Human chromosome 16p11.2 microdeletion is among the most common gene copy number variations (CNVs) known to confer risk for intellectual disability (ID) and autism spectrum disorder (ASD) and affects an estimated 3 in 10 000 people. Caused by a single copy deletion of ~27 genes, 16p11.2 microdeletion syndrome is characterized by ID, impaired language, communication and socialization skills, and ASD. Studies in animal models where a single copy of the syntenic 16p11.2 region has been deleted have revealed morphological, behavioral, and electrophysiological abnormalities. Previous studies suggested the possibility of some overlap in the mechanisms of pathophysiology in 16p11.2 microdeletion syndrome and fragile X syndrome. Improvements in fragile X phenotypes have been observed following chronic treatment with R-baclofen, a selective agonist of GABAB receptors. We were therefore motivated to investigate the effects of chronic oral R-baclofen administration in two independently generated mouse models of 16p11.2 microdeletion syndrome. In studies performed across two independent laboratories, we found that chronic activation of GABAB receptors improved performance on a series of cognitive and social tasks known to be impaired in two different 16p11.2 deletion mouse models. Our findings suggest that R-baclofen may have clinical utility for some of the core symptoms of human 16p11.2 microdeletion syndrome.

Summary Synaptic protein synthesis is essential for modification of the brain by experience and is aberrant in several genetically-defined disorders, notably fragile X, a heritable cause of autism and intellectual disability. Neural activity directs local protein synthesis via activation of metabotropic glutamate receptor 5 (mGlu5), yet how mGlu5 couples to the intracellular signaling pathways that regulate mRNA translation is poorly understood. Here, we provide evidence that β-arrestin2 mediates mGlu5-stimulated protein synthesis in the hippocampus and show that genetic reduction of β-arrestin2 corrects aberrant synaptic plasticity and cognition in the Fmr1−/y mouse model of fragile X. Importantly, reducing β-arrestin2 does not induce psychotomimetic activity associated with full mGlu5 inhibitors, and does not affect Gq signaling. Thus, in addition to identifying a key requirement for mGlu5-stimulated protein synthesis, these data suggest that β-arrestin2-biased negative modulators of mGlu5 offer significant advantages over first-generation inhibitors for the treatment of fragile X and related disorders.

SynGAP is a postsynaptic density (PSD) protein that binds to PDZ domains of the scaffold protein PSD-95. We previously reported that heterozygous deletion of Syngap1 in mice is correlated with increased steady-state levels of other key PSD proteins that bind PSD-95, although the level of PSD-95 remains constant (Walkup et al., 2016). For example, the ratio to PSD-95 of Transmembrane AMPA-Receptor-associated Proteins (TARPs), which mediate binding of AMPA-type glutamate receptors to PSD-95, was increased in young Syngap1+/-mice. Here we show that only females and not males show a highly significant correlation between an increase in TARP and a decrease in synGAP in the PSDs of Syngap1+/-rodents. The data reveal a sex difference in the adaptation of the PSD scaffold to synGAP haploinsufficiency.

2SynGAP is a postsynaptic density (PSD) protein that binds to PDZ domains of the scaffold protein PSD-3 95. We previously reported that heterozygous deletion of synGAP in mice is correlated with increased 4 steady-state levels of other key PSD proteins that bind PSD-95, although the level of PSD-95 remains 5 constant (Walkup et al., 2016). For example, the ratio to PSD-95 of Transmembrane AMPA-Receptor-6 associated Proteins (TARPs), which mediate binding of AMPA-type glutamate receptors to PSD-95, was 7 increased in young synGAP +/-mice. Here we show that a highly significant increase in TARP in the PSDs 8 of young synGAP +/-rodents is present only in females and not in males. The data reveal a sex difference in 9 the adaptation of the PSD scaffold to synGAP heterozygosity. 10 42 PSD fractions prepared from pooled forebrains of synGAP +/-(HET) and of wild type (WT) mice (5 WT 43 males and 1 WT female; 4 HET males and 2 HET females, ranging in age from 7.9 weeks to 12.4 weeks) 44The ratio of synGAP to PSD-95 was 25% reduced in PSDs from the HET mice compared to WT. As we 45 had predicted, the average ratio of TARPs to PSD-95 showed a small (12%) but significant increase in 46 PSDs from the HET animals compared to WT (Walkup et al., 2016). We also found a small but 47 significant increase in the ratio of LRRTM2 (14%) and neuroligin-2 (9%) to PSD-95. The ratio of 48 neuroligin-1 to PSD-95 was unchanged. 49To attempt to reproduce these findings with a larger data set and to examine the biological variability in 50 the relationship between the amount of synGAP and the amount of TARP in PSD fractions, we devised a 51 method to isolate PSD fractions from individual WT and HET rodents and we measured the ratios of 52 synGAP and TARPs to PSD-95 in each individual PSD fraction. We also measured the ratios of GluN2B, 53 neuroligin1, and neuroligin2 to PSD-95. We were able to study mice and rat HETs by using a new rat 54 mutant in which one copy of the synGAP gene is inactivated by the CRISPR-Cas9 method. 55When the data was averaged over all WT and HET animals in this large data set, the TARP/PSD-95 56 ratio in PSDs was not different between WT and HET animals. Nonetheless, we confirmed our earlier 57 conclusion by using the more powerful Spearman's rank correlation coefficient to show a statistically 58 significant inverse correlation between the synGAP/PSD-95 and TARPs/PSD-95 ratios in individual PSD 59 fractions from HET rodents. However, to our surprise, when data from HET rodents are separated by sex, 60 the inverse correlation is present only in females and not in males. The large and highly significant 93 ( Fig. 2 A and B), except for 7 wk old female mice in which the ratio of TARPs to PSD-95 was 94 significantly reduced compared to WT. This value may have been influenced by lower overall expression 95 of TARPS in 7 week old mice. We also noted more variability in the averaged ratios of TARP to PSD-95 96 for females ( Fig 2B, right) compared to males (Fig. 2B, middle). Taken as a whole, it appears that the 97 average...

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