Yongling Zhu

Inactivity Produces Increases in Neurotransmitter Release and Synapse Size

When hippocampal synapses in culture are pharmacologically silenced for several days, synaptic strength increases. The structural correlate of this change in strength is an increase in the size of the synapses, with all synaptic components--active zone, postsynaptic density, and bouton--becoming larger. Further, the number of docked vesicles and the total number of vesicles per synapse increases, although the number of docked vesicles per area of active zone is unchanged. In parallel with these anatomical changes, the physiologically measured size of the readily releasable pool (RRP) and the release probability are increased. Ultrastructural analysis of individual synapses in which the RRP was previously measured reveals that, within measurement error, the same number of vesicles are docked as are estimated to be in the RRP.

Optical measurement of synaptic glutamate spillover and reuptake by linker optimized glutamate-sensitive fluorescent reporters

Genetically encoded sensors of glutamate concentration are based on FRET between cyan and yellow fluorescent proteins bracketing a bacterial glutamate-binding protein. Such sensors have yet to find quantitative applications in neurons, because of poor response amplitude in physiological buffers or when expressed on the neuronal cell surface. We have improved our glutamate-sensing fluorescent reporter (GluSnFR) by systematic optimization of linker sequences and glutamate affinities. Using SuperGluSnFR, which exhibits a 6.2-fold increase in response magnitude over the original GluSnFR, we demonstrate quantitative optical measurements of the time course of synaptic glutamate release, spillover, and reuptake in cultured hippocampal neurons with centisecond temporal and spine-sized spatial resolution. During burst firing, functionally significant spillover persists for hundreds of milliseconds. These glutamate levels appear sufficient to prime NMDA receptors, potentially affecting dendritic spike initiation and computation. Stimulation frequency-dependent modulation of spillover suggests a mechanism for nonsynaptic neuronal communication.

mGluR5 Has a Critical Role in Inhibitory Learning

The mechanisms that contribute to the extinction of previously acquired memories are not well understood. These processes, often referred to as inhibitory learning, are thought to be parallel learning mechanisms that require a reacquisition of new information and suppression of previously acquired experiences in order to adapt to novel situations. Using newly generated metabotropic glutamate receptor 5 (mGluR5) knock-out mice, we investigated the role of mGluR5 in the acquisition and reversal of an associative conditioned task and a spatial reference task. We found that acquisition of fear conditioning is partially impaired in mice lacking mGluR5. More markedly, we found that extinction of both contextual and auditory fear was completely abolished in mGluR5 knock-out mice. In the Morris Water Maze test (MWM), mGluR5 knock-out mice exhibited mild deficits in the rate of acquisition of the regular water maze task, but again had significant deficits in the reversal task, despite overall spatial memory being intact. Together, these results demonstrate that mGluR5 is critical to the function of neural circuits that are required for inhibitory learning mechanisms, and suggest that targeting metabotropic receptors may be useful in treating psychiatric disorders in which aversive memories are inappropriately retained.

Two pathways of synaptic vesicle retrieval revealed by single-vesicle imaging.

Synaptic vesicle recycling is essential for maintaining efficient synaptic transmission. Detailed dissection of single-vesicle recycling still remains a major challenge. We have developed a fluorescent pH reporter that permits us to follow the fate of individual vesicles at hippocampal synapses after exocytosis. Here we show that, during low-frequency stimulation, single-vesicle fusion leads to two distinct vesicle internalizations, instead of one, as in general perception: one by a fast endocytosis pathway ( approximately 3 s), the other by a slow endocytosis pathway (after 10 s). The exocytosed vesicular proteins are preferentially recaptured in both pathways. RNAi knockdown of clathrin inhibits both pathways. As stimulation frequency increases, the number of endocytosed vesicles begins to match antecedent exocytosis. Meanwhile, the slow endocytosis is accelerated and becomes the predominant pathway. These results reveal that two pathways of endocytosis are orchestrated during neuronal activity, establishing a highly efficient endocytosis at central synapses.

Identification of Sequence Motifs That Target Neuronal Nicotinic Receptors to Dendrites and Axons

Neuronal nicotinic acetylcholine receptors (nAChRs) belong to a family of ligand-gated ion channels that play important roles in central and peripheral nervous systems. The subcellular distribution of neuronal nAChRs has important implications for function and is not well understood. Here, we analyzed the targeting of two major types of neuronal nAChRs by expressing epitope-tagged subunits in cultured hippocampal neurons. Surprisingly, the α7 nAChR (α7) and α4/β2 nAChR (α4β2) displayed distinct patterns of expression, with α7 targeted preferentially to the somatodendritic compartments, whereas α4β2 was localized to both axonal and dendritic domains. When fused to CD4 or IL2RA (interleukin 2 receptor α subunit) proteins, which are normally distributed ubiquitously, the M3–M4 intracellular loop from the α7 subunit promoted dendritic expression, whereas the homologous M3–M4 loop from the α4 subunit led to surface axonal expression. Systemic screening and alanine substitution further identified a 25-residue leucine motif ([DE]XXXL[LI]) containing an axonal targeting sequence within the α4 loop and a 48-residue dileucine and tyrosine motif (YXXØ) containing a dendritic targeting sequence from the α7 loop. These results provide valuable information in understanding diverse roles of neuronal nAChRs in mediating and modulating synaptic transmission, synaptic plasticity, and nicotine addiction.

A C-Terminal Determinant of GluR6 Kainate Receptor Trafficking

Intracellular trafficking of ionotropic glutamate receptors is regulated predominantly by determinants in the cytoplasmic C-terminal domain of the subunit proteins. Although AMPA receptors are found at the vast majority of excitatory synapses, synaptic kainate receptors exhibit a much more restricted distribution, suggesting that specific mechanisms exist for selective trafficking of these receptor proteins. In this report, we define a critical forward trafficking motif that is necessary for surface expression of the glutamate receptor 6 (GluR6) kainate receptor as well as chimeric proteins containing only the GluR6 C-terminal domain. The trafficking determinant was identified by tracking surface expression of green fluorescent protein-tagged GluR6 receptors with confocal immunofluorescence in COS-7 cells and cultured neurons and patch-clamp electrophysiology in human embryonic kidney 293 cells. Serial truncation and alanine site mutagenesis of the GluR6 subunit C terminus localized the critical motif to a seven amino acid stretch of predominantly basic residues. Alanine mutation of the trafficking motif reduced kainate receptor current amplitudes by >90% and resulted in retention of the mutated receptors in the endoplasmic reticulum. This forward trafficking domain is the first such identified for kainate receptors.

Genetically Targeted Binary Labeling of Retinal Neurons

A major stumbling block to understanding neural circuits is the extreme anatomical and functional diversity of interneurons. Subsets of interneurons can be targeted for manipulation using Cre mouse lines, but Cre expression is rarely confined to a single interneuron type. It is essential to have a strategy that further restricts labeling in Cre driver lines. We now describe an approach that combines Cre driver mice, recombinant adeno-associated virus, and rabies virus to produce sparse but binary labeling of select interneurons—frequently only a single cell in a large region. We used this approach to characterize the retinal amacrine and ganglion cell types in five GABAergic Cre mouse (Mus musculus) lines, and identified two new amacrine cell types: an asymmetric medium-field type and a wide-field type. We also labeled several wide-field amacrine cell types that have been previously identified based on morphology but whose connectivity and function had not been systematically studied due to lack of genetic markers. All Cre-expressing amacrine cells labeled with an antibody to GABA. Cre-expressing RGCs lacked GABA labeling and included classically defined as well as recently identified types. In addition to the retina, our technique leads to sparse labeling of neurons in the cortex, lateral geniculate nucleus, and superior colliculus, and can be used to express optogenetic tools such as channelrhodopsin and protein sensors such as GCaMP. The Cre drivers identified in this study provide genetic access to otherwise hard to access cell types for systematic analysis including anatomical characterization, physiological recording, optogenetic and/or chemical manipulation, and circuit mapping.

Synaptotagmin mutants Y311N and K326/327A alter the calcium dependence of neurotransmission

Synaptotagmin I, a calcium-binding synaptic vesicle protein, is thought to act as the calcium sensor for fast neurotransmission, but what synaptotagmin I does, upon binding calcium, to trigger exocytosis is still unknown. To begin to examine the role of synaptotagmin Is interactions with calcium-dependent binding partners, three mutant versions of synaptotagmin I reported to affect calcium-dependent self-oligomerization (Y311N, K327A, and K326/327A) were expressed in cultured mouse hippocampal neurons lacking endogenous synaptotagmin I, and effects on neurotransmission were evaluated by comparison with transmission rescued by wild-type synaptotagmin I. All three mutants reduced transmitter release. To separate effects on calcium binding from effects on calcium-dependent oligomerization, we measured the calcium dependence of exocytosis for two of the mutants. Both showed apparent calcium affinity much lower than wild-type, a reduction sufficient to account for the neurotransmission defects. We conclude that self-oligomerization is unlikely to play any significant role in triggering synaptic vesicle exocytosis.

Potentiating mGluR5 function with a positive allosteric modulator enhances adaptive learning

Metabotropic glutamate receptor 5 (mGluR5) plays important roles in modulating neural activity and plasticity and has been associated with several neuropathological disorders. Previous work has shown that genetic ablation or pharmacological inhibition of mGluR5 disrupts fear extinction and spatial reversal learning, suggesting that mGluR5 signaling is required for different forms of adaptive learning. Here, we tested whether ADX47273, a selective positive allosteric modulator (PAM) of mGluR5, can enhance adaptive learning in mice. We found that systemic administration of the ADX47273 enhanced reversal learning in the Morris Water Maze, an adaptive task. In addition, we found that ADX47273 had no effect on single-session and multi-session extinction, but administration of ADX47273 after a single retrieval trial enhanced subsequent fear extinction learning. Together these results demonstrate a role for mGluR5 signaling in adaptive learning, and suggest that mGluR5 PAMs represent a viable strategy for treatment of maladaptive learning and for improving behavioral flexibility.

Organizational motifs for ground squirrel cone bipolar cells

In daylight vision, parallel processing starts at the cone synapse. Cone signals flow to On and Off bipolar cells, which are further divided into types according to morphology, immunocytochemistry, and function. The axons of the bipolar cell types stratify at different levels in the inner plexiform layer (IPL) and can interact with costratifying amacrine and ganglion cells. These interactions endow the ganglion cell types with unique functional properties. The wiring that underlies the interactions among bipolar, amacrine, and ganglion cells is poorly understood. It may be easier to elucidate this wiring if organizational rules can be established. We identify 13 types of cone bipolar cells in the ground squirrel, 11 of which contact contiguous cones, with the possible exception of short‐wavelength‐sensitive cones. Cells were identified by antibody labeling, tracer filling, and Golgi‐like filling following transduction with an adeno‐associated virus encoding for green fluorescent protein. The 11 bipolar cell types displayed two organizational patterns. In the first pattern, eight to 10 of the 11 types came in pairs with partially overlapping axonal stratification. Pairs shared morphological, immunocytochemical, and functional properties. The existence of similar pairs is a new motif that might have implications for how signals first diverge from a cone to bipolar cells and then reconverge onto a costratifying ganglion cell. The second pattern is a mirror symmetric organization about the middle of the IPL involving at least seven bipolar cell types. This anatomical symmetry may be associated with a functional symmetry in On and Off ganglion cell responses. J. Comp. Neurol. 520:2864–2887, 2012.