ABSTRACT
Cortical information storage requires combined changes in connectivity and synaptic strength between neurons, but the signaling mechanisms underlying this two-step wiring plasticity are unknown. Because acute 17beta-estradiol (E2) modulates cortical memory, we examined its effects on spine morphogenesis, AMPA receptor trafficking, and GTPase signaling in cortical neurons. Acute E2 application resulted in a rapid, transient increase in spine density, accompanied by temporary formation of silent synapses through reduced surface GluR1. These rapid effects of E2 were dependent on a Rap/AF-6/ERK1/2 pathway. Intriguingly, NMDA receptor (NMDAR) activation after E2 treatment potentiated silent synapses and elevated spine density for as long as 24 h. Hence, we show that E2 transiently increases neuronal connectivity by inducing dynamic nascent spines that "sample" the surrounding neuropil and that subsequent NMDAR activity is sufficient to stabilize or "hold" E2-mediated effects. This work describes a form of two-step wiring plasticity relevant for cortical memory and identifies targets that may facilitate recovery from brain injuries.
Subject(s)
Estradiol/pharmacology , Estrogens/pharmacology , Neuronal Plasticity/drug effects , Receptors, N-Methyl-D-Aspartate/metabolism , Synapses/physiology , Animals , Cells, Cultured , Dendritic Spines/drug effects , Enzymes/metabolism , Neuronal Plasticity/physiology , Neurons/cytology , Neurons/metabolism , Protein Transport/drug effects , Rats , Rats, Sprague-Dawley , Receptors, AMPA/metabolism , Receptors, Glutamate/metabolism , Signal Transduction/drug effects , Synapses/drug effects , Time FactorsABSTRACT
Remodeling of central excitatory synapses is crucial for synapse maturation and plasticity, and contributes to neurodevelopmental and psychiatric disorders. Remodeling of dendritic spines and the associated synapses has been postulated to require the coordination of adhesion with spine morphology and stability; however, the molecular mechanisms that functionally link adhesion molecules with regulators of dendritic spine morphology are mostly unknown. Here, we report that spine size and N-cadherin content are tightly coordinated. In rat mature cortical pyramidal neurons, N-cadherin-dependent adhesion modulates the morphology of existing spines by recruiting the Rac1 guanine-nucleotide exchange factor kalirin-7 to synapses through the scaffolding protein AF-6/afadin. In pyramidal neurons, N-cadherin, AF-6, and kalirin-7 colocalize at synapses and participate in the same multiprotein complexes. N-cadherin clustering promotes the reciprocal interaction and recruitment of N-cadherin, AF-6, and kalirin-7, increasing the content of Rac1 and in spines and PAK (p21-activated kinase) phosphorylation. N-cadherin-dependent spine enlargement requires AF-6 and kalirin-7 function. Conversely, disruption of N-cadherin leads to thin, long spines, with reduced Rac1 contact, caused by uncoupling of N-cadherin, AF-6, and kalirin-7 from each other. By dynamically linking N-cadherin with a regulator of spine plasticity, this pathway allows synaptic adhesion molecules to rapidly coordinate spine remodeling associated with synapse maturation and plasticity. This study hence identifies a novel mechanism whereby cadherins, a major class of synaptic adhesion molecules, signal to the actin cytoskeleton to control the morphology of dendritic spines, and outlines a mechanism that underlies the coordination of synaptic adhesion with spine morphology.
Subject(s)
Dendritic Spines/physiology , Guanine Nucleotide Exchange Factors/physiology , Microfilament Proteins/physiology , Neurons/cytology , Synapses/physiology , Analysis of Variance , Animals , Antibodies/pharmacology , Cadherins/immunology , Cadherins/metabolism , Cadherins/pharmacology , Cell Line, Transformed , Cells, Cultured , Cerebral Cortex/cytology , Dendritic Spines/drug effects , Disks Large Homolog 4 Protein , Embryo, Mammalian , Green Fluorescent Proteins/biosynthesis , Guanine Nucleotide Exchange Factors/genetics , Humans , Immunoprecipitation/methods , Intracellular Signaling Peptides and Proteins/metabolism , LIM Domain Proteins , Membrane Proteins/metabolism , Microfilament Proteins/genetics , Models, Biological , Mutation/physiology , Rats , Rats, Sprague-Dawley , Receptors, AMPA/metabolism , Signal Transduction/physiology , Synapses/drug effects , Transfection/methodsABSTRACT
Activity-dependent rapid structural and functional modifications of central excitatory synapses contribute to synapse maturation, experience-dependent plasticity, and learning and memory and are associated with neurodevelopmental and psychiatric disorders. However, the signal transduction mechanisms that link glutamate receptor activation to intracellular effectors that accomplish structural and functional plasticity are not well understood. Here we report that NMDA receptor activation in pyramidal neurons causes CaMKII-dependent phosphorylation of the guanine-nucleotide exchange factor (GEF) kalirin-7 at residue threonine 95, regulating its GEF activity, leading to activation of small GTPase Rac1 and rapid enlargement of existing spines. Kalirin-7 also interacts with AMPA receptors and controls their synaptic expression. By demonstrating that kalirin expression and spine localization are required for activity-dependent spine enlargement and enhancement of AMPAR-mediated synaptic transmission, our study identifies a signaling pathway that controls structural and functional spine plasticity.
