Recruitment of parvalbumin and somatostatin interneuron inputs to adult born dentate granule neurons.

GABA is a key regulator of adult-born dentate granule cell (abDGC) maturation so mapping the functional connectivity between abDGCs and local interneurons is required to understand their development and integration into the hippocampal circuit. We recorded from birthdated abDGCs in mice and photoactivated parvalbumin (PV) and somatostatin (SST) interneurons to map the timing and strength of inputs to abDGCs during the first 4 weeks after differentiation. abDGCs received input from PV interneurons in the first week, but SST inputs were not detected until the second week. Analysis of desynchronized quantal events established that the number of GABAergic synapses onto abDGCs increased with maturation, whereas individual synaptic strength was constant. Voluntary wheel running in mice scaled the GABAergic input to abDGCs by increasing the number of synaptic contacts from both interneuron types. This demonstrates that GABAergic innervation to abDGCs develops during a prolonged post-mitotic period and running scales both SST and PV synaptic afferents.

A Schizophrenia-Linked KALRN Coding Variant Alters Neuron Morphology, Protein Function, and Transcript Stability.

BACKGROUND: Large-scale genetic studies have revealed that rare sequence variants, including single nucleotide variants (SNVs), in glutamatergic synaptic genes are enriched in schizophrenia patients. However, the majority are too rare to show any association with disease and have not been examined functionally. One such SNV, KALRN-P2255T, displays a penetrance that greatly exceeds that of previously identified schizophrenia-associated SNVs. Therefore, we sought to characterize its effects on the function of kalirin (Kal)-9, a dual Ras-related C3 botulinum toxin substrate 1 and Ras homologue gene family, member A (RhoA) guanine nucleotide exchange factor, upregulated in human schizophrenia brain tissue. METHODS: Kal9 was overexpressed in primary rat cortical neurons or human embryonic kidney 293 (HEK293) cells. The effects of the P2255T variant on dendritic branching, dendritic spine morphology, protein and messenger RNA stability, and catalytic activity were examined. RESULTS: Kal9-P2255T leads to diminished basal dendritic branching and dendritic spine size, compared with wild-type Kal9. The P2255T SNV directly affected Kal9 protein function, causing increased RhoA activation in HEK293 cells, but had no effect on Ras-related C3 botulinum toxin substrate 1 activation. Consistent with human postmortem findings, we found that Kal9-P2255T protein levels were higher than those of wild-type Kal9 in neurons. Increased messenger RNA stability was detected in HEK293 cells, indicating that this was the cause of the higher protein levels. When analyzed together, increased intrinsic RhoA guanine nucleotide exchange factor catalytic activity combined with increased messenger RNA expression led to net enhancement of RhoA activation, known to negatively impact neuronal morphology. CONCLUSIONS: Taken together, our data reveal a novel mechanism for disease-associated SNVs and provide a platform for modeling morphological changes in mental disorders.

Development of GABAergic Inputs Is Not Altered in Early Maturation of Adult Born Dentate Granule Neurons in Fragile X Mice.

Fragile X syndrome (FXS) is the most common form of inherited mental retardation and the most common known cause of autism. Loss of fragile X mental retardation protein (FMRP) in mice (Fmr1 KO) leads to altered synaptic and circuit maturation in the hippocampus that is correlated with alterations in hippocampal-dependent behaviors. Previous studies have demonstrated that loss of FMRP increased the rate of proliferation of progenitor cells and altered their fate specification in adult Fmr1 KO mice. While these studies clearly demonstrate a role for FMRP in adult neurogenesis in the hippocampus, it is not known whether the functional synaptic maturation and integration of adult-born dentate granule cells (abDGCs) into hippocampal circuits is affected in Fmr1 KO mice. Here, we used retroviral labeling to birthdate abDGCs in Fmr1 KO mice which allowed us to perform targeted patch clamp recording to measure the development of synaptic inputs to these neurons at precise time points after differentiation. The frequency and amplitude of spontaneous GABAergic events increased over the first three weeks after differentiation; however, this normal development of GABAergic synapses was not altered in Fmr1 KO mice. Furthermore, the relatively depolarized GABA reversal potential (EGABA) in immature abDGCs was unaffected by loss of FMRP as was the development of dendritic arbor of the adult generated neurons. These studies systematically characterized the functional development of abDGCs during the first four weeks after differentiation and demonstrate that the maturation of GABAergic synaptic inputs to these neurons is not grossly affected by the loss of FMRP.

Delayed Maturation of Fast-Spiking Interneurons Is Rectified by Activation of the TrkB Receptor in the Mouse Model of Fragile X Syndrome.

Fragile X syndrome (FXS) is a neurodevelopmental disorder that is a leading cause of inherited intellectual disability, and the most common known cause of autism spectrum disorder. FXS is broadly characterized by sensory hypersensitivity and several developmental alterations in synaptic and circuit function have been uncovered in the sensory cortex of the mouse model of FXS (Fmr1 KO). GABA-mediated neurotransmission and fast-spiking (FS) GABAergic interneurons are central to cortical circuit development in the neonate. Here we demonstrate that there is a delay in the maturation of the intrinsic properties of FS interneurons in the sensory cortex, and a deficit in the formation of excitatory synaptic inputs on to these neurons in neonatal Fmr1 KO mice. Both these delays in neuronal and synaptic maturation were rectified by chronic administration of a TrkB receptor agonist. These results demonstrate that the maturation of the GABAergic circuit in the sensory cortex is altered during a critical developmental period due in part to a perturbation in BDNF-TrkB signaling, and could contribute to the alterations in cortical development underlying the sensory pathophysiology of FXS.SIGNIFICANCE STATEMENT Fragile X (FXS) individuals have a range of sensory related phenotypes, and there is growing evidence of alterations in neuronal circuits in the sensory cortex of the mouse model of FXS (Fmr1 KO). GABAergic interneurons are central to the correct formation of circuits during cortical critical periods. Here we demonstrate a delay in the maturation of the properties and synaptic connectivity of interneurons in Fmr1 KO mice during a critical period of cortical development. The delays both in cellular and synaptic maturation were rectified by administration of a TrkB receptor agonist, suggesting reduced BDNF-TrkB signaling as a contributing factor. These results provide evidence that the function of fast-spiking interneurons is disrupted due to a deficiency in neurotrophin signaling during early development in FXS.

Subchronic phencyclidine treatment in adult mice increases GABAergic transmission and LTP threshold in the hippocampus.

Repeated administration of non-competitive N-methyl-d-aspartate (NMDA) receptor antagonists such as phencyclidine (PCP) to rodents causes long-lasting deficits in cognition and memory, and has effects on behaviors that have been suggested to be models of the cognitive impairment associated with schizophrenia (CIAS). Despite this being a widely studied animal model, little is known about the long lasting changes in synapses and circuits that underlie the altered behaviors. Here we examined synaptic transmission ex-vivo in the hippocampus of mice after a subchronic PCP (scPCP) administration regime. We found that after at least one week of drug free washout period when mice have impaired cognitive function, the threshold for long-term potentiation (LTP) of CA1 excitatory synapses was elevated. This elevated LTP threshold was directly related to increased inhibitory input to CA1 pyramidal cells through increased activity of GABAergic neurons. These results suggest repeated PCP administration causes a long-lasting metaplastic change in the inhibitory circuits in the hippocampus that results in impaired LTP, and could contribute to the deficits in hippocampal-dependent memory in PCP-treated mice. Changes in GABA signaling have been described in patients with schizophrenia, therefore our results support using scPCP as a model of CIAS. This article is part of the Special Issue entitled 'Synaptopathy--from Biology to Therapy'.

Epac2 Mediates cAMP-Dependent Potentiation of Neurotransmission in the Hippocampus.

Presynaptic terminal cAMP elevation plays a central role in plasticity at the mossy fiber-CA3 synapse of the hippocampus. Prior studies have identified protein kinase A as a downstream effector of cAMP that contributes to mossy fiber LTP (MF-LTP), but the potential contribution of Epac2, another cAMP effector expressed in the MF synapse, has not been considered. We investigated the role of Epac2 in MF-CA3 neurotransmission using Epac2(-/-) mice. The deletion of Epac2 did not cause gross alterations in hippocampal neuroanatomy or basal synaptic transmission. Synaptic facilitation during short trains was not affected by loss of Epac2 activity; however, both long-term plasticity and forskolin-mediated potentiation of MFs were impaired, demonstrating that Epac2 contributes to cAMP-dependent potentiation of transmitter release. Examination of synaptic transmission during long sustained trains of activity suggested that the readily releasable pool of vesicles is reduced in Epac2(-/-) mice. These data suggest that cAMP elevation uses an Epac2-dependent pathway to promote transmitter release, and that Epac2 is required to maintain the readily releasable pool at MF synapses in the hippocampus.