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

Maternal Loss of Ube3a Impairs Experience-Driven Dendritic Spine Maintenance in the Developing Visual Cortex

Dendritic spines are a morphological feature of the majority of excitatory synapses in the mammalian neocortex and are motile structures with shapes and lifetimes that change throughout development. Proper cortical development and function, including cortical contributions to learning and memory formation, require appropriate experience-dependent dendritic spine remodeling. Dendritic spine abnormalities have been reported for many neurodevelopmental disorders, including Angelman syndrome (AS), which is caused by the loss of the maternally inherited UBE3A allele (encoding ubiquitin protein ligase E3A). Prior studies revealed that UBE3A protein loss leads to reductions in dendritic spine density and diminished excitatory synaptic transmission. However, the decrease in spine density could come from either a reduction in spine formation or an increase in spine elimination. Here, we used acute and longitudinal in vivo two-photon microscopy to investigate developmental and experience-dependent changes in the numbers, dynamics, and morphology of layer 5 pyramidal neuron apical dendritic spines in the primary visual cortex of control and AS model mice (Ube3am−/p+ mice). We found that neurons in AS model mice undergo a greater elimination of dendritic spines than wild-type mice during the end of the first postnatal month. However, when raised in darkness, spine density and dynamics were indistinguishable between control and AS model mice, which indicates that decreased spine density in AS model mice reflects impaired experience-driven spine maintenance. Our data thus demonstrate an experience-dependent anatomical substrate by which the loss of UBE3A reduces dendritic spine density and disrupts cortical circuitry

Transsynaptic Signaling by Activity-Dependent Cleavage of Neuroligin-1

Adhesive contact between pre- and postsynaptic neurons initiates synapse formation during brain development and provides a natural means of trans-synaptic signaling. Numerous adhesion molecules and their role during synapse development have been described in detail. However, once established, the mechanisms of adhesive disassembly and its function in regulating synaptic transmission have been unclear. Here, we report that synaptic activity induces acute proteolytic cleavage of neuroligin-1 (NLG1), a postsynaptic adhesion molecule at glutamatergic synapses. NLG1 cleavage is triggered by NMDA receptor activation, requires Ca2+/calmodulin-dependent protein kinase, and is mediated by proteolytic activity of matrix metalloprotease 9 (MMP9). Cleavage of NLG1 occurs at single activated spines, is regulated by neural activity in vivo, and causes rapid destabilization of its presynaptic partner neurexin-1β (NRX1β). In turn, NLG1 cleavage depresses synaptic transmission by abruptly reducing presynaptic release probability. Thus, local proteolytic control of synaptic adhesion tunes synaptic transmission during brain development and plasticity

Presynaptic NMDA Receptor Mechanisms for Enhancing Spontaneous Neurotransmitter Release

NMDA receptors (NMDARs) are required for experience-driven plasticity during formative periods of brain development and are critical for neurotransmission throughout postnatal life. Most NMDAR functions have been ascribed to postsynaptic sites of action, but there is now an appreciation that presynaptic NMDARs (preNMDARs) can modulate neurotransmitter release in many brain regions, including the neocortex. Despite these advances, the cellular mechanisms by which preNMDARs can affect neurotransmitter release are largely unknown. Here we pharmacologically interrogated preNMDAR functions to determine how these receptors promote spontaneous neurotransmitter release in mouse primary visual cortex. Our results provide three new insights into the mechanisms by which preNMDARs can function. First, preNMDARs can tonically enhance spontaneous neurotransmitter release with minimal extracellular Ca(2+) or with major sources of intracellular Ca(2+) blocked. Second, lowering extracellular Na(+) levels significantly reduces the contribution of preNMDARs to spontaneous transmitter release. Third, preNMDAR enhance transmitter release in part through protein kinase C (PKC) signaling. These data demonstrate that preNMDARs can act through novel pathways to promote neurotransmitter release in the absence of action potentials

Maternal Loss of Ube3a Impairs Experience-Driven Dendritic Spine Maintenance in the Developing Visual Cortex

Dendritic spines are a morphological feature of the majority of excitatory synapses in the mammalian neocortex and are motile structures with shapes and lifetimes that change throughout development. Proper cortical development and function, including cortical contributions to learning and memory formation, require appropriate experience-dependent dendritic spine remodeling. Dendritic spine abnormalities have been reported for many neurodevelopmental disorders, including Angelman syndrome (AS), which is caused by the loss of the maternally inherited UBE3A allele (encoding ubiquitin protein ligase E3A). Prior studies revealed that UBE3A protein loss leads to reductions in dendritic spine density and diminished excitatory synaptic transmission. However, the decrease in spine density could come from either a reduction in spine formation or an increase in spine elimination. Here, we used acute and longitudinal in vivo two-photon microscopy to investigate developmental and experience-dependent changes in the numbers, dynamics, and morphology of layer 5 pyramidal neuron apical dendritic spines in the primary visual cortex of control and AS model mice (Ube3a(m−/p+) mice). We found that neurons in AS model mice undergo a greater elimination of dendritic spines than wild-type mice during the end of the first postnatal month. However, when raised in darkness, spine density and dynamics were indistinguishable between control and AS model mice, which indicates that decreased spine density in AS model mice reflects impaired experience-driven spine maintenance. Our data thus demonstrate an experience-dependent anatomical substrate by which the loss of UBE3A reduces dendritic spine density and disrupts cortical circuitry. SIGNIFICANCE STATEMENT Reduced dendritic spine densities are common in the neurodevelopmental disorder Angelman syndrome (AS). Because prior reports were based on postmortem tissue, it was unknown whether this anatomical deficit arises from decreased spine formation and/or increased spine elimination. Here, we used in vivo two-photon imaging to track spines over multiple days in a mouse model of AS. We found that spine formation is normal, but experience-dependent spine maintenance is reduced in the visual cortex of AS model mice. Our data pinpoint the anatomical process underlying the loss of dendritic spines, which can account for the decreased excitatory synaptic connectivity associated with AS. Therefore, normalizing spine maintenance is a potential therapeutic strategy

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