1828G > A mutation. Brain magnetic resonance imaging on day 10 of life revealed increased T2 signal within the subcortical and periventricular white matter, an immature gyral pattern, large periventricular cysts with mass effect on the lateral ventricles, and dilatation of the occipital and temporal horns. Magnetic resonance spectroscopy showed reduced creatine and NAA peaks, a relatively high choline peak and no lactate peak. These findings were observed prior to the neonate experiencing any episodes of decompensation with lactic acidosis. The presence of these brain anomalies at this gestational age, prior to any metabolic decompensation, supports the essential role of PC in normal brain morphogenesis and the resulting in-utero brain anomalies secondary to its deficiency. Our experience with this affected premature infant and many others we have managed with the same founder mutation suggests that the clinical phenotypes of the type A and the more severe type B PC deficient patients are on a spectrum rather than distinct subtypes.

Sonic hedgehog expands neural stem cells in the neocortical region leading to an expanded and wrinkled neocortical surface

An expanded and folded neocortex is characteristic of higher mammals, including humans and other primates. The neocortical surface area was dramatically enlarged during the course of mammalian brain evolution from lissencephalic to gyrencephalic mammals, and this bestowed higher cognitive functions especially to primates, including humans. In this study, we generated transgenic (Tg) mice in which the expression of Sonic hedgehog (Shh) could be controlled in neural stem cells (NSCs) and neural progenitors by using the Tet-on system. Shh overexpression during embryogenesis promoted the symmetric proliferative division of NSCs in the neocortical region, leading to the expansion of lateral ventricles and tangential extension of the ventricular zone. Moreover, Shh-overexpressing Tg mice showed dramatic expansion of the neocortical surface area and exhibited a wrinkled brain when overexpression was commenced at early stages of neural development. These results indicate that Shh is able to increase the neocortical NSCs and contribute to expansion of the neocortex.

A Method for Parasagittal Sectioning for Neuroanatomical Quantification of Brain Structures in the Adult Mouse

In this article, we present a standardized protocol for fast and robust neuroanatomical phenotyping of the adult mouse brain, which complements a previously published article (doi: 10.1002/cpmo.12) in Current Protocols in Mouse Biology. It is aimed at providing an experimental pipeline within an academic research setting from experimental work to data analysis. Our analysis focuses on one single parasagittal plane, covering the majority of brain regions involved in higher order cognitions such as the cortex, hippocampus, and cerebellum, for a total of 166 parameters of area, length, and cell-level measurements in contrast to 78 parameters in our previously published coronal screen. Benefits of using parasagittal analysis for large-scale neuroanatomic screens are discussed. © 2018 by John Wiley & Sons, Inc.

Universality in human cortical folding in health and disease

The folding of the cortex in mammalian brains across species has recently been shown to follow a universal scaling law that can be derived from a simple physics model. However, it was yet to be determined whether this law also applies to the morphological diversity of different individuals in a single species, in particular with respect to factors, such as age, sex, and disease. To this end, we derived and investigated the cortical morphology from magnetic resonance images (MRIs) of over 1,000 healthy human subjects from three independent public databases. Our results show that all three MRI datasets follow the scaling law obtained from the comparative neuroanatomical data, which strengthens the case for the existence of a common mechanism for cortical folding. Additionally, for comparable age groups, both male and female brains scale in exactly the same way, despite systematic differences in size and folding. Furthermore, age introduces a systematic shift in the offset of the scaling law. In the model, this shift can be interpreted as changes in the mechanical forces acting on the cortex. We also applied this analysis to a dataset derived from comparable cohorts of Alzheimer's disease patients and healthy subjects of similar age. We show a systematically lower offset and a possible change in the exponent for Alzheimer's disease subjects compared with the control cohort. Finally, we discuss implications of the changes in offset and exponent in the data and relate it to existing literature. We, thus, provide a possible mechanistic link between previously independent observations.

Endogenous gradients of resting potential instructively pattern embryonic neural tissue via Notch signaling and regulation of proliferation

Biophysical forces play important roles throughout embryogenesis, but the roles of spatial differences in cellular resting potentials during large-scale brain morphogenesis remain unknown. Here, we implicate endogenous bioelectricity as an instructive factor during brain patterning in Xenopus laevis. Early frog embryos exhibit a characteristic hyperpolarization of cells lining the neural tube; disruption of this spatial gradient of the transmembrane potential (Vmem) diminishes or eliminates the expression of early brain markers, and causes anatomical mispatterning of the brain, including absent or malformed regions. This effect is mediated by voltage-gated calcium signaling and gap-junctional communication. In addition to cell-autonomous effects, we show that hyperpolarization of transmembrane potential (Vmem) in ventral cells outside the brain induces upregulation of neural cell proliferation at long range. Misexpression of the constitutively active form of Notch, a suppressor of neural induction, impairs the normal hyperpolarization pattern and neural patterning; forced hyperpolarization by misexpression of specific ion channels rescues brain defects induced by activated Notch signaling. Strikingly, hyperpolarizing posterior or ventral cells induces the production of ectopic neural tissue considerably outside the neural field. The hyperpolarization signal also synergizes with canonical reprogramming factors (POU and HB4), directing undifferentiated cells toward neural fate in vivo. These data identify a new functional role for bioelectric signaling in brain patterning, reveal interactions between Vmem and key biochemical pathways (Notch and Ca(2+) signaling) as the molecular mechanism by which spatial differences of Vmem regulate organogenesis of the vertebrate brain, and suggest voltage modulation as a tractable strategy for intervention in certain classes of birth defects.

Gyrification from constrained cortical expansion

The exterior of the mammalian brain--the cerebral cortex--has a conserved layered structure whose thickness varies little across species. However, selection pressures over evolutionary time scales have led to cortices that have a large surface area to volume ratio in some organisms, with the result that the brain is strongly convoluted into sulci and gyri. Here we show that the gyrification can arise as a nonlinear consequence of a simple mechanical instability driven by tangential expansion of the gray matter constrained by the white matter. A physical mimic of the process using a layered swelling gel captures the essence of the mechanism, and numerical simulations of the brain treated as a soft solid lead to the formation of cusped sulci and smooth gyri similar to those in the brain. The resulting gyrification patterns are a function of relative cortical expansion and relative thickness (compared with brain size), and are consistent with observations of a wide range of brains, ranging from smooth to highly convoluted. Furthermore, this dependence on two simple geometric parameters that characterize the brain also allows us to qualitatively explain how variations in these parameters lead to anatomical anomalies in such situations as polymicrogyria, pachygyria, and lissencephalia.

Thyroid hormone treated astrocytes induce maturation of cerebral cortical neurons through modulation of proteoglycan levels

Proper brain neuronal circuitry formation and synapse development is dependent on specific cues, either genetic or epigenetic, provided by the surrounding neural environment. Within these signals, thyroid hormones (T3 and T4) play crucial role in several steps of brain morphogenesis including proliferation of progenitor cells, neuronal differentiation, maturation, migration, and synapse formation. The lack of thyroid hormones during childhood is associated with several impair neuronal connections, cognitive deficits, and mental disorders. Many of the thyroid hormones effects are mediated by astrocytes, although the mechanisms underlying these events are still unknown. In this work, we investigated the effect of 3, 5, 3′-triiodothyronine-treated (T3-treated) astrocytes on cerebral cortex neuronal differentiation. Culture of neural progenitors from embryonic cerebral cortex mice onto T3-treated astrocyte monolayers yielded an increment in neuronal population, followed by enhancement of neuronal maturation, arborization and neurite outgrowth. In addition, real time PCR assays revealed an increase in the levels of the heparan sulfate proteoglycans, Glypican 1 (GPC-1) and Syndecans 3 e 4 (SDC-3 e SDC-4), followed by a decrease in the levels of the chondroitin sulfate proteoglycan, Versican. Disruption of glycosaminoglycan chains by chondroitinase AC or heparanase III completely abolished the effects of T3-treated astrocytes on neuronal morphogenesis. Our work provides evidence that astrocytes are key mediators of T3 actions on cerebral cortex neuronal development and identified potential molecules and pathways involved in neurite extension; which might eventually contribute to a better understanding of axonal regeneration, synapse formation, and neuronal circuitry recover.

Vax1, a novel homeobox-containing gene, directs development of the basal forebrain and visual system

The novel homeobox-containing gene Vax1, a member of the Emx/Not gene family, is specifically expressed in the developing basal forebrain and optic nerve. Here, we show that Vax1 is essential for normal development of these structures. Mice carrying a targeted mutation of Vax1 show dysgenesis of the optic nerve, coloboma, defects in the basal telencephalon, and lobar holoprosencephaly. With the help of molecular markers we determined that in the developing visual system, the absence of Vax1 results in a proximal expansion of the activity of Pax6 and Rx. This observation suggests that Vax1 may interfere negatively with the expression of Pax6 and Rx. In reciprocal gain-of-function experiments, injection of Xvax1 mRNA or Shh into Xenopus embryos primarily affects the brain at the level of the eye primordium. Consistent with the loss-of-function results, the injection of Xvax1 results in a down-regulation of Rx. Similarly, Shh injection expands the Vax1 and Pax2 territory at the expense of the Pax6 and Rx region. On the basis of these results, we propose a model for a molecular cascade involved in the establishment of structures of the visual system.