Gender-specific effect of Mthfr genotype and neonatal vigabatrin interaction on synaptic proteins in mouse cortex

Sex Differences in Autism Spectrum Disorder: Diagnostic, Neurobiological, and Behavioral Features

Autism Spectrum Disorder (ASD) is a complex neurodevelopmental disorder with a worldwide prevalence of about 1%, characterized by impairments in social interaction, communication, repetitive patterns of behaviors, and can be associated with hyper- or hypo-reactivity of sensory stimulation and cognitive disability. ASD comorbid features include internalizing and externalizing symptoms such as anxiety, depression, hyperactivity, and attention problems. The precise etiology of ASD is still unknown and it is undoubted that the disorder is linked to some extent to both genetic and environmental factors. It is also well-documented and known that one of the most striking and consistent finding in ASD is the higher prevalence in males compared to females, with around 70% of ASD cases described being males. The present review looked into the most significant studies that attempted to investigate differences in ASD males and females thus trying to shade some light on the peculiar characteristics of this prevalence in terms of diagnosis, imaging, major autistic-like behavior and sex-dependent uniqueness. The study also discussed sex differences found in animal models of ASD, to provide a possible explanation of the neurological mechanisms underpinning the different presentation of autistic symptoms in males and females.

Dietary folic acid deficiency impacts hippocampal morphology and cortical acetylcholine metabolism in adult male and female mice

OBJECTIVE

One-carbon (1C) metabolism is a metabolic network that integrates nutritional signals with biosynthesis, redox homeostasis, and epigenetics. There are sex differences in hepatic 1C metabolism, however, it is unclear whether sex differences in 1C impact the brain. The aim of this study was to investigate if sex modulates the effects of dietary folic acid deficiency, the main component of 1C, in brain tissue using a mouse model.

METHODS

Male and female C57Bl/6J mice were placed on a folic acid deficient (FD) or control diet (CD) at six weeks until six months of aged. After which brain tissue and serum were collected for analysis. In brain tissue, hippocampal volume, morphology, and apoptosis as well as cortical acetylcholine metabolism were measured.

RESULTS

Male and female FD mice had reduced serum levels of folate. Both males and females maintained on a FD showed a decrease in the thickness of the hippocampal CA1-CA3 region. Interestingly, there was a sex difference in the levels of active caspase-3 within the CA3 region of the hippocampus. In cortical tissue, there were increased levels of neuronal ChAT and reduced levels of AChE in FD females and male mice.

CONCLUSIONS

The results indicated that FD impacts hippocampal morphology and cortical neuronal acetylcholine metabolism. The data from our study indicate that there was only one sex difference and that was in hippocampal apoptosis. Our study provides little evidence that sex modulates the effects of dietary folate deficiency on hippocampal morphology and cortical neuronal acetylcholine metabolism.

Genetics and Epigenetics of One-Carbon Metabolism Pathway in Autism Spectrum Disorder: A Sex-Specific Brain Epigenome?

Autism spectrum disorder (ASD) is a complex neurodevelopmental condition affecting behavior and communication, presenting with extremely different clinical phenotypes and features. ASD etiology is composite and multifaceted with several causes and risk factors responsible for different individual disease pathophysiological processes and clinical phenotypes. From a genetic and epigenetic side, several candidate genes have been reported as potentially linked to ASD, which can be detected in about 10–25% of patients. Folate gene polymorphisms have been previously associated with other psychiatric and neurodegenerative diseases, mainly focused on gene variants in the DHFR gene (5q14.1; rs70991108, 19bp ins/del), MTHFR gene (1p36.22; rs1801133, C677T and rs1801131, A1298C), and CBS gene (21q22.3; rs876657421, 844ins68). Of note, their roles have been scarcely investigated from a sex/gender viewpoint, though ASD is characterized by a strong sex gap in onset-risk and progression. The aim of the present review is to point out the molecular mechanisms related to intracellular folate recycling affecting in turn remethylation and transsulfuration pathways having potential effects on ASD. Brain epigenome during fetal life necessarily reflects the sex-dependent different imprint of the genome-environment interactions which effects are difficult to decrypt. We here will focus on the DHFR, MTHFR and CBS gene-triad by dissecting their roles in a sex-oriented view, primarily to bring new perspectives in ASD epigenetics.

The influence of choline treatment on behavioral and neurochemical autistic-like phenotype in Mthfr-deficient mice

Imbalanced one carbon metabolism and aberrant autophagy is robustly reported in patients with autism. Polymorphism in the gene methylenetetrahydrofolate reductase (Mthfr), encoding for a key enzyme in this pathway is associated with an increased risk for autistic-spectrum-disorders (ASDs). Autistic-like core and associated behaviors have been described, with contribution of both maternal and offspring Mthfr^+/- genotype to the different domains of behavior. Preconception and prenatal supplementation with methyl donor rich diet to human subjects and mice reduced the risk for developing autism and autistic-like behavior, respectively. Here we tested the potential of choline supplementation to Mthfr-deficient mice at young-adulthood to reduce behavioral and neurochemical changes reminiscent of autism characteristics. We show that offspring of Mthfr^+/- mothers, whether wildtype or heterozygote, exhibit autistic-like behavior, altered brain p62 protein levels and LC3-II/LC3-I levels ratio, both, autophagy markers. Choline supplementation to adult offspring of Mthfr^+/- mothers for 14 days counteracted characteristics related to repetitive behavior and anxiety both in males and in females and improved social behavior solely in male mice. Choline treatment also normalized deviant cortical levels of the autophagy markers measured in male mice. The results demonstrate that choline supplementation even at adulthood, not tested previously, to offspring of Mthfr-deficient mothers, attenuates the autistic-like phenotype. If this proof of concept is replicated it might promote translation of these results to treatment recommendation for children with ASDs bearing similar genetic/metabolic make-up.

Molecular causes of sex-specific deficits in rodent models of neurodevelopmental disorders

Neurodevelopmental disorders (NDDs) such as intellectual disability and autism spectrum disorder consistently show a male bias in prevalence, but it remains unclear why males and females are affected with different frequency. While many behavioral studies of transgenic NDD models have focused only on males, the requirement by the National Institutes of Health to consider sex as a biological variable has promoted the comparison of male and female performance in wild-type and mutant animals. Here, we review examples of rodent models of NDDs in which sex-specific deficits were identified in molecular, physiological, and/or behavioral responses, showing sex differences in susceptibility to disruption of genes mutated in NDDs. Haploinsufficiency in genes involved in mechanisms such as synaptic function (GABRB3 and NRXN1), chromatin remodeling (CHD8, EMHT1, and ADNP), and intracellular signaling (CC2D1A and ERK1) lead to more severe behavioral outcomes in males. However, in the absence of behavioral deficits, females can still present with cellular and electrophysiological changes that could be due to compensatory mechanisms or differential allocation of molecular and cellular functions in the two sexes. By contrasting these findings with mouse models where females are more severely affected (MTHFR and AMBRA1), we propose a framework to approach the study of sex-specific deficits possibly leading to sex bias in NDDs.

Effect of supplementation with methyl-donor nutrients on neurodevelopment and cognition: considerations for future research

Pregnancy represents a critical period in fetal development, such that the prenatal environment can, in part, establish a lifelong trajectory of health or disease for the offspring. Poor nutrition (macro- or micronutrient deficiencies) can adversely affect brain development and significantly increase offspring risk for metabolic and neurological disease development. The concentration of dietary methyl-donor nutrients is known to alter DNA methylation in the brain, and alterations in DNA methylation can have long-lasting effects on gene expression and neuronal function. The decreased availability of methyl-donor nutrients to the developing fetus in models of poor maternal nutrition is one mechanism hypothesized to link maternal malnutrition and disease risk in offspring. Animal studies indicate that supplementation of both maternal and postnatal (early- and later-life) diets with methyl-donor nutrients can attenuate disease risk in offspring; however, clinical research is more equivocal. The objective of this review is to summarize how specific methyl-donor nutrient deficiencies and excesses during pre- and postnatal life alter neurodevelopment and cognition. Emphasis is placed on reviewing the current literature, highlighting challenges within nutrient supplementation research, and considering potential strategies to ensure robust findings in future studies.

Sex-specific Behavioral Features of Rodent Models of Autism Spectrum Disorder

Sex is an important factor in understanding the clinical presentation, management, and developmental trajectory of children with neuropsychiatric disorders. While much is known about the clinical and neurobehavioral profiles of males with neuropsychiatric disorders, surprisingly little is known about females in this respect. Animal models may provide detailed mechanistic information about sex differences in autism spectrum disorder (ASD) in terms of manifestation, disease progression, and development of therapeutic options. This review aims to widen our understanding of the role of sex in autism spectrum disorder, by summarizing and comparing behavioral characteristics of animal models. Our current understanding of how differences emerge in boys and girls with neuropsychiatric disorders is limited: Information derived from animal studies will stimulate future research on the role of biological maturation rates, sex hormones, sex-selective protective (or aggravating) factors and psychosocial factors, which are essential to devise sex precision medicine and to improve diagnostic accuracy. Moreover, there is a strong need of novel strategies to elucidate the major mechanisms leading to sex-specific autism features, as well as novel models or methods to examine these sex differences.

Glutamatergic synapse protein composition of wild-type mice is sensitive to in utero MTHFR genotype and the timing of neonatal vigabatrin exposure

The enzyme methylenetetrahydrofolate-reductase (MTHFR) is part of the homocysteine and folate metabolic pathways. In utero, Mthfr-deficient environment has been reported as a risk factor for neurodevelopmental disorders such as autism and neural tube defects. Neonatal disruption of the GABAergic system is also associated with behavioral outcomes. The interaction between Mthfr deficiency and neonatal exposure to the GABA-potentiating drug vigabatrin (GVG) in mice alters anxiety, memory, and social behavior in a gender-dependent manner. In addition, a gender-dependent enhancement of proteins implicated in excitatory synapse plasticity in the cerebral cortex was shown. Here we show that in utero MTHFR deficiency is sufficient to alter the levels of glutamate receptor subunits GluR1, GluR2, and NR2B in the cerebral cortex and hippocampus of adult offspring with a WT genotype. In addition, FMRP1, CAMKII α and γ, and NLG1 levels in WT offspring were vulnerable to the in utero genotype. These effects depend on brain region and the cellular compartment tested. The effect of in utero MTHFR deficiency varies with the age of neonatal GVG exposure to modify GluR1, NR2A, reelin, CAMKII α, and NLG1 levels. These changes in molecular composition of the glutamatergic synapse were associated with increased anxiety-like behavior. Complex, multifactorial disorders of the nervous system show significant association with several genetic and environmental factors. Our data exemplify the contribution of an in utero MTHFR-deficient environment and early exposure to an antiepileptic drug to the basal composition of the glutamatergic synapses. The robust effect is expected to alter synapse function and plasticity and the cortico-hippocampal circuitry.

Increased susceptibility to mild neonatal stress in MTHFR deficient mice

Early life stress is shown to have a life-span outcome on human and animal behavior, increasing the risk for psychopathology. The gene methylenetetrahydrofolate reductase (MTHFR), which encodes for a key enzyme in one carbon metabolism, shows a high prevalence of polymorphism in patients with developmental disorders. Here we examined the hypothesis that MTHFR deficiency results in an increased susceptibility of the developing brain to mild neonatal stress (NS). Mild NS failed to alter corticosterone levels in young and adult Wt mice. However, an elevated level of corticosterone was found in the MTHFR deficient-NS female, exemplifying enhanced sensitivity to NS. Behavioral phenotyping of Wt and MTHFR deficient mice provides evidence that the effect of mild NS may be amplified by the MTHFR deficient genotype. Distinct behavioral characteristics were altered in male and female mice. In general, three patterns of influence on mice behavior were observed: (1) an additive suppressive effect of NS and MTHFR deficiency on exploration and activity was evident in females; (2) stress related parameters were significantly sensitive to genotype in females, presenting an interaction between genotype and sex; (3) various aspects of behavior in a social setting were modified preferably in males by genotype, NS and the interaction between the two, while females exhibited a smaller effect that was restricted to NS with no genotype effect. Overall, our results support an interaction between mild NS, the MTHFR genotype and sex. We suggest using this animal model to study the molecular mechanism linking these two risk factors and their involvement in neurodevelopmental disorders such as schizophrenia and autism.

Re-Conceptualizing “Sex” and “Gender” in the Human Brain

When is an observed dissimilarity between brains of females and males a “sex” difference and when is it a “gender” difference? The aim of this conceptual paper is to pinpoint the understandings of “sex” and “gender” within neuropsychological research, as these terms implicitly lead to overlapping and nonspecific associations when variables concerning female and male characteristics are operationalized. Also, it is argued, following a central approach within gender studies, that it is impossible for the variables of “sex” and “gender” to be categorized as solely biological or solely social components or to be measured or recorded as such, and for this reason, they should in fact be regarded as a unity and designated as sex/gender.