Social Isolation Alters Social and Mating Behavior in the R451C Neuroligin Mouse Model of Autism

A quantitative weight-of-evidence review of preclinical studies examining the potential developmental neurotoxicity of acetaminophen

Acetaminophen [paracetamol; N-acetyl-para-aminophenol (APAP)] is an antipyretic/analgesic commonly used in the treatment of fever and mild to moderate pain, headache, myalgia, and dysmenorrhea. Recent literature has questioned the safety of acetaminophen use during pregnancy, with an emphasis on whether exposure to the developing nervous system results in behavioral changes consistent with autism spectrum disorder (ASD), attention-deficit/hyperactivity disorder (ADHD), and/or other cognitive deficits in the offspring. No previous review has used a fully detailed, quantitative weight-of-evidence (QWoE) approach to critically examine the preclinical acetaminophen data with regards to potential developmental neurotoxicity (DNT). Following regulatory guidance, a QWoE framework using prespecified scoring criteria was developed consistent with previous approaches to characterize potential adverse DNT outcomes with considerations for biological relevance of the response to adverse outcomes (outcome score) and the strength of methods and study design (methods score). Considerations for the methods score included (1) experimental design, (2) details/reliability of measurement(s), (3) data transparency, and (4) translational/methodological relevance. Considerations for the outcome score included response-related (1) statistical significance, (2) dose-response, (3) relevance/reliability/magnitude, (4) plausibility, and (5) translational relevance, including consideration of systemic toxicity/hepatotoxicity and therapeutic and/or non-systemically toxic doses and durations of use. Application of this QWoE framework to the 34 in vivo studies identified that assess the potential DNT of acetaminophen resulted in 188 QWoE entries documented across 11 DNT endpoints: social behavior, stereotypic behavior, behavioral rigidity, attention/impulsivity, hyperactivity, anxiety-like behavior, sensorimotor function, spatial learning/memory, nonspatial learning/memory, neuroanatomy, and neurotransmission. For each endpoint, the mean outcome score and methods score were calculated for total entries and for entries segregated by sex to assist in determining data quality and potential adversity. Informed by all 188 entries, the QWoE analysis demonstrated data of moderate quality showing no consistent evidence of DNT in male and female rodents following exposure to acetaminophen at therapeutic and/or nonsystemically toxic doses. Although some of the DNT endpoints (behavioral rigidity, attention/impulsivity, spatial learning/memory, neuroanatomy, and neurotransmission) generally displayed a more limited dataset and/or relatively lower data quality, similar conclusions were drawn based on results indicating a lack of biological relevance and reliability of reported adverse effects. Overall, this QWoE analysis on the preclinical in vivo data demonstrates no consistent evidence of adverse effects following developmental exposure to acetaminophen at therapeutic and/or non-systemically toxic doses on the structure and function of the nervous system, including neuroanatomical, neurotransmission, and behavioral endpoints.

The role of gene-environment interactions in social dysfunction: Focus on preclinical evidence from mouse studies

Human and animal research has demonstrated that genetic and environmental factors can strongly modulate behavioral function, including the expression of social behaviors and their dysfunctionalities. Several genes have been linked to pathologies characterized by alterations in social behaviors, e.g., aggressive/antisocial personality disorder (ASPD), or autism spectrum disorder (ASD). Environmental stimulation (e.g., physical exercise, environmental enrichment) or adversity (e.g., chronic stress, social isolation) may respectively improve or impair social interactions. While the independent contribution of genetic and environmental factors to social behaviors has been assessed in a variety of human and animal studies, the impact of their interactive effects on social functions has been less extensively investigated. Genetic mutations and environmental changes can indeed influence each other through complex mutual effects, e.g., inducing synergistic, antagonistic or interactive behavioral outcomes. This complexity is difficult to be disentangled in human populations, thus encouraging studies in animal models, especially in the mouse species which is the most suitable for genetic manipulations. Here we review the available preclinical evidence on the impact of gene-environment interactions on social behaviors and their dysfunction, focusing on studies in laboratory mice. We included findings combining naturally occurring mutations, selectively bred or transgenic mice with multiple environmental manipulations, including positive (environmental enrichment, physical exercise) and aversive (social isolation, maternal separation, and stress) experiences. The impact of these results is critically discussed in terms of their generalizability across mouse models and social tests, as well as their implications for human studies on social dysfunction.

Impacts of adverse childhood experiences on individuals with autism spectrum disorder

Individuals with autism spectrum disorder (ASD) are more likely to experience adverse childhood experiences (ACEs) compared with typically developing (TD) individuals, which predisposes them to an elevated risk of mental health issues. This review elucidates the profound impact of ACEs on individuals with ASD by synthesizing findings from a plethora of epidemiologic and biological studies, encompassing genetics, epigenetics, and neuroimaging. Despite the limited number of studies explicitly focusing on this intersection, the extant literature consistently demonstrates that ASD individuals are disproportionately affected by ACEs, leading to significant deterioration in mental health and brain function. Furthermore, the nature and extent of the effects of ACEs appear to diverge between ASD and TD populations, underscoring the necessity for tailored clinical and research approaches. Understanding these complex and intertwined interactions is imperative for advancing both clinical practice and research, with the goal of mitigating the adverse outcomes associated with ACEs in ASD individuals.

Effect of the social environment on olfaction and social skills in wild-type and a mouse model of autism

Autism spectrum disorders (ASD) are complex, polygenic and heterogenous neurodevelopmental conditions. The severity of autism-associated variants is influenced by environmental factors, particularly social experiences during the critical neurodevelopmental period. While early behavioral interventions have shown efficacy in some children with autism, pharmacological support for core features - impairments in social interaction and communication, and stereotyped or restricted behaviors - is currently lacking. In this study, we examined how the social environment influences both wild-type (WT) and Shank3 knockout (KO) mice, a model reflecting core autism-like traits. Our findings revealed that chronic social isolation enhanced social interaction and olfactory neuron responses in WT animals. Furthermore, it restored impairments in social novelty preference and olfactory function, as well as self-grooming in Shank3 KO mice. Conversely, an enriched social environment heightened social interest toward novel conspecifics in WT mice, but elicited the opposite effect in Shank3 KO mice. Notably, Shank3 KO mice displayed distinct social responses when exposed to WT or Shank3 KO mice. These results offer novel insights that could favor the implementation of behavioral interventions and inclusive classroom programs for children with ASD.

Attenuated responses to attention-modulating drugs in the neuroligin-3R451C mouse model of autism

Attention deficits are frequently reported within the clinical autism population. Despite not being a core diagnostic feature, some aetiological theories place atypical attention at the centre of autism development. Drugs used to treat attention dysfunction are therefore increasingly prescribed to autistic patients, though currently off-label with uncertain efficacy. We utilised a rodent-translated touchscreen test of sustained attention in mice carrying an autism-associated R451C mutation in the neuroligin-3 gene (Nlgn3R451C). In doing so, we replicated their cautious but accurate response profile and probed it using two widely prescribed attention-modulating drugs: methylphenidate (MPH) and atomoxetine (ATO). In wild-type mice, acute administration of MPH (3 mg/kg) promoted impulsive responding at the expense of accuracy, while ATO (3 mg/kg) broadly reduced impulsive responding. These drug effects were absent in Nlgn3R451C mice, other than a small reduction in blank touches to the screen following ATO administration. The absence of drug effects in Nlgn3R451C mice likely arises from their altered behavioural baseline and underlying neurobiology, highlighting caveats to the use of classic attention-modulating drugs across disorders and autism subsets. It further suggests that altered dopaminergic and/or norepinephrinergic systems may drive behavioural differences in the Nlgn3R451C mouse model of autism, supporting further targeted investigation.

Mice with an autism-associated R451C mutation in neuroligin-3 show intact attention orienting but atypical responses to methylphenidate and atomoxetine in the mouse-Posner task

RATIONALE

Atypical attention orienting has been associated with some autistic symptoms, but the neural mechanisms remain unclear. The human Posner task, a classic attention orienting paradigm, was recently adapted for use with mice, supporting the investigation of the neurobiological underpinnings of atypical attention orienting in preclinical mouse models.

OBJECTIVE

The current study tested mice expressing the autism-associated R451C gene mutation in neuroligin-3 (NL3) on the mouse-Posner (mPosner) task.

METHODS

NL3R451C and wild-type (WT) mice were trained to respond to a validly or invalidly cued target on a touchscreen. The cue was a peripheral non-predictive flash in the exogenous task and a central spatially predictive image in the endogenous task. The effects of dopaminergic- and noradrenergic-modulating drugs, methylphenidate and atomoxetine, on task performance were assessed.

RESULTS

In both tasks, mice were quicker and more accurate in the validly versus invalidly cued trials, consistent with results in the human Posner task. NL3R451C and WT mice showed similar response times and accuracy but responded differently when treated with methylphenidate and atomoxetine. Methylphenidate impaired exogenous attention disengagement in NL3R451C mice but did not significantly affect WT mice. Atomoxetine impaired endogenous orienting in WT mice but did not significantly affect NL3R451C mice.

CONCLUSIONS

NL3R451C mice demonstrated intact attention orienting but altered responses to the pharmacological manipulation of the dopaminergic and noradrenergic networks. These findings expand our understanding of the NL3R451C mutation by suggesting that this mutation may lead to selective alterations in attentional processes.

Impaired cecal motility and secretion alongside expansion of gut-associated lymphoid tissue in the Nlgn3R451C mouse model of autism

Individuals with Autism Spectrum Disorder (ASD; autism) commonly present with gastrointestinal (GI) illness in addition to core diagnostic behavioural traits. The appendix, or cecum in mice, is important for GI homeostasis via its function as a key site for fermentation and a microbial reservoir. Even so, the role of the appendix and cecum in autism-associated GI symptoms remains uninvestigated. Here, we studied mice with an autism-associated missense mutation in the post-synaptic protein neuroligin-3 (Nlgn3R451C), which impacts brain and enteric neuronal activity. We assessed for changes in cecal motility using a tri-cannulation video-imaging approach in ex vivo preparations from wild-type and Nlgn3R451C mice. We investigated cecal permeability and neurally-evoked secretion in wild-type and Nlgn3R451C tissues using an Ussing chamber set-up. The number of cecal patches in fresh tissue samples were assessed and key immune populations including gut macrophages and dendritic cells were visualised using immunofluorescence. Nlgn3R451C mice displayed accelerated cecal motor complexes and reduced cecal weight in comparison to wildtype littermates. Nlgn3R451C mice also demonstrated reduced neurally-evoked cecal secretion in response to the nicotinic acetylcholine receptor agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP), but permeability was unchanged. We observed an increase in the number of cecal patches in Nlgn3R451C mice, however the cellular morphologies of key immune populations studied were not significantly altered. We show that the R451C nervous system mutation leads to cecal dysmotility, impaired secretion, and neuro-immune alterations. Together, these results suggest that the R451C mutation disrupts the gut-brain axis with GI dysfunction in autism.

Neuroligin-3: A Circuit-Specific Synapse Organizer That Shapes Normal Function and Autism Spectrum Disorder-Associated Dysfunction

Chemical synapses provide a vital foundation for neuron-neuron communication and overall brain function. By tethering closely apposed molecular machinery for presynaptic neurotransmitter release and postsynaptic signal transduction, circuit- and context- specific synaptic properties can drive neuronal computations for animal behavior. Trans-synaptic signaling via synaptic cell adhesion molecules (CAMs) serves as a promising mechanism to generate the molecular diversity of chemical synapses. Neuroligins (Nlgns) were discovered as postsynaptic CAMs that can bind to presynaptic CAMs like Neurexins (Nrxns) at the synaptic cleft. Among the four (Nlgn1-4) or five (Nlgn1-3, Nlgn4X, and Nlgn4Y) isoforms in rodents or humans, respectively, Nlgn3 has a heterogeneous expression and function at particular subsets of chemical synapses and strong association with non-syndromic autism spectrum disorder (ASD). Several lines of evidence have suggested that the unique expression and function of Nlgn3 protein underlie circuit-specific dysfunction characteristic of non-syndromic ASD caused by the disruption of Nlgn3 gene. Furthermore, recent studies have uncovered the molecular mechanism underlying input cell-dependent expression of Nlgn3 protein at hippocampal inhibitory synapses, in which trans-synaptic signaling of specific alternatively spliced isoforms of Nlgn3 and Nrxn plays a critical role. In this review article, we overview the molecular, anatomical, and physiological knowledge about Nlgn3, focusing on the circuit-specific function of mammalian Nlgn3 and its underlying molecular mechanism. This will provide not only new insight into specific Nlgn3-mediated trans-synaptic interactions as molecular codes for synapse specification but also a better understanding of the pathophysiological basis for non-syndromic ASD associated with functional impairment in Nlgn3 gene.

Mice with an autism-associated R451C mutation in neuroligin-3 show a cautious but accurate response style in touchscreen attention tasks

One of the earliest identifiable features of autism spectrum disorder (ASD) is altered attention. Mice expressing the ASD-associated R451C mutation in synaptic adhesion protein neuroligin-3 (NL3) exhibit impaired reciprocal social interactions and repetitive and restrictive behaviours. The role of this mutation in attentional abnormalities has not been established. We assessed attention in male NL3^R451C mice using two well-established tasks in touchscreen chambers. In the 5-choice serial reaction task, rodents were trained to attend to light stimuli that appear in any one of five locations. While no differences between NL3^R451C and WT mice were seen in accuracy or omissions, slower response times and quicker reward collection latencies were seen across all training and probe trials. In the rodent continuous-performance test, animals were required to discriminate, and identify a visual target pattern over multiple distractor stimuli. NL3^R451C mice displayed enhanced ability to attend to stimuli when task-load was low during training and baseline but lost this advantage when difficulty was increased by altering task parameters in probe trials. NL3^R451C mice made less responses to the distractor stimuli, exhibiting lower false alarm rates during all training stages and in probe trials. Slower response times and quicker reward latencies were consistently seen in NL3^R451C mice in the rCPT. Slower response times are a major cognitive phenotype reported in ASD patients and are indicative of slower processing speed. Enhanced attention has been shown in a subset of ASD patients and we have demonstrated this phenotype also exists in the NL3^R451C mouse model.

Proper synaptic adhesion signaling in the control of neural circuit architecture and brain function

Trans-synaptic cell-adhesion molecules are critical for governing various stages of synapse development and specifying neural circuit properties via the formation of multifarious signaling pathways. Recent studies have pinpointed the putative roles of trans-synaptic cell-adhesion molecules in mediating various cognitive functions. Here, we review the literature on the roles of a diverse group of central synaptic organizers, including neurexins (Nrxns), leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs), and their associated binding proteins, in regulating properties of specific type of synapses and neural circuits. In addition, we highlight the findings that aberrant synaptic adhesion signaling leads to alterations in the structures, transmission, and plasticity of specific synapses across diverse brain areas. These results seem to suggest that proper trans-synaptic signaling pathways by Nrxns, LAR-RPTPs, and their interacting network is likely to constitute central molecular complexes that form the basis for cognitive functions, and that these complexes are heterogeneously and complexly disrupted in many neuropsychiatric and neurodevelopmental disorders.

The Role of Synaptic Cell Adhesion Molecules and Associated Scaffolding Proteins in Social Affiliative Behaviors

Social affiliative behaviors-engagement in positive (i.e., nonaggressive) social approach and reciprocal social interactions with a conspecific-comprise a construct within the National Institute of Mental Health Research Domain Criteria Social Processes Domain. These behaviors are disrupted in multiple human neurodevelopmental and neuropsychiatric disorders, such as autism, schizophrenia, social phobia, and others. Human genetic studies have strongly implicated synaptic cell adhesion molecules (sCAMs) in several such disorders that involve marked reductions, or other dysregulations, of social affiliative behaviors. Here, we review the literature on the role of sCAMs in social affiliative behaviors. We integrate findings pertaining to synapse structure and morphology, neurotransmission, postsynaptic signaling pathways, and neural circuitry to propose a multilevel model that addresses the impact of a diverse group of sCAMs, including neurexins, neuroligins, protocadherins, immunoglobulin superfamily proteins, and leucine-rich repeat proteins, as well as their associated scaffolding proteins, including SHANKs and others, on social affiliative behaviors. This review finds that the disruption of sCAMs often manifests in changes in social affiliative behaviors, likely through alterations in synaptic maturity, pruning, and specificity, leading to excitation/inhibition imbalance in several key regions, namely the medial prefrontal cortex, basolateral amygdala, hippocampus, anterior cingulate cortex, and ventral tegmental area. Unraveling the complex network of interacting sCAMs in glutamatergic synapses will be an important strategy for elucidating the mechanisms of social affiliative behaviors and the alteration of these behaviors in many neuropsychiatric and neurodevelopmental disorders.

Autism-associated synaptic mutations impact the gut-brain axis in mice

Interactions between the gut microbiome and the brain affect mood and behaviour in health and disease. Using preclinical animal models, recent discoveries begin to explain how bacteria in the gut influence our mood as well as highlighting new findings relevant to autism. Autism-associated gene mutations known to alter synapse function in the CNS also affect inflammatory response and modify the enteric nervous system resulting in abnormal gastrointestinal motility and structure. Strikingly, these mutations additionally affect the gut microbiome in mice. This review describes the changes in gut physiology and microbiota in mouse models of autism with modified synapse function. The rationale for different regions of the gastrointestinal tract having variable susceptibility to dysfunction is also discussed. To dissect underlying biological mechanisms involving gut-brain axis dysfunction in preclinical models, a range of multidisciplinary approaches are required. This research will provide insights into the role of the gut-brain axis in health and neurodevelopmental disorders including autism.

Evidence for a Contribution of the Nlgn3/Cyfip1/Fmr1 Pathway in the Pathophysiology of Autism Spectrum Disorders

Autism Spectrum Disorders (ASD) are characterized by heterogeneity both in their presentation and their genetic aetiology. In order to discover points of convergence common to different cases of ASD, attempts were made to identify the biological pathways genes associated with ASD contribute to. Many of these genes were found to play a role in neuronal and synaptic development and function. Among these genes are FMR1, CYFIP1 and NLGN3, all present at the synapse and reliably linked to ASD. In this review, we evaluate the evidence for the contribution of these genes to the same biological pathway responsible for the regulation of structural and physiological plasticity. Alterations in dendritic spine density and turnover, as well as long-term depression (LTD), were found in mouse models of mutations of all three genes. This overlap in the phenotypes associated with these mouse models likely arises from the molecular interaction between the protein products of FMR1, CYFIP1, and NLG3. A number of other proteins linked to ASD are also likely to participate in these pathways, resulting in further downstream effects. Overall, a synaptic pathway centered around FMR1, CYFIP1, and NLG3 is likely to contribute to the phenotypes associated with structural and physiological plasticity characteristic of ASD.

Encounters between pairs of unfamiliar dogs in a dog park

INTRODUCTION

The aim of this study was to explore spontaneous social interactions between dyads of unfamiliar adult dogs. Although intraspecific encounters are frequent events in the life of pet dogs, the factors that might influence encounters, such as sex, dyad composition, reproductive status, age, and state of cohabitation (keeping the dogs singly or in groups), remained unexplored.

METHODS

In this study, we assigned unfamiliar, non-aggressive dogs to three types of dyads defined by sex and size. We observed their unrestrained, spontaneous behaviors in an unfamiliar dog park, where only the two dogs, the owners, and experimenter were present.

RESULTS

We found that the dogs, on average, spent only 17% of the time (less than 1 min) in proximity. Sex, dyad composition, reproductive status, and age influenced different aspects of the interactions in dyads. Female dogs were more likely to initiate the first contact in their dyad but later approached the partner less frequently, were less likely to move apart, and displayed less scent marking. Following and moving apart were more frequent in male-male interactions. Neutered dogs spent more time following the other dog and sniffed other dogs more frequently. The time companion dogs spent in proximity and number of approaches decreased with age.

CONCLUSION

The study provides guidance for dog owners about the outcomes of intraspecific encounters based on the dog's age, sex, and reproductive status, as well as the sex of the interacting partner.

Neurodevelopmental synaptopathies: Insights from behaviour in rodent models of synapse gene mutations

The genomic revolution has begun to unveil the enormous complexity and heterogeneity of the genetic basis of neurodevelopmental disorders such as such epilepsy, intellectual disability, autism spectrum disorder and schizophrenia. Increasingly, human mutations in synapse genes are being identified across these disorders. These neurodevelopmental synaptopathies highlight synaptic homeostasis pathways as a convergence point underlying disease mechanisms. Here, we review some of the key pre- and postsynaptic genes in which penetrant human mutations have been identified in neurodevelopmental disorders for which genetic rodent models have been generated. Specifically, we focus on the main behavioural phenotypes that have been documented in these animal models, to consolidate our current understanding of how synapse genes regulate key behavioural and cognitive domains. These studies provide insights into better understanding the basis of the overlapping genetic and cognitive heterogeneity observed in neurodevelopmental disorders.

Synaptic Neurexin Complexes: A Molecular Code for the Logic of Neural Circuits

Synapses are specialized junctions between neurons in brain that transmit and compute information, thereby connecting neurons into millions of overlapping and interdigitated neural circuits. Here, we posit that the establishment, properties, and dynamics of synapses are governed by a molecular logic that is controlled by diverse trans-synaptic signaling molecules. Neurexins, expressed in thousands of alternatively spliced isoforms, are central components of this dynamic code. Presynaptic neurexins regulate synapse properties via differential binding to multifarious postsynaptic ligands, such as neuroligins, cerebellin/GluD complexes, and latrophilins, thereby shaping the input/output relations of their resident neural circuits. Mutations in genes encoding neurexins and their ligands are associated with diverse neuropsychiatric disorders, especially schizophrenia, autism, and Tourette syndrome. Thus, neurexins nucleate an overall trans-synaptic signaling network that controls synapse properties, which thereby determines the precise responses of synapses to spike patterns in a neuron and circuit and which is vulnerable to impairments in neuropsychiatric disorders.