Glycine Receptors Support Excitatory Neurotransmitter Release in Developing Mouse Visual Cortex.

Glycine receptors (GlyRs) are found in most areas of the brain, and their dysfunction can cause severe neurological disorders. While traditionally thought of as inhibitory receptors, presynaptic-acting GlyRs (preGlyRs) can also facilitate glutamate release under certain circumstances, although the underlying molecular mechanisms are unknown. In the current study, we sought to better understand the role of GlyRs in the facilitation of excitatory neurotransmitter release in mouse visual cortex. Using whole-cell recordings, we found that preGlyRs facilitate glutamate release in developing, but not adult, visual cortex. The glycinergic enhancement of neurotransmitter release in early development depends on the high intracellular to extracellular Cl(-) gradient maintained by the Na(+)-K(+)-2Cl(-) cotransporter and requires Ca(2+) entry through voltage-gated Ca(2+) channels. The glycine transporter 1, localized to glial cells, regulates extracellular glycine concentration and the activation of these preGlyRs. Our findings demonstrate a developmentally regulated mechanism for controlling excitatory neurotransmitter release in the neocortex

Subcellular organization of UBE3A in human cerebral cortex.

BackgroundLoss of UBE3A causes Angelman syndrome, whereas excess UBE3A activity appears to increase the risk for autism. Despite this powerful association with neurodevelopmental disorders, there is still much to be learned about UBE3A, including its cellular and subcellular organization in the human brain. The issue is important, since UBE3A's localization is integral to its function.MethodsWe used light and electron microscopic immunohistochemistry to study the cellular and subcellular distribution of UBE3A in the adult human cerebral cortex. Experiments were performed on multiple tissue sources, but our results focused on optimally preserved material, using surgically resected human temporal cortex of high ultrastructural quality from nine individuals.ResultsWe demonstrate that UBE3A is expressed in both glutamatergic and GABAergic neurons, and to a lesser extent in glial cells. We find that UBE3A in neurons has a non-uniform subcellular distribution. In somata, UBE3A preferentially concentrates in euchromatin-rich domains within the nucleus. Electron microscopy reveals that labeling concentrates in the head and neck of dendritic spines and is excluded from the PSD. Strongest labeling within the neuropil was found in axon terminals.ConclusionsBy highlighting the subcellular compartments in which UBE3A is likely to function in the human neocortex, our data provide insight into the diverse functional capacities of this E3 ligase. These anatomical data may help to elucidate the role of UBE3A in Angelman syndrome and autism spectrum disorder

Postsynaptic distribution of IRSp53 in spiny excitatory and inhibitory neurons

The 53 kDa insulin receptor substrate protein (IRSp53) is highly enriched in the brain. Despite evidence that links mutations of IRSp53 with autism and other neuropsychiatric problems, the functional significance of this protein remains unclear. We used light and electron microscopic immunohistochemistry to demonstrate that IRSp53 is expressed throughout the adult rat brain. Labeling concentrated selectively in dendritic spines, where it was associated with the postsynaptic density (PSD). Surprisingly, its organization within the PSD of spiny excitatory neurons of neocortex and hippocampus differed from that within spiny inhibitory neurons of neostriatum and cerebellar cortex. The present data support previous suggestions that IRSp53 is involved in postsynaptic signaling, while hinting that its signaling role may differ in different types of neurons

“Fast” plasma membrane calcium pump PMCA2a concentrates in GABAergic terminals in the adult rat brain

The plasma membrane Ca2+-ATPases (PMCA) represent the major high-affinity Ca2+ extrusion system in the brain. PMCAs comprise four isoforms and over 20 splice variants. Their different functional properties may permit different PMCA splice variants to accommodate different kinds of local [Ca2+] transients, but for a specific PMCA to play a unique role in local Ca2+ handling it must be targeted to the appropriate subcellular compartment. We used immunohistochemistry to study the spatial distribution of PMCA2a–one of the two major carboxyl-terminal splice variants of PMCA2–in the adult rat brain, testing whether this isoform, with especially high basal activity, is targeted to specific subcellular compartments. In striking contrast to the widespread distribution of PMCA2 as a whole, we found that PMCA2a is largely restricted to parvalbumin-positive inhibitory presynaptic terminals throughout the brain. The only major exception to this targeting pattern was in the cerebellar cortex, where PMCA2a also concentrates postsynaptically, in the spines of Purkinje cells. We propose that the fast Ca2+ activation kinetics and high Vmax of PMCA2a make this pump especially suited for rapid clearance of presynaptic Ca2+ in fast-spiking inhibitory nerve terminals, which face severe transient calcium loads

A Critical Role for Myosin IIB in Dendritic Spine Morphology and Synaptic Function

Dendritic spines show rapid motility and plastic morphology, which may mediate information storage in the brain. It is presently believed that polymerization/depolymerization of actin is the primary determinant of spine motility and morphogenesis. Here, we show that myosin IIB, a molecular motor that binds and contracts actin filaments, is essential for normal spine morphology and dynamics and represents a distinct biophysical pathway to control spine size and shape. Myosin IIB is enriched in the postsynaptic density (PSD) of neurons. Pharmacologic or genetic inhibition of myosin IIB alters protrusive motility of spines, destabilizes their classical mushroom-head morphology, and impairs excitatory synaptic transmission. Thus, the structure and function of spines is regulated by an actin-based motor in addition to the polymerization state of actin

Maternal Loss of Ube3a Produces an Excitatory/Inhibitory Imbalance through Neuron Type-Specific Synaptic Defects

Angelman syndrome (AS) is a neurodevelopmental disorder caused by loss of the maternally inherited allele of UBE3A. AS model mice, which carry a maternal Ube3a null mutation (Ube3am−/p+), recapitulate major features of AS in humans, including enhanced seizure susceptibility. Excitatory neurotransmission onto neocortical pyramidal neurons is diminished in Ube3am−/p+ mice, seemingly at odds with enhanced seizure susceptibility. We show here that inhibitory drive onto neocortical pyramidal neurons is more severely decreased in Ube3am−/p+ mice. This inhibitory deficit follows the loss of excitatory inputs and appears to arise from defective presynaptic vesicle cycling in multiple interneuron populations. In contrast, excitatory and inhibitory synaptic inputs onto inhibitory interneurons are largely normal. Our results indicate that there are neuron type-specific synaptic deficits in Ube3am−/p+ mice despite the presence of Ube3a in all neurons. These deficits result in excitatory/inhibitory imbalance at cellular and circuit levels and may contribute to seizure susceptibility in AS

Synaptic Localization of Nitric Oxide Synthase and Soluble Guanylyl Cyclase in the Hippocampus

Functional evidence suggests that nitric oxide released from CA1 pyramidal cells can act as a retrograde messenger to mediate hippocampal long-term potentiation, but the failure to find neuronal nitric oxide synthase (NOS-I) in the dendritic spines of these cells has cast doubt on this suggestion. We hypothesized that NOS-I may be in spines but in a form inaccessible to antibody when using standard histological fixation procedures. Supporting this hypothesis, we found that after a weak fixation protocol shown previously to enhance staining of synaptic proteins, CA1 pyramidal cells exhibit clear immunoreactivity for NOS-I. Confocal microscopy revealed that numerous dendritic spines in the stratum radiatum contained the NR2 subunit of the NMDA receptor and the adaptor protein postsynaptic density-95, and a subset of these spines also contained NOS-I. Quantitative studies showed that only approximately 8% of synaptic puncta (identified by synaptophysin staining) were associated with NOS-I, and approximately 9% contained the beta subunit of soluble guanylyl cyclase (sGC), a major target of NO. However, the majority of NOS-I-positive synaptic puncta was associated with sGC and vice versa. Postembedding immunogold electron microscopy showed that NOS-I concentrates just inside the postsynaptic plasma membrane of asymmetric axospinous synapses in the stratum radiatum of CA1, whereas sGCbeta concentrates just inside the presynaptic membrane. Together, these findings support the possibility that NO may act as a retrograde messenger to help mediate homosynaptic plasticity in a subpopulation of synapses in the stratum radiatum of CA1

Identification of an elaborate complex mediating postsynaptic inhibition

Inhibitory synapses dampen neuronal activity through postsynaptic hyperpolarization. The composition of the inhibitory postsynapse and the mechanistic basis of its regulation, however, remains poorly understood. We used an in vivo chemico-genetic proximity-labeling approach to discover inhibitory postsynaptic proteins. Quantitative mass spectrometry not only recapitulated known inhibitory postsynaptic proteins, but also revealed a large network of new proteins, many of which are either implicated in neurodevelopmental disorders or are of unknown function. CRISPR-depletion of one of these previously uncharacterized proteins, InSyn1, led to decreased postsynaptic inhibitory sites, reduced frequency of miniature inhibitory currents, and increased excitability in the hippocampus. Our findings uncover a rich and functionally diverse assemblage of previously unknown proteins that regulate postsynaptic inhibition and might contribute to developmental brain disorders

The sodium-driven chloride/bicarbonate exchanger in presynaptic terminals

The sodium-driven chloride/bicarbonate exchanger (NDCBE), a member of the SLC4 family of bicarbonate transporters, was recently found to modulate excitatory neurotransmission in hippocampus. By using light and electron microscopic immunohistochemistry, we demonstrate here that NDCBE is expressed throughout the adult rat brain, and selectively concentrates in presynaptic terminals, where it is closely associated with synaptic vesicles. NDCBE is in most glutamatergic axon terminals, and is also present in the terminals of parvalbumin-positive γ-aminobutyric acid (GABA)ergic cells. These findings suggest that NDCBE can regulate glutamatergic transmission throughout the brain, and point to a role for NDCBE as a possible regulator of GABAergic neurotransmission