Protein synthesis as a modifiable target for autism-related dendritic spine pathophysiologies

The intersection of sleep and synaptic translation in synaptic plasticity deficits in neurodevelopmental disorders

Individuals with neurodevelopmental disorders experience persistent sleep deficits, and there is increasing evidence that sleep dysregulation is an underlying cause, rather than merely an effect, of the synaptic and behavioral defects observed in these disorders. At the molecular level, dysregulation of the synaptic proteome is a common feature of neurodevelopmental disorders, though the mechanism connecting these molecular and behavioral phenotypes is an ongoing area of investigation. A role for eIF2α in shifting the local proteome in response to changes in the conditions at the synapse has emerged. Here, we discuss recent progress in characterizing the intersection of local synaptic translation and sleep and propose a reciprocal mechanism of dysregulation in the development of synaptic plasticity defects in neurodevelopmental disorders.

Modulation of PI3K/Akt/GSK3β signaling cascade through G protein-coupled receptor 55 (GPR55) activation: Prenatal lysophosphatidylinositol attenuates valproic acid-induced synaptic abnormalities and mitochondrial dysfunction

AIMS

Dysregulation of PI3K/Akt/GSK3β signaling has been implicated in various neurological disorders, including autism spectrum disorder (ASD). G protein-coupled receptor 55 (GPR55) has recently emerged as a potential regulator of this signaling cascade. This study explores the intricate modulation of the PI3K/Akt/GSK3β signaling cascade via GPR55 activation and its potential therapeutic implications in the context of autism-associated neuronal impairments.

MAIN METHODS

Valproic acid (VPA) was administered on embryonic day 12 (E12) to induce ASD, and lysophosphatidylinositol (LPI), a GPR55 agonist, was used prenatally to modulate the receptor activity. Golgi-cox staining was performed to observe neuronal morphology, and Hematoxylin and eosin (H and E) staining was carried out to quantify damaged neurons. Enzyme-linked immunosorbent assay (ELISA) was implemented to identify molecular mediators involved in neuroprotection.

KEY FINDINGS

Prenatal VPA exposure resulted in significant abnormalities in synaptic development, which were further evidenced by impairments in social interaction and cognitive function. When LPI was administered, most of the synaptic abnormalities were alleviated, as reflected by higher neuron and dendritic spine count. LPI treatment also reduced cytoplasmic cytochrome c concentration and related neuronal cell death. Mechanistically, GPR55 activation by LPI increases the expression of phospho-Akt and phospho-GSK3β, leading to the activation of this signaling in the process of rescuing synaptic abnormalities and mitochondria-mediated neuronal apoptosis.

SIGNIFICANCE

The observed therapeutic effects of GPR55 activation shed light on its significance as a prospective target for ameliorating mitochondrial dysfunction and dendritic spine loss, offering novel prospects for developing targeted interventions to alleviate the neuropathological causes of ASD.

Sex bias in social deficits, neural circuits and nutrient demand in Cttnbp2 autism models

Autism spectrum disorders caused by both genetic and environmental factors are strongly male-biased neuropsychiatric conditions. However, the mechanism underlying the sex bias of autism spectrum disorders remains elusive. Here, we use a mouse model in which the autism-linked gene Cttnbp2 is mutated to explore the potential mechanism underlying the autism sex bias. Autism-like features of Cttnbp2 mutant mice were assessed via behavioural assays. C-FOS staining identified sex-biased brain regions critical to social interaction, with their roles and connectivity then validated by chemogenetic manipulation. Proteomic and bioinformatic analyses established sex-biased molecular deficits at synapses, prompting our hypothesis that male-biased nutrient demand magnifies Cttnbp2 deficiency. Accordingly, intakes of branched-chain amino acids (BCAA) and zinc were experimentally altered to assess their effect on autism-like behaviours. Both deletion and autism-linked mutation of Cttnbp2 result in male-biased social deficits. Seven brain regions, including the infralimbic area of the medial prefrontal cortex (ILA), exhibit reduced neural activity in male mutant mice but not in females upon social stimulation. ILA activation by chemogenetic manipulation is sufficient to activate four of those brain regions susceptible to Cttnbp2 deficiency and consequently to ameliorate social deficits in male mice, implying an ILA-regulated neural circuit is critical to male-biased social deficits. Proteomics analysis reveals male-specific downregulated proteins (including SHANK2 and PSD-95, two synaptic zinc-binding proteins) and female-specific upregulated proteins (including RRAGC) linked to neuropsychiatric disorders, which are likely relevant to male-biased deficits and a female protective effect observed in Cttnbp2 mutant mice. Notably, RRAGC is an upstream regulator of mTOR that senses BCAA, suggesting that mTOR exerts a beneficial effect on females. Indeed, increased BCAA intake activates the mTOR pathway and rescues neuronal responses and social behaviours of male Cttnbp2 mutant mice. Moreover, mutant males exhibit greatly increased zinc demand to display normal social behaviours. Mice carrying an autism-linked Cttnbp2 mutation exhibit male-biased social deficits linked to specific brain regions, differential synaptic proteomes and higher demand for BCAA and zinc. We postulate that lower demand for zinc and BCAA are relevant to the female protective effect. Our study reveals a mechanism underlying sex-biased social defects and also suggests a potential therapeutic approach for autism spectrum disorders.

Neuronal differentiation reporter mice as a new methodology for detecting in vivo developmental neurotoxicity

Current in vivo developmental neurotoxicity (DNT) tests are not performed routinely for chemical risk assessment because they are time and resource intensive and require many animals. Therefore, new methodologies are required that can detect and evaluate the DNT potential of chemicals in a more simple, quantitative, and objective manner. Toward this end, we generated transgenic mice expressing reporter genes (luciferase and lacZ) under the control of the rat synapsin 1 promoter (Syn-Rep mice) and evaluated their usefulness as a DNT detection tool. Brain luciferase expression levels in Syn-Rep mice increased dramatically from just before to after birth, peaked early in the postnatal period, subsequently decreased sharply, and then remained low after weaning. This pattern is analogous to the generally recognized temporal changes in synapse numbers in the developing mammal brain. To evaluate further the responsiveness of Syn-Rep mice during DNT induction, we administered valproic acid (VPA), a reference DNT-inducing chemical, to pregnant mice and evaluated its effect on reporter gene expression in the developing brains of Syn-Rep pups. In vivo luminescence in the brains of VPA-exposed pups was significantly lower than in controls from postnatal days 4 to 13. Moreover, luciferase activity in the prefrontal cortexes of 8-week-old VPA-exposed offspring was significantly lower than in controls, reflecting the reduced number of neurons in the prefrontal cortex. These results suggest that the Syn-Rep mice are potentially useful tools for streamlined detection of chemical-induced DNT in the developing mammalian brain.

Profiling of Sexually Dimorphic Genes in Neural Cells to Identify Eif2s3y, Whose Overexpression Causes Autism-Like Behaviors in Male Mice

Many neurological disorders exhibit sex differences and sex-specific therapeutic responses. Unfortunately, significant amounts of studies investigating molecular and cellular mechanisms underlying these neurological disorders use primary cell cultures with undetermined sexes; and this may be a source for contradictory results among different studies and impair the validity of study conclusion. Herein, we comprehensively compared sexual dimorphism of gene expression in primary neurons, astrocytes, and microglia derived from neonatal mouse brains. We found that overall sexually dimorphic gene numbers were relatively low in these primary cells, with microglia possessing the most (264 genes), neurons possessing the medium (69 genes), and astrocytes possessing the least (30 genes). KEGG analysis indicated that sexually dimorphic genes in these three cell types were strongly enriched for the immune system and immune-related diseases. Furthermore, we identified that sexually dimorphic genes shared by these primary cells dominantly located on the Y chromosome, including Ddx3y, Eif2s3y, Kdm5d, and Uty. Finally, we demonstrated that overexpression of Eif2s3y increased synaptic transmission specifically in male neurons and caused autism-like behaviors specifically in male mice. Together, our results demonstrate that the sex of primary cells should be considered when these cells are used for studying the molecular mechanism underlying neurological disorders with sex-biased susceptibility, especially those related to immune dysfunction. Moreover, our findings indicate that dysregulation of sexually dimorphic genes on the Y chromosome may also result in autism and possibly other neurological disorders, providing new insights into the genetic driver of sex differences in neurological disorders.