2016Alycia Halladay
Making sense of toxic chemicals and risk of autism
Human neuroimaging studies suggest that aberrant neural connectivity underlies behavioural deficits in autism spectrum disorders (ASDs), but the molecular and neural circuit mechanisms underlying ASDs remain elusive. Here, we describe a complete knockout mouse model of the autism-associated Shank3 gene, with a deletion of exons 4-22 (Δe4-22). Both mGluR5-Homer scaffolds and mGluR5-mediated signalling are selectively altered in striatal neurons. These changes are associated with perturbed function at striatal synapses, abnormal brain morphology, aberrant structural connectivity and ASD-like behaviour. In vivo recording reveals that the cortico-striatal-thalamic circuit is tonically hyperactive in mutants, but becomes hypoactive during social behaviour. Manipulation of mGluR5 activity attenuates excessive grooming and instrumental learning differentially, and rescues impaired striatal synaptic plasticity in Δe4-22(-/-) mice. These findings show that deficiency of Shank3 can impair mGluR5-Homer scaffolding, resulting in cortico-striatal circuit abnormalities that underlie deficits in learning and ASD-like behaviours. These data suggest causal links between genetic, molecular, and circuit mechanisms underlying the pathophysiology of ASDs.
Shank3 is a multi-domain, synaptic scaffolding protein that organizes proteins in the postsynaptic density of excitatory synapses. Clinical studies suggest that ∼ 0.5% of autism spectrum disorder (ASD) cases may involve SHANK3 mutation/deletion. Patients with SHANK3 mutations exhibit deficits in cognition along with delayed/impaired speech/language and repetitive and obsessive/compulsive-like (OCD-like) behaviors. To examine how mutation/deletion of SHANK3 might alter brain function leading to ASD, we have independently created mice with deletion of Shank3 exons 4-9, a region implicated in ASD patients. We find that homozygous deletion of exons 4-9 (Shank3(e4-9) KO) results in loss of the two highest molecular weight isoforms of Shank3 and a significant reduction in other isoforms. Behaviorally, both Shank3(e4-9) heterozygous (HET) and Shank3(e4-9) KO mice display increased repetitive grooming, deficits in novel and spatial object recognition learning and memory, and abnormal ultrasonic vocalizations. Shank3(e4-9) KO mice also display abnormal social interaction when paired with one another. Analysis of synaptosome fractions from striata of Shank3(e4-9) KO mice reveals decreased Homer1b/c, GluA2, and GluA3 expression. Both Shank3(e4-9) HET and KO demonstrated a significant reduction in NMDA/AMPA ratio at excitatory synapses onto striatal medium spiny neurons. Furthermore, Shank3(e4-9) KO mice displayed reduced hippocampal LTP despite normal baseline synaptic transmission. Collectively these behavioral, biochemical and physiological changes suggest Shank3 isoforms have region-specific roles in regulation of AMPAR subunit localization and NMDAR function in the Shank3(e4-9) mutant mouse model of autism.
Multiple hypothalamic neuronal populations that regulate energy balance have been identified. Although hypothalamic glia exist in abundance and form intimate structural connections with neurons, their roles in energy homeostasis are less known. Here we show that selective Ca2+ activation of glia in the mouse arcuate nucleus (ARC) reversibly induces increased food intake while disruption of Ca2+ signaling pathway in ARC glia reduces food intake. The specific activation of ARC glia enhances the activity of agouti-related protein/neuropeptide Y (AgRP/NPY)-expressing neurons but induces no net response in pro-opiomelanocortin (POMC)-expressing neurons. ARC glial activation non-specifically depolarizes both AgRP/NPY and POMC neurons but a strong inhibitory input to POMC neurons balances the excitation. When AgRP/NPY neurons are inactivated, ARC glial activation fails to evoke any significant changes in food intake. Collectively, these results reveal an important role of ARC glia in the regulation of energy homeostasis through its interaction with distinct neuronal subtype-specific pathways.
Keywords: arcuate nucleus; feeding; glial cell; mouse; neuroscience.
Introduction: We recently uncovered a signaling mechanism by which the endocannabinoid anandamide mediates the action of oxytocin, a neuropeptide that is crucial for social behavior, to control social reward. Oxytocin signaling has been implicated in autism spectrum disorder (ASD), and social reward is a key aspect of social functioning that is thought to be disrupted in ASD. Therefore, as a proof of principle for the core component of ASD-social impairment-we tested an endocannabinoid-enhancing compound on two widely studied mouse models of ASD, the BTBR and fmr1-/- (model of Fragile X Syndrome). Methods: We used the established three-chambered social approach test. We specifically increased the activity of anandamide by administering the compound URB597, a selective inhibitor of fatty acid amide hydrolase (FAAH), the hydrolytic enzyme for anandamide. Results: Remarkably, we found that FAAH blockade completely reversed the social impairment in both mouse models. CB1 receptor blockade prevented the prosocial action of FAAH inhibition in BTBR mice. These results were likely independent of effects on anxiety, as FAAH inhibition did not alter the performance of BTBR mice in the elevated plus maze. Conclusions: The results suggest that increasing anandamide activity at CB1 receptors improves ASD-related social impairment and identify FAAH as a novel therapeutic target for ASD.
Keywords: CB1; Fragile X Syndrome; autism spectrum disorders; fatty acid amide hydrolase; social approach.
Summary: Epigenetic mechanisms have the potential to help elucidate biology connecting experiential risk factors with neuropsychiatric disease manifestation. Cross-tissue studies as well as advanced epidemiologic methods should be employed to more effectively conduct neuropsychiatric epigenetic research.
Keywords: Blood; DNA methylation; Epigenetics; Neuropsychiatric disorders; Tissue.