AUTS2 Regulates RNA Metabolism and Dentate Gyrus Development in Mice

Research advances of polycomb group proteins in regulating mammalian development

Polycomb group (PcG) proteins are a subset of epigenetic factors that are highly conserved throughout evolution. In mammals, PcG proteins can be classified into two muti-proteins complexes: Polycomb repressive complex 1 (PRC1) and PRC2. Increasing evidence has demonstrated that PcG complexes play critical roles in the regulation of gene expression, genomic imprinting, chromosome X-inactivation, and chromatin structure. Accordingly, the dysfunction of PcG proteins is tightly orchestrated with abnormal developmental processes. Here, we summarized and discussed the current knowledge of the biochemical and molecular functions of PcG complexes, especially the PRC1 and PRC2 in mammalian development including embryonic development and tissue development, which will shed further light on the deep understanding of the basic knowledge of PcGs and their functions for reproductive health and developmental disorders.

Isolated loss of the AUTS2 long isoform, brain-wide or targeted to Calbindin-lineage cells, generates a specific suite of brain, behavioral, and molecular pathologies

Rearrangements within the AUTS2 region are associated with a rare syndromic disorder with intellectual disability, developmental delay, and behavioral abnormalities as core features. In addition, smaller regional variants are linked to wide range of neuropsychiatric disorders, underscoring the gene's essential role in brain development. Like many essential neurodevelopmental genes, AUTS2 is large and complex, generating distinct long (AUTS2-l) and short (AUTS2-s) protein isoforms from alternative promoters. Although evidence suggests unique isoform functions, the contributions of each isoform to specific AUTS2-linked phenotypes have not been clearly resolved. Furthermore, Auts2 is widely expressed across the developing brain, but cell populations most central to disease presentation have not been determined. In this study, we focused on the specific roles of AUTS2-l in brain development, behavior, and postnatal brain gene expression, showing that brain-wide AUTS2-l ablation leads to specific subsets of the recessive pathologies associated with mutations in 3' exons (exons 8-19) that disrupt both major isoforms. We identify downstream genes that could explain expressed phenotypes including hundreds of putative direct AUTS2-l target genes. Furthermore, in contrast to 3' Auts2 mutations which lead to dominant hypoactivity, AUTS2-l loss-of-function is associated with dominant hyperactivity and repetitive behaviors, phenotypes exhibited by many human patients. Finally, we show that AUTS2-l ablation in Calbindin 1-expressing cell lineages is sufficient to yield learning/memory deficits and hyperactivity with abnormal dentate gyrus granule cell maturation, but not other phenotypic effects. These data provide new clues to in vivo AUTS2-l functions and novel information relevant to genotype-phenotype correlations in the human AUTS2 region.

Keeping the balance: Trade-offs between human brain evolution, autism, and schizophrenia

The unique qualities of the human brain are a product of a complex evolutionary process. Evolution, famously described by François Jacob as a “tinkerer,” builds upon existing genetic elements by modifying and repurposing them for new functions. Genetic changes in DNA may lead to the emergence of new genes or cause altered gene expression patterns. Both gene and regulatory element mutations may lead to new functions. Yet, this process may lead to side-effects. An evolutionary trade-off occurs when an otherwise beneficial change, which is important for evolutionary success and is under strong positive selection, concurrently results in a detrimental change in another trait. Pleiotropy occurs when a gene affects multiple traits. Antagonistic pleiotropy is a phenomenon whereby a genetic variant leads to an increase in fitness at one life-stage or in a specific environment, but simultaneously decreases fitness in another respect. Therefore, it is conceivable that the molecular underpinnings of evolution of highly complex traits, including brain size or cognitive ability, under certain conditions could result in deleterious effects, which would increase the susceptibility to psychiatric or neurodevelopmental diseases. Here, we discuss possible trade-offs and antagonistic pleiotropies between evolutionary change in a gene sequence, dosage or activity and the susceptibility of individuals to autism spectrum disorders and schizophrenia. We present current knowledge about genes and alterations in gene regulatory landscapes, which have likely played a role in establishing human-specific traits and have been implicated in those diseases.

auts2 Features and Expression Are Highly Conserved during Evolution Despite Different Evolutionary Fates Following Whole Genome Duplication

The AUTS2 gene plays major roles during brain development and is associated with various neuropathologies including autism. Data in non-mammalian species are scarce, and the aim of our study was to provide a comprehensive analysis of auts2 evolution in teleost fish, which are widely used for in vivo functional analysis and biomedical purposes. Comparative genomics in 78 species showed that auts2a and auts2b originate from the teleost-specific whole genome duplication (TGD). auts2a, which is highly similar to human AUTS2, was almost systematically retained following TGD. In contrast, auts2b, which encodes for a shorter protein similar to a short human AUTS2 isoform, was lost more frequently and independently during evolution. RNA-seq analysis in 10 species revealed a highly conserved profile with predominant expression of both genes in the embryo, brain, and gonads. Based on protein length, conserved domains, and expression profiles, we speculate that the long human isoform functions were retained by auts2a, while the short isoform functions were retained by auts2a and/or auts2b, depending on the lineage/species. auts2a showed a burst in expression during medaka brain formation, where it was expressed in areas of the brain associated with neurodevelopmental disorders. Together, our data suggest a strong conservation of auts2 functions in vertebrates despite different evolutionary scenarios in teleosts.

The Relationship between the Aberrant Long Non-Coding RNA-Mediated Competitive Endogenous RNA Network and Alzheimer’s Disease Pathogenesis

Alzheimer’s disease (AD) is a common neurodegenerative disorder characterized by cognitive dysfunction. The role of long non-coding RNAs (lncRNAs) with the action of competitive endogenous RNA (ceRNA) in AD remains unclear. The present study aimed to identify significantly differentially expressed lncRNAs (SDELs) and establish lncRNA-associated ceRNA networks via RNA sequencing analysis and a quantitative real-time Polymerase Chain Reaction (qPCR) assay using transgenic mice with five familial AD mutations. A total of 53 SDELs in the cortex and 51 SDELs in the hippocampus were identified, including seven core SDELs common to both regions. The functions and pathways were then investigated through the potential target genes of SDELs via Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses, which indicate biological effects, action distributions, and pathological transductions associated with AD. Based on the ceRNA hypothesis, integrated ceRNA networks in the cortex and hippocampus of lncRNA-miRNA-mRNA were constructed. The core SDEL-mediated ceRNA relationship was established and the expression of these RNAs was verified by qPCR. The results identified lncRNA ENSMUST00000127786 and highlighted miRNAs and mRNAs as potential key mediators in AD. These findings provide AD-derived lncRNA-mediated ceRNA profiles, and further experimental evidence is needed to confirm these identified ceRNA regulatory relationships.

AUTS2 Syndrome: Molecular Mechanisms and Model Systems

AUTS2 syndrome is a genetic disorder that causes intellectual disability, microcephaly, and other phenotypes. Syndrome severity is worse when mutations involve 3' regions (exons 9-19) of the AUTS2 gene. Human AUTS2 protein has two major isoforms, full-length (1259 aa) and C-terminal (711 aa), the latter produced from an alternative transcription start site in exon 9. Structurally, AUTS2 contains the putative "AUTS2 domain" (∼200 aa) conserved among AUTS2 and its ohnologs, fibrosin, and fibrosin-like-1. Also, AUTS2 contains extensive low-complexity sequences and intrinsically disordered regions, features typical of RNA-binding proteins. During development, AUTS2 is expressed by specific progenitor cell and neuron types, including pyramidal neurons and Purkinje cells. AUTS2 localizes mainly in cell nuclei, where it regulates transcription and RNA metabolism. Some studies have detected AUTS2 in neurites, where it may regulate cytoskeletal dynamics. Neurodevelopmental functions of AUTS2 have been studied in diverse model systems. In zebrafish, auts2a morphants displayed microcephaly. In mice, excision of different Auts2 exons (7, 8, or 15) caused distinct phenotypes, variously including neonatal breathing abnormalities, cerebellar hypoplasia, dentate gyrus hypoplasia, EEG abnormalities, and behavioral changes. In mouse embryonic stem cells, AUTS2 could promote or delay neuronal differentiation. Cerebral organoids, derived from an AUTS2 syndrome patient containing a pathogenic missense variant in exon 9, exhibited neocortical growth defects. Emerging technologies for analysis of human cerebral organoids will be increasingly useful for understanding mechanisms underlying AUTS2 syndrome. Questions for future research include whether AUTS2 binds RNA directly, how AUTS2 regulates neurogenesis, and how AUTS2 modulates neural circuit formation.

Auts2 deletion involves in DG hypoplasia and social recognition deficit: The developmental and neural circuit mechanisms

The involvement of genetic risk and the underlying developmental and neural circuit mechanisms in autism-related social deficit are largely unclear. Here, we report that deletion of AUTS2, a high-susceptibility gene of ASDs, caused postnatal dentate gyrus (DG) hypoplasia, which was closely relevant to social recognition deficit. Furthermore, a previously unknown mechanism for neural cell migration in postnatal DG development was identified, in which Auts2-related signaling played a vital role as the transcription repressor. Moreover, the supramammillary nucleus (SuM)-DG-CA3 neural circuit was found to be involved in social recognition and affected in Auts2-deleted mice due to DG hypoplasia. Correction of DG-CA3 synaptic transmission by using a pharmacological approach or chemo/optogenetic activation of the SuM-DG circuit restored the social recognition deficit in Auts2-deleted mice. Our findings demonstrated the vital role of Auts2 in postnatal DG development, and this role was critical for SuM-DG-CA3 neural circuit-mediated social recognition behavior.

AUTS2 Gene: Keys to Understanding the Pathogenesis of Neurodevelopmental Disorders

Neurodevelopmental disorders (NDDs), including autism spectrum disorders (ASD) and intellectual disability (ID), are a large group of neuropsychiatric illnesses that occur during early brain development, resulting in a broad spectrum of syndromes affecting cognition, sociability, and sensory and motor functions. Despite progress in the discovery of various genetic risk factors thanks to the development of novel genomics technologies, the precise pathological mechanisms underlying the onset of NDDs remain elusive owing to the profound genetic and phenotypic heterogeneity of these conditions. Autism susceptibility candidate 2 (AUTS2) has emerged as a crucial gene associated with a wide range of neuropsychological disorders, such as ASD, ID, schizophrenia, and epilepsy. AUTS2 has been shown to be involved in multiple neurodevelopmental processes; in cell nuclei, it acts as a key transcriptional regulator in neurodevelopment, whereas in the cytoplasm, it participates in cerebral corticogenesis, including neuronal migration and neuritogenesis, through the control of cytoskeletal rearrangements. Postnatally, AUTS2 regulates the number of excitatory synapses to maintain the balance between excitation and inhibition in neural circuits. In this review, we summarize the knowledge regarding AUTS2, including its molecular and cellular functions in neurodevelopment, its genetics, and its role in behaviors.