Cortical circuit and behavioural pathophysiology in rodent models of SYNGAP1 haploinsufficiency
MetadataShow full item record
SYNGAP1 haploinsufficiency is one of the most common monogenic causes of nonsyndromic moderate to severe intellectual disability (NSID) and autism (Hamdan et al., 2009; Pinto et al., 2010). De novo truncating or frameshift mutations in the SYNGAP1 gene lead to the loss of the encoded protein Synaptic GTPase activating protein (SynGAP), one of the most abundant of postsynaptic proteins (Hamdan et al., 2011). SynGAP, present at excitatory and inhibitory synapses (Kim et al., 1998), acts as a key regulator of highly conserved signaling pathways linked to AMPA- and NMDA-receptor dependent plasticity at the post synaptic density (Krapivisky et al., 2004; Vazquez et al., 2004). The Syngap mouse model has been extensively used to understand the pathophysiology underlying abnormal SynGAP-mediated signaling. Syngap heterozygous (het) mice demonstrate a range of physiological and behavioural abnormalities from development to adulthood (Komiyama et al., 2002; Muhia et al., 2010). However, recent advances in techniques for genome manipulation have allowed for the generation of rat models of neurodevelopmental disorders, including Syngap; enabling phenotypes to be validated across species and to address cognitive and social dysfunction, using paradigms that are more difficult to assess in mice. In this study, we examined the pathophysiology associated with a heterozygous deletion of the C2 and catalytic GAP domain of the protein, in Long-Evans rats (het). In contrast with het mice, het rats do not present with hyperactivity and can be habituated to an open field environment. To examine associative recognition memory, we tested the rats in five spontaneous exploration tasks for short-term and long-term memory, object-recognition (OR), object-location (OL), object-place (OP), object-context (OC) and object-place-context (OPC). Both groups were able to perform short-term memory tasks, but only wild type rats performed above chance in OL with a 24hour delay, suggesting deficits in long- term spatial memory. We also tested if partial loss of the GAP domain in SynGAP affects social behaviour in rats and we found that het rats exhibited impaired short- term social memory, with no signs of social isolation. These findings do not fully recapitulate previous abnormalities reported in the mouse model of SYNGAP1 haploinsufficiency, suggesting that some key behavioural phenotypes may be species-specific. Furthermore, based on physiological deficits that Syngap het mice exhibit, such as alterations in mEPSC/mIPSC amplitude and frequency and evoked cortical hyperexcitability in vitro (Guo et al., 2009; Ozkan et al., 2014), we also aimed to test if in vivo neuronal activity and circuit properties are altered. Using two-photon calcium imaging in awake mice, we focused on two areas of the cortex; a primary sensory area, the binocular region of the visual cortex (V1), and an association area, the medial posterior parietal cortex (PPC). Both areas have been found to maintain activity during visual discrimination tasks but to present with divergent activity trajectories (Harvey et al., 2012; Goard et al., 2016). We found preliminary evidence that neurons in layer 2-3 of the PPC of Syngap mice are hypoactive in basal conditions when animals are still in the dark, compared to wild type controls. When we assessed whether that changes when animals are running, we found that during locomotion neurons of both genotypes increase their activity, consistent with previous findings in wild type mice (McGinley et al., 2015; Pakan et al., 2016). However, this response gain is exaggerated in Syngap het neurons of the PPC. In contrast to above findings in PPC, results in V1 show that layer 2-3 neurons are hyperactive during both behavioural states, suggesting seemingly different computations of these two cortical areas. This work provides the first evidence for a dysregulated neuronal circuit in vivo in both visual and parietal cortex of Syngap mice, two areas critical for sensory processing that has been found to be affected in individuals with NSID and autism (Joosten and Bundy, 2010). We also provide first evidence of the effect of loss of SynGAP activity in behaviour of rats, complimenting existing data in the literature in a species-specific manner and providing greater insight into sensory and cognitive dysfunction associated with dysregulation in SynGAP-mediated signaling.