Shankopathies in the Developing Brain in Autism Spectrum Disorders

Targeting TrkB-PSD-95 coupling to mitigate neurological disorders

Tropomyosin receptor kinase B (TrkB) signaling plays a pivotal role in dendritic growth and dendritic spine formation to promote learning and memory. The activity-dependent release of brain-derived neurotrophic factor at synapses binds to pre- or postsynaptic TrkB resulting in the strengthening of synapses, reflected by long-term potentiation. Postsynaptically, the association of postsynaptic density protein-95 with TrkB enhances phospholipase Cγ-Ca2+/calmodulin-dependent protein kinase II and phosphatidylinositol 3-kinase-mechanistic target of rapamycin signaling required for long-term potentiation. In this review, we discuss TrkB-postsynaptic density protein-95 coupling as a promising strategy to magnify brain-derived neurotrophic factor signaling towards the development of novel therapeutics for specific neurological disorders. A reduction of TrkB signaling has been observed in neurodegenerative disorders, such as Alzheimer's disease and Huntington's disease, and enhancement of postsynaptic density protein-95 association with TrkB signaling could mitigate the observed deficiency of neuronal connectivity in schizophrenia and depression. Treatment with brain-derived neurotrophic factor is problematic, due to poor pharmacokinetics, low brain penetration, and side effects resulting from activation of the p75 neurotrophin receptor or the truncated TrkB.T1 isoform. Although TrkB agonists and antibodies that activate TrkB are being intensively investigated, they cannot distinguish the multiple human TrkB splicing isoforms or cell type-specific functions. Targeting TrkB-postsynaptic density protein-95 coupling provides an alternative approach to specifically boost TrkB signaling at localized synaptic sites versus global stimulation that risks many adverse side effects.

Increased gene dosage of RFWD2 causes autistic-like behaviors and aberrant synaptic formation and function in mice

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impaired social interactions, communication deficits and repetitive behaviors. A study of autistic human subjects has identified RFWD2 as a susceptibility gene for autism, and autistic patients have 3 copies of the RFWD2 gene. The role of RFWD2 as an E3 ligase in neuronal functions, and its contribution to the pathophysiology of ASD, remain unknown. We generated RFWD2 knockin mice to model the human autistic condition of high gene dosage of RFWD2. We found that heterozygous knockin (Rfwd2+/-) male mice exhibited the core symptoms of autism. Rfwd2+/- male mice showed deficits in social interaction and communication, increased repetitive and anxiety-like behavior, and spatial memory deficits, whereas Rfwd2+/- female mice showed subtle deficits in social communication and spatial memory but were normal in anxiety-like, repetitive, and social behaviors. These autistic-like behaviors in males were accompanied by a reduction in dendritic spine density and abnormal synaptic function on layer II/III pyramidal neurons in the prelimbic area of the medial prefrontal cortex (mPFC), as well as decreased expression of synaptic proteins. Impaired social behaviors in Rfwd2+/- male mice were rescued by the expression of ETV5, one of the major substrates of RFWD2, in the mPFC. These findings indicate an important role of RFWD2 in the pathogenesis of autism.

From synapses to circuits: What mouse models have taught us about how autism spectrum disorder impacts hippocampal function

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder that impacts a variety of cognitive and behavioral domains. While a genetic component of ASD has been well-established, none of the numerous syndromic genes identified in humans accounts for more than 1% of the clinical patients. Due to this large number of target genes, numerous mouse models of the disorder have been generated. However, the focus on distinct brain circuits, behavioral phenotypes and diverse experimental approaches has made it difficult to synthesize the overwhelming number of model animal studies into concrete throughlines that connect the data across levels of investigation. Here we chose to focus on one circuit, the hippocampus, and one hypothesis, a shift in excitatory/inhibitory balance, to examine, from the level of the tripartite synapse up to the level of in vivo circuit activity, the key commonalities across disparate models that can illustrate a path towards a better mechanistic understanding of ASD's impact on hippocampal circuit function.

"Comparative evaluation of different chemical agents induced Autism Spectrum Disorder in experimental Wistar rats"

Autism Spectrum Disorder (ASD) is a complex neurodevelopmental condition with uncertain etiology and pathophysiology. Several studies revealed that the commonly used animal models like Valproic Acid (VPA) and Propionic Acid (PPA) do not precisely represent the disease as the human patient does. The current study was conducted on different chemically (VPA, PPA, Poly I:C, Dioxin (2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)) & Chlorpyrifos (CPF)) induced ASD-like animal models and validated the best suitable experimental animal model, which would closely resemble with clinical features of the ASD. This validated model might help to explore the pathophysiology of ASD. This study included rat pups prenatally exposed to VPA, PPA, Poly I:C, Dioxin & CPF within GD9 to GD15 doses. The model groups were validated through developmental and behavioral parameters, Gene Expressions, Oxidative Stress, and Pro-inflammatory and Anti-inflammatory cytokines levels. Developmental and neurobehavioral parameters showed significant changes in model groups compared to the control. In oxidative stress parameters and neuro-inflammatory cytokines levels, model groups exhibited high oxidative stress and neuro-inflammation compared to control groups. Gene expression profile of ASD-related genes showed significant downregulation in model groups compared to the control group. Moreover, the Poly I:C group showed more significant results than other model groups. The comparison of available ASD-like experimental animal models showed that the Poly I:C induced model represented the exact pathophysiology of ASD as the human patient does. Poly I:C was reported in the maternal immune system activation via the inflammatory cytokines pathway, altering embryonic development and causing ASD in neonates.

Inhibitory dysfunction and social processing difficulties in autism: A comprehensive narrative review

The primary inhibitory neurotransmitter γ-aminobutyric acid (GABA) has a prominent role in regulating neural development and function, with disruption to GABAergic signalling linked to behavioural phenotypes associated with neurodevelopmental disorders, particularly autism. Such neurochemical disruption, likely resulting from diverse genetic and molecular mechanisms, particularly during early development, can subsequently affect the cellular balance of excitation and inhibition in neuronal circuits, which may account for the social processing difficulties observed in autism and related conditions. This comprehensive narrative review integrates diverse streams of research from several disciplines, including molecular neurobiology, genetics, epigenetics, and systems neuroscience. In so doing it aims to elucidate the relevance of inhibitory dysfunction to autism, with specific focus on social processing difficulties that represent a core feature of this disorder. Many of the social processing difficulties experienced in autism have been linked to higher levels of the excitatory neurotransmitter glutamate and/or lower levels of inhibitory GABA. While current therapeutic options for social difficulties in autism are largely limited to behavioural interventions, this review highlights the psychopharmacological studies that explore the utility of GABA modulation in alleviating such difficulties.

Transmembrane protein 184B (TMEM184B) promotes expression of synaptic gene networks in the mouse hippocampus

In Alzheimer's Disease (AD) and other dementias, hippocampal synaptic dysfunction and loss contribute to the progression of memory impairment. Recent analysis of human AD transcriptomes has provided a list of gene candidates that may serve as drivers of disease. One such candidate is the membrane protein TMEM184B. To evaluate whether TMEM184B contributes to neurological impairment, we asked whether loss of TMEM184B in mice causes gene expression or behavior alterations, focusing on the hippocampus. Because one major risk factor for AD is age, we compared young adult (5-month-old) and aged (15-month-old) wild type and Tmem184b-mutant mice to assess the dual contributions of age and genotype. TMEM184B loss altered expression of pre- and post-synaptic transcripts by 5 months and continued through 15 months, specifically affecting genes involved in synapse assembly and neural development. Wnt-activated enhancer elements were enriched among differentially expressed genes, suggesting an intersection with this pathway. Few differences existed between young adult and aged mutants, suggesting that transcriptional effects of TMEM184B loss are relatively constant. To understand how TMEM184B disruption may impact behaviors, we evaluated memory using the novel object recognition test and anxiety using the elevated plus maze. Young adult Tmem184b-mutant mice show normal object discrimination, suggesting a lack of memory impairment at this age. However, mutant mice showed decreased anxiety, a phenotype seen in some neurodevelopmental disorders. Taken together, our data suggest that TMEM184B is required for proper synaptic gene expression and anxiety-related behavior and is more likely to be linked to neurodevelopmental disorders than to dementia.

The Central Noradrenergic System in Neurodevelopmental Disorders: Merging Experimental and Clinical Evidence

The aim of this article is to highlight the potential role of the locus-coeruleus-noradrenergic (LC-NA) system in neurodevelopmental disorders (NdDs). The LC is the main brain noradrenergic nucleus, key in the regulation of arousal, attention, and stress response, and its early maturation and sensitivity to perinatal damage make it an interesting target for translational research. Clinical data shows the involvement of the LC-NA system in several NdDs, suggesting a pathogenetic role in the development of such disorders. In this context, a new neuroimaging tool, LC Magnetic Resonance Imaging (MRI), has been developed to visualize the LC in vivo and assess its integrity, which could be a valuable tool for exploring morphological alterations in NdD in vivo in humans. New animal models may be used to test the contribution of the LC-NA system to the pathogenic pathways of NdD and to evaluate the efficacy of NA-targeting drugs. In this narrative review, we provide an overview of how the LC-NA system may represent a common pathophysiological and pathogenic mechanism in NdD and a reliable target for symptomatic and disease-modifying drugs. Further research is needed to fully understand the interplay between the LC-NA system and NdD.

Super-resolution fluorescence microscopic imaging in pathogenesis and drug treatment of neurological disease

Since super-resolution fluorescence microscopic technology breaks the diffraction limit that has existed for a long time in optical imaging, it can observe the process of synapses formed between nerve cells and the protein aggregation related to neurological disease. Thus, super-resolution fluorescence microscopic imaging has significantly impacted several industries, including drug development and pathogenesis research, and it is anticipated that it will significantly alter the future of life science research. Here, we focus on several typical super-resolution fluorescence microscopic technologies, introducing their benefits and drawbacks, as well as applications in several common neurological diseases, in the hope that their services will be expanded and improved in the pathogenesis and drug treatment of neurological diseases.

Targeting Shank3 deficiency and paresthesia in autism spectrum disorder: A brief review

Autism spectrum disorder (ASD) includes a group of multifactorial neurodevelopmental disorders characterized by impaired social communication, social interaction, and repetitive behaviors. Several studies have shown an association between cases of ASD and mutations in the genes of SH3 and multiple ankyrin repeat domain protein 3 (SHANK3). These genes encode many cell adhesion molecules, scaffold proteins, and proteins involved in synaptic transcription, protein synthesis, and degradation. They have a profound impact on all aspects of synaptic transmission and plasticity, including synapse formation and degeneration, suggesting that the pathogenesis of ASD may be partially attributable to synaptic dysfunction. In this review, we summarize the mechanism of synapses related to Shank3 in ASD. We also discuss the molecular, cellular, and functional studies of experimental models of ASD and current autism treatment methods targeting related proteins.

Glutamatergic synapse in autism: a complex story for a complex disorder

Autism spectrum disorder (ASD) is a neurodevelopmental disorder whose pathophysiological mechanisms are still unclear. Hypotheses suggest a role for glutamate dysfunctions in ASD development, but clinical studies investigating brain and peripheral glutamate levels showed heterogenous results leading to hypo- and hyper-glutamatergic hypotheses of ASD. Recently, studies proposed the implication of elevated mGluR5 densities in brain areas in the pathophysiology of ASD. Thus, our objective was to characterize glutamate dysfunctions in adult subjects with ASD by quantifying (1) glutamate levels in the cingulate cortex and periphery using proton magnetic resonance spectroscopy and metabolomics, and (2) mGluR5 brain density in this population and in a validated animal model of ASD (prenatal exposure to valproate) at developmental stages corresponding to childhood and adolescence in humans using positron emission tomography. No modifications in cingulate Glu levels were observed between individuals with ASD and controls further supporting the difficulty to evaluate modifications in excitatory transmission using spectroscopy in this population, and the complexity of its glutamate-related changes. Our imaging results showed an overall increased density in mGluR5 in adults with ASD, that was only observed mostly subcortically in adolescent male rats prenatally exposed to valproic acid, and not detected in the stage corresponding to childhood in the same animals. This suggest that clinical changes in mGluR5 density could reflect the adaptation of the glutamatergic dysfunctions occurring earlier rather than being key to the pathophysiology of ASD.

The emerging roles of Shank3 in cardiac function and dysfunction

Shank3 is a member of the Shank family proteins (Shank1-3), which are abundantly present in the postsynaptic density (PSD) of neuronal excitatory synapses. As a core scaffold in the PSD, Shank3 plays a critical role in organizing the macromolecular complex, ensuring proper synaptic development and function. Clinically, various mutations of the SHANK3 gene are causally associated with brain disorders such as autism spectrum disorders and schizophrenia. However, recent in vitro and in vivo functional studies and expression profiling in various tissues and cell types suggest that Shank3 also plays a role in cardiac function and dysfunction. For example, Shank3 interacts with phospholipase Cβ1b (PLCβ1b) in cardiomyocytes, regulating its localization to the sarcolemma and its role in mediating Gq-induced signaling. In addition, changes in cardiac morphology and function associated with myocardial infarction and aging have been investigated in a few Shank3 mutant mouse models. This review highlights these results and potential underlying mechanisms, and predicts additional molecular functions of Shank3 based on its protein interactors in the PSD, which are also highly expressed and function in the heart. Finally, we provide perspectives and possible directions for future studies to better understand the roles of Shank3 in the heart.

Striatal increase of dopamine receptor 2 density in idiopathic and syndromic mouse models of autism spectrum disorder

Autism spectrum disorder (ASD) comprises a wide range of neurodevelopmental phenotypes united by impaired social interaction and repetitive behavior. Environmental and genetic factors are associated with the pathogenesis of ASD, while other cases are classified as idiopathic. The dopaminergic system has a profound impact in the modulation of motor and reward-motivated behaviors, and defects in dopaminergic circuits are implicated in ASD. In our study, we compare three well-established mouse models of ASD, one idiopathic, the BTBR strain, and two syndromic, Fmr1 and Shank3 mutants. In these models, and in humans with ASD, alterations in dopaminergic metabolism and neurotransmission were highlighted. Still, accurate knowledge about the distribution of dopamine receptor densities in the basal ganglia is lacking. Using receptor autoradiography, we describe the neuroanatomical distribution of D1 and D2 receptors in dorsal and ventral striatum at late infancy and adulthood in the above-mentioned models. We show that D1 receptor binding density is different among the models irrespective of the region. A significant convergence in increased D2 receptor binding density in the ventral striatum at adulthood becomes apparent in BTBR and Shank3 lines, and a similar trend was observed in the Fmr1 line. Altogether, our results confirm the involvement of the dopaminergic system, showing defined alterations in dopamine receptor binding density in three well-established ASD lines, which may provide a plausible explanation to some of the prevalent traits of ASD. Moreover, our study provides a neuroanatomical framework to explain the utilization of D2-acting drugs such as Risperidone and Aripiprazole in ASD.

The Effects of Prenatal and Postnatal Exposure to 50-Hz and 3 mT Electromagnetic Field on Rat Testicular Development

Background and objectives: It has been shown that electromagnetic fields (EMFs) have negative effects on the reproductive system. The biological effects of EMF on the male reproductive system are controversial and vary depending on the frequency and exposure time. Although a limited number of studies have focused on the structural and functional effects of EMF, the effects of prenatal and postnatal EMF exposure on testes are not clear. We aimed to investigate the effects of 50-Hz, 3-mT EMF exposure (5 days/wk, 4 h/day) during pre- and postnatal periods on testis development. Materials and Methods: Pups from three groups of Sprague-Dawley pregnant rats were used: Sham, EMF-28 (EMF-exposure applied during pregnancy and until postnatal day 28), EMF-42 (EMF-exposure applied during pregnancy and until postnatal day 42). The testis tissues and blood samples of male offspring were collected on the postnatal day 42. Results: Morphometric analyses showed a decrease in seminiferous tubule diameter as a result of testicular degeneration in the EMF-42 group. Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) levels were decreased in the EMF-42 group. Lipid peroxidation levels were increased in both EMF groups, while antioxidant levels were decreased only in the EMF-28 group. We found decreased levels of vascular endothelial growth factor (VEGF) and insulin-like growth factor-1 (IGF1) in the EMF-42 group, and decreased levels of the SRC homology 3 (SH3) and multiple ankyrin repeat domain (SHANK3) in the EMF-28 group in the testis tissue. Conclusions: EMF exposure during pre- and postnatal periods may cause deterioration in the structure and function of testis and decrease in growing factors that would affect testicular functions in male rat pups. In addition to the oxidative stress observed in testis, decreased SHANK3, VEGF, and IGF1 protein levels suggests that these proteins may be mediators in testis affected by EMF exposure. This study shows that EMF exposure during embryonic development and adolescence can cause apoptosis and structural changes in the testis.

The Role of Zinc and NMDA Receptors in Autism Spectrum Disorders

NMDA-type glutamate receptors are critical for synaptic plasticity in the central nervous system. Their unique properties and age-dependent arrangement of subunit types underpin their role as a coincidence detector of pre- and postsynaptic activity during brain development and maturation. NMDAR function is highly modulated by zinc, which is co-released with glutamate and concentrates in postsynaptic spines. Both NMDARs and zinc have been strongly linked to autism spectrum disorders (ASDs), suggesting that NMDARs are an important player in the beneficial effects observed with zinc in both animal models and children with ASDs. Significant evidence is emerging that these beneficial effects occur via zinc-dependent regulation of SHANK proteins, which form the backbone of the postsynaptic density. For example, dietary zinc supplementation enhances SHANK2 or SHANK3 synaptic recruitment and rescues NMDAR deficits and hypofunction in Shank3^ex13-16-/- and Tbr1^+/- ASD mice. Across multiple studies, synaptic changes occur in parallel with a reversal of ASD-associated behaviours, highlighting the zinc-dependent regulation of NMDARs and glutamatergic synapses as therapeutic targets for severe forms of ASDs, either pre- or postnatally. The data from rodent models set a strong foundation for future translational studies in human cells and people affected by ASDs.

Exposure to Antibiotics and Neurodevelopmental Disorders: Could Probiotics Modulate the Gut-Brain Axis?

In order to develop properly, the brain requires the intricate interconnection of genetic factors and pre-and postnatal environmental events. The gut-brain axis has recently raised considerable interest for its involvement in regulating the development and functioning of the brain. Consequently, alterations in the gut microbiota composition, due to antibiotic administration, could favor the onset of neurodevelopmental disorders. Literature data suggest that the modulation of gut microbiota is often altered in individuals affected by neurodevelopmental disorders. It has been shown in animal studies that metabolites released by an imbalanced gut-brain axis, leads to alterations in brain function and deficits in social behavior. Here, we report the potential effects of antibiotic administration, before and after birth, in relation to the risk of developing neurodevelopmental disorders. We also review the potential role of probiotics in treating gastrointestinal disorders associated with gut dysbiosis after antibiotic administration, and their possible effect in ameliorating neurodevelopmental disorder symptoms.

Altered synaptic protein expression, aberrant spine morphology, and impaired spatial memory in Dlgap2 mutant mice, a genetic model of autism spectrum disorder

A microdeletion of approximately 2.4 Mb at the 8p23 terminal region has been identified in a Taiwanese autistic boy. Among the products transcribed/translated from genes mapped in this region, the reduction of DLGAP2, a postsynaptic scaffold protein, might be involved in the pathogenesis of autism spectrum disorder (ASD). DLGAP2 protein was detected in the hippocampus yet abolished in homozygous Dlgap2 knockout (Dlgap2 KO) mice. In this study, we characterized the hippocampal phenotypes in Dlgap2 mutant mice. Dlgap2 KO mice exhibited impaired spatial memory, indicating poor hippocampal function in the absence of DLGAP2. Aberrant expressions of postsynaptic proteins, including PSD95, SHANK3, HOMER1, GluN2A, GluR2, mGluR1, mGluR5, βCAMKII, ERK1/2, ARC, BDNF, were noticed in Dlgap2 mutant mice. Further, the spine density was increased in Dlgap2 KO mice, while the ratio of mushroom-type spines was decreased. We also observed a thinner postsynaptic density thickness in Dlgap2 KO mice at the ultrastructural level. These structural changes found in the hippocampus of Dlgap2 KO mice might be linked to impaired hippocampus-related cognitive functions such as spatial memory. Mice with Dlgap2 deficiency, showing signs of intellectual disability, a common co-occurring condition in patients with ASD, could be a promising animal model which may advance our understanding of ASD.