Pathogenic SYNGAP1 mutations impair cognitive development by disrupting maturation of dendritic spine synapses.

Mutations that cause intellectual disability (ID) and autism spectrum disorder (ASD) are commonly found in genes that encode for synaptic proteins. However, it remains unclear how mutations that disrupt synapse function impact intellectual ability. In the SYNGAP1 mouse model of ID/ASD, we found that dendritic spine synapses develop prematurely during the early postnatal period. Premature spine maturation dramatically enhanced excitability in the developing hippocampus, which corresponded with the emergence of behavioral abnormalities. Inducing SYNGAP1 mutations after critical developmental windows closed had minimal impact on spine synapse function, whereas repairing these pathogenic mutations in adulthood did not improve behavior and cognition. These data demonstrate that SynGAP protein acts as a critical developmental repressor of neural excitability that promotes the development of life-long cognitive abilities. We propose that the pace of dendritic spine synapse maturation in early life is a critical determinant of normal intellectual development.

Input-specific regulation of hippocampal circuit maturation by non-muscle myosin IIB.

Myh9 and Myh10, which encode two major isoforms of non-muscle myosin II expressed in the brain, have emerged as risk factors for developmental brain disorders. Myosin II motors regulate neuronal cytoskeletal dynamics leading to optimization of synaptic plasticity and memory formation. However, the role of these motor complexes in brain development remains poorly understood. Here, we disrupted the in vivo expression of Myh9 and/or Myh10 in developing hippocampal neurons to determine how these motors contribute to circuit maturation in this brain area important for cognition. We found that Myh10 ablation in early postnatal, but not mature, CA1 pyramidal neurons reduced excitatory synaptic function in the Schaffer collateral pathway, whereas more distal inputs to CA1 neurons were relatively unaffected. Myh10 ablation in young neurons also selectively impaired the elongation of oblique dendrites that receive Schaffer collateral inputs, whereas the structure of distal dendrites was normal. We observed normal spine density and spontaneous excitatory currents in these neurons, indicating that Myh10 KO impaired proximal pathway synaptic maturation through disruptions to dendritic development rather than post-synaptic strength or spine morphogenesis. To address possible redundancy and/or compensation by other Myosin II motors expressed in neurons, we performed similar experiments in Myh9 null neurons. In contrast to findings in Myh10 mutants, evoked synaptic function in young Myh9 KO hippocampal neurons was normal. Data obtained from double Myh9/Myh10 KO neurons largely resembled the MyH10 KO synaptic phenotype. These data indicate that Myosin IIB is a key molecular factor that guides input-specific circuit maturation in the developing hippocampus. Non-muscle myosin II is an actin binding protein with three isoforms in the brain (IIA, IIB and IIC) encoded by the myh9, myh10, and myh14 genes in mice, respectively. We have studied the structure and the function of hippocampal CA1 neurons missing NMIIB and/or NMIIA proteins at different times during development. We have discovered that NMIIB is the major isoform regulating Schaffer collateral inputs, and that this regulation is restricted to early postnatal development.

SYNGAP1 links the maturation rate of excitatory synapses to the duration of critical-period synaptic plasticity.

Critical periods of developmental plasticity contribute to the refinement of neural connections that broadly shape brain development. These windows of plasticity are thought to be important for the maturation of perception, language, and cognition. Synaptic properties in cortical regions that underlie critical periods influence the onset and duration of windows, although it remains unclear how mechanisms that shape synapse development alter critical-period properties. In this study, we demonstrate that inactivation of a single copy of syngap1, which causes a surprisingly common form of sporadic, non-syndromic intellectual disability with autism in humans, induced widespread early functional maturation of excitatory connections in the mouse neocortex. This accelerated functional maturation was observed across distinct areas and layers of neocortex and directly influenced the duration of a critical-period synaptic plasticity associated with experience-dependent refinement of cortical maps. These studies support the idea that genetic control over synapse maturation influences the duration of critical-period plasticity windows. These data also suggest that critical-period duration links synapse maturation rates to the development of intellectual ability.

SYNGAP1 heterozygosity disrupts sensory processing by reducing touch-related activity within somatosensory cortex circuits.

In addition to cognitive impairments, neurodevelopmental disorders often result in sensory processing deficits. However, the biological mechanisms that underlie impaired sensory processing associated with neurodevelopmental disorders are generally understudied and poorly understood. We found that SYNGAP1 haploinsufficiency in humans, which causes a sporadic neurodevelopmental disorder defined by cognitive impairment, autistic features, and epilepsy, also leads to deficits in tactile-related sensory processing. In vivo neurophysiological analysis in Syngap1 mouse models revealed that upper-lamina neurons in somatosensory cortex weakly encode information related to touch. This was caused by reduced synaptic connectivity and impaired intrinsic excitability within upper-lamina somatosensory cortex neurons. These results were unexpected, given that Syngap1 heterozygosity is known to cause circuit hyperexcitability in brain areas more directly linked to cognitive functions. Thus, Syngap1 heterozygosity causes a range of circuit-specific pathologies, including reduced activity within cortical neurons required for touch processing, which may contribute to sensory phenotypes observed in patients.

A role for the subiculum in the brain motivation/reward circuitry.

The ventral subiculum (vSUB) is an interface between the hippocampal formation and structures in the brain reward circuitry, such as the nucleus accumbens (NAc) and prefrontal cortex (PFC). The vSUB powerfully activates the dopamine system, particularly in response to novelty. This activity is both necessary and sufficient to elevate nucleus accumbens dopamine levels triggered by a novel stimulus. Direct stimulation of the vSUB increases the population of active dopamine neurons in the ventral tegmental area and dopamine levels in the accumbens via a multisynaptic route relayed through the nucleus accumbens. Furthermore, activity in the vSUB is correlated with drug-seeking behaviour such that vSUB inhibition attenuates cocaine-primed reinstatement of drug-seeking, while brief vSUB activation triggers reinstatement cocaine-seeking. We report that acute cocaine alters vSUB pyramidal neuron activity by inducing a frequency-dependent output mode transition from bursting to single-spiking. We suggest that under normal conditions bursting and output mode switching (bursting to single-spiking) may be needed for proper routing of information in and out of the vSUB. We propose that psychostimulants disrupt bursting and output mode switching leading to inappropriate dopamine/novelty signaling that is necessary to set motivational states and direct attention and ultimately, behaviour.

Prioritizing the development of mouse models for childhood brain disorders.

Mutations in hundreds of genes contribute to cognitive and behavioral dysfunction associated with developmental brain disorders (DBDs). Due to the sheer number of risk factors available for study combined with the cost of developing new animal models, it remains an open question how genes should be prioritized for in-depth neurobiological investigations. Recent reviews have argued that priority should be given to frequently mutated genes commonly found in sporadic DBD patients. Intrigued by this idea, we explored to what extent "high priority" risk factors have been studied in animals in an effort to assess their potential for generating valuable preclinical models capable of advancing the neurobiological understanding of DBDs. We found that in-depth whole animal studies are lacking for many high priority genes, with relatively few neurobiological studies performed in construct valid animal models aimed at understanding the pathological substrates associated with disease phenotypes. However, some high priority risk factors have been extensively studied in animal models and they have generated novel insights into DBD patho-neurobiology while also advancing early pre-clinical therapeutic treatment strategies. We suggest that prioritizing model development toward genes frequently mutated in non-specific DBD populations will accelerate the understanding of DBD patho-neurobiology and drive novel therapeutic strategies. This article is part of the Special Issue entitled 'Synaptopathy--from Biology to Therapy'.

Pharmacological Selectivity Within Class I Histone Deacetylases Predicts Effects on Synaptic Function and Memory Rescue.

Histone deacetylases (HDACs) are promising therapeutic targets for neurological and psychiatric disorders that impact cognitive ability, but the relationship between various HDAC isoforms and cognitive improvement is poorly understood, particularly in mouse models of memory impairment. A goal shared by many is to develop HDAC inhibitors with increased isoform selectivity in order to reduce unwanted side effects, while retaining procognitive effects. However, studies addressing this tack at the molecular, cellular and behavioral level are limited. Therefore, we interrogated the biological effects of class I HDAC inhibitors with varying selectivity and assessed a subset of these compounds for their ability to regulate transcriptional activity, synaptic function and memory. The HDAC-1, -2, and -3 inhibitors, RGFP963 and RGFP968, were most effective at stimulating synaptogenesis, while the selective HDAC3 inhibitor, RGFP966, with known memory enhancing abilities, had minimal impact. Furthermore, RGFP963 increased hippocampal spine density, while HDAC3 inhibition was ineffective. Genome-wide gene expression analysis by RNA sequencing indicated that RGFP963 and RGFP966 induce largely distinct transcriptional profiles in the dorsal hippocampus of mature mice. The results of bioinformatic analyses were consistent with RGFP963 inducing a transcriptional program that enhances synaptic efficacy. Finally, RGFP963, but not RGFP966, rescued memory in a mouse model of Alzheimer's Disease. Together, these studies suggest that the specific memory promoting properties of class I HDAC inhibitors may depend on isoform selectivity and that certain pathological brain states may be more receptive to HDAC inhibitors that improve network function by enhancing synapse efficacy.

Reduced cognition in Syngap1 mutants is caused by isolated damage within developing forebrain excitatory neurons.

Syngap1 haploinsufficiency is a common cause of sporadic intellectual disability. Syngap1 mutations disrupt developing pyramidal neurons, although it remains unclear if this process contributes to cognitive abnormalities. Here, we found that haploinsufficiency restricted to forebrain glutamatergic neurons was sufficient to disrupt cognition and removing mutations from this population prevented cognitive abnormalities. In contrast, manipulating Syngap1 function in GABAergic neurons had no effect on cognition, excitability, or neurotransmission, highlighting the specificity of Syngap1 mutations within forebrain excitatory neurons. Interestingly, cognitive abnormalities were reliably predicted by the emergence of enhanced excitatory synaptic function in mature superficial cortical pyramidal cells, which was a neurophysiological disruption caused by Syngap1 dysfunction in developing, but not adult, forebrain neurons. We conclude that reduced cognition in Syngap1 mutants is caused by isolated damage to developing forebrain glutamatergic neurons. This damage triggers secondary disruptions to synaptic homeostasis in mature cortical pyramidal cells, which perpetuates brain dysfunction into adulthood.

Dopamine modulates an mGluR5-mediated depolarization underlying prefrontal persistent activity.

The intrinsic properties of neurons that enable them to maintain depolarized, persistently activated states in the absence of sustained input are poorly understood. In short-term memory tasks, individual prefrontal cortical (PFC) neurons can maintain persistent action potential output during delay periods between informative cues and behavioral responses. Dopamine and drugs of abuse alter PFC function and working memory, possibly by modulating intrinsic neuronal properties. Here we used patch-clamp recording of layer 5 PFC pyramidal neurons to identify a postsynaptic depolarization that was evoked by action potential bursts and mediated by metabotropic glutamate receptor 5 (mGluR5). This depolarization occurred in the absence of recurrent synaptic activity and was reduced by a dopamine D1 receptor (D1R) protein kinase A pathway. After behavioral sensitization to cocaine, the depolarization was substantially diminished and D1R modulation was lost. We propose that burst-evoked intrinsic depolarization is a form of short-term cellular memory that is modulated by dopamine and cocaine experience.

Corticolimbic expression of TRPC4 and TRPC5 channels in the rodent brain.

The canonical transient receptor potential (TRPC) channels are a family of non-selective cation channels that are activated by increases in intracellular Ca(2+) and G(q)/phospholipase C-coupled receptors. We used quantitative real-time PCR, in situ hybridization, immunoblots and patch-clamp recording from several brain regions to examine the expression of the predominant TRPC channels in the rodent brain. Quantitative real-time PCR of the seven TRPC channels in the rodent brain revealed that TRPC4 and TRPC5 channels were the predominant TRPC subtypes in the adult rat brain. In situ hybridization histochemistry and immunoblotting further resolved a dense corticolimbic expression of the TRPC4 and TRPC5 channels. Total protein expression of HIP TRPC4 and 5 proteins increased throughout development and peaked late in adulthood (6-9 weeks). In adults, TRPC4 expression was high throughout the frontal cortex, lateral septum (LS), pyramidal cell layer of the hippocampus (HIP), dentate gyrus (DG), and ventral subiculum (vSUB). TRPC5 was highly expressed in the frontal cortex, pyramidal cell layer of the HIP, DG, and hypothalamus. Detailed examination of frontal cortical layer mRNA expression indicated TRPC4 mRNA is distributed throughout layers 2-6 of the prefrontal cortex (PFC), motor cortex (MCx), and somatosensory cortex (SCx). TRPC5 mRNA expression was concentrated specifically in the deep layers 5/6 and superficial layers 2/3 of the PFC and anterior cingulate. Patch-clamp recording indicated a strong metabotropic glutamate-activated cation current-mediated depolarization that was dependent on intracellular Ca(2+)and inhibited by protein kinase C in brain regions associated with dense TRPC4 or 5 expression and absent in regions lacking TRPC4 and 5 expression. Overall, the dense corticolimbic expression pattern suggests that these Gq/PLC coupled nonselective cation channels may be involved in learning, memory, and goal-directed behaviors.