ABSTRACT
The balance between excitatory and inhibitory synapses is a tightly regulated process that requires differential recruitment of proteins that dictate the specificity of newly formed contacts. However, factors that control this process remain unidentified. Here we show that members of the neuroligin (NLG) family, including NLG1, NLG2, and NLG3, drive the formation of both excitatory and inhibitory presynaptic contacts. The enrichment of endogenous NLG1 at excitatory contacts and NLG2 at inhibitory synapses supports an important in vivo role for these proteins in the development of both types of contacts. Immunocytochemical and electrophysiological analysis showed that the effects on excitatory and inhibitory synapses can be blocked by treatment with a fusion protein containing the extracellular domain of neurexin-1beta. We also found that overexpression of PSD-95, a postsynaptic binding partner of NLGs, resulted in a shift in the distribution of NLG2 from inhibitory to excitatory synapses. These findings reveal a critical role for NLGs and their synaptic partners in controlling the number of inhibitory and excitatory synapses. Furthermore, relative levels of PSD-95 alter the ratio of excitatory to inhibitory synaptic contacts by sequestering members of the NLG family to excitatory synapses.
Subject(s)
Membrane Proteins/physiology , Nerve Tissue Proteins/metabolism , Nerve Tissue Proteins/physiology , Synapses/metabolism , Animals , Blotting, Western , Cell Adhesion Molecules, Neuronal , Cells, Cultured , Cloning, Molecular , DNA, Complementary/metabolism , Disks Large Homolog 4 Protein , Electrophysiology , Gene Library , Green Fluorescent Proteins/metabolism , Guanylate Kinases , Hippocampus/cytology , Hippocampus/metabolism , Image Processing, Computer-Assisted , Immunohistochemistry , Intracellular Signaling Peptides and Proteins , Membrane Proteins/metabolism , Mice , Microscopy, Fluorescence , Models, Biological , Multigene Family , Nerve Tissue Proteins/chemistry , Neurons/metabolism , Protein Binding , Rats , Rats, Wistar , Recombinant Fusion Proteins/chemistry , TransfectionABSTRACT
Factors that control differentiation of presynaptic and postsynaptic elements into excitatory or inhibitory synapses are poorly defined. Here we show that the postsynaptic density (PSD) proteins PSD-95 and neuroligin-1 (NLG) are critical for dictating the ratio of excitatory-to-inhibitory synaptic contacts. Exogenous NLG increased both excitatory and inhibitory presynaptic contacts and the frequency of miniature excitatory and inhibitory synaptic currents. In contrast, PSD-95 overexpression enhanced excitatory synapse size and miniature frequency, but reduced the number of inhibitory synaptic contacts. Introduction of PSD-95 with NLG augmented synaptic clustering of NLG and abolished NLG effects on inhibitory synapses. Interfering with endogenous PSD-95 expression alone was sufficient to reduce the ratio of excitatory-to-inhibitory synapses. These findings elucidate a mechanism by which the amounts of specific elements critical for synapse formation control the ratio of excitatory-to-inhibitory synaptic input.
Subject(s)
Cerebellum/physiology , Nerve Tissue Proteins/physiology , Neurons/cytology , Neurons/physiology , Synapses/physiology , Animals , Cell Adhesion Molecules/genetics , Cell Adhesion Molecules/physiology , Cell Adhesion Molecules, Neuronal , Cells, Cultured , Cerebellum/cytology , Disks Large Homolog 4 Protein , Guanylate Kinases , Hippocampus/cytology , Hippocampus/physiology , Intracellular Signaling Peptides and Proteins , Membrane Proteins/genetics , Membrane Proteins/physiology , Mice , Microscopy , Nerve Tissue Proteins/genetics , Polymerase Chain Reaction , Synapses/ultrastructure , TransfectionABSTRACT
Dynamic regulation of AMPA-type glutamate receptors represents a primary mechanism for controlling synaptic strength, though mechanisms for this process are poorly understood. The palmitoylated postsynaptic density protein, PSD-95, regulates synaptic plasticity and associates with the AMPA receptor trafficking protein, stargazin. Here, we identify palmitate cycling on PSD-95 at the synapse and find that palmitate turnover on PSD-95 is regulated by glutamate receptor activity. Acutely blocking palmitoylation disperses synaptic clusters of PSD-95 and causes a selective loss of synaptic AMPA receptors. We also find that rapid glutamate-mediated AMPA receptor internalization requires depalmitoylation of PSD-95. In a nonneuronal model system, clustering of PSD-95, stargazin, and AMPA receptors is also regulated by ongoing palmitoylation of PSD-95 at the plasma membrane. These studies suggest that palmitate cycling on PSD-95 can regulate synaptic strength and regulates aspects of activity-dependent plasticity.
