Turnover of PSD molecules in hippocampal neurons

Shigeo Okabe

Abstract

Remodeling of synaptic structure is important in both development of functional neural circuits and modulation of preexisting connections. A variety of proteins are recruited to the postsynaptic density (PSD), an electron dense thickening of the postsynaptic membrane, and are thought to play critical roles in the modulation of synaptic functions. We previously showed continual remodeling of postsynaptic structure containing PSD-95 in cultured hippocampal neurons (1) and synchronized accumulation of synaptic vesicle proteins and PSD-95 at synaptic junctional sites (2). Long-term observation of dissociated cultures of hippocampal neurons isolated from transgenic embryos expressing GFP-tagged PSD revealed synchronized assembly and disassembly of PSDs, which were regulated by cAMP-dependent signaling (4). These observations, together with imaging experiments performed in other laboratories, collectively indicated very rapid assembly/disassembly of PSD proteins in the developing hippocampal network (5).

A variety of proteins, including membrane proteins, scaffolding proteins and cytoskeletal proteins, are recruited to the PSD and are thought to play distinct roles in organizing the complex molecular architecture. Although both PSD-95 and PSD-Zip45 (also known as Homer 1c) are PSD scaffolding proteins of excitatory synapses in the hippocampus, postsynaptic calcium increase selectively regulates the distribution of PSD-Zip45 (3). This result suggests that mechanism of assembly/disassembly of individual PSD scaffolding proteins is distinct and their turnover can be regulated independently. Our time-lapse analysis of multiple PSD proteins in hippocampal neurons revealed distinct kinetics of individual proteins. Transmembrane proteins, such as glutamate receptors, and lipid-anchored scaffolding proteins, such as PSD-95, are thought to function as linkers between the postsynaptic membrane and the cytoplasmic matrix and can potentially stabilize the PSD structure. Less dynamic behavior of PSD-95 at the postsynaptic sites is consistent with this notion. However, analyses of culture preparations from genetically manipulated mice and pharmacological treatments indicated that both glutamate receptors and PSD-95 are dispensable for the PSD stability. In contrast, manipulation that disrupts F-actin induced rapid disassembly of multiple PSD scaffolding proteins, including PSD-Zip45 (Homer 1c), GKAP, and cortactin-binding protein (Shank family proteins). These scaffolding proteins showed more dynamic behavior than PSD-95 and the fractions that showed actin-dependent dispersion were comparable with rapid-exchangeable fractions measured by fluorescence recovery after photobleaching analysis. In contrast, PSD-95 was resistant to pharmacological disruption of F-actin. Integrity of F-actin in dendritic spines is indispensable for the organization of PSD scaffolding proteins possibly through its interaction with the cytoplasmic surface of PSDs.

Okabe, S., Kim, H., Miwa, A., Kuriu, T., and H. Okado. Continual remodeling of postsynaptic density and its regulation by synaptic activity. Nature Neuroscience, 2, 804-811, 1999.

Okabe, S., Miwa, A., and H. Okado. Spine formation and correlated assembly of presynaptic and postsynaptic molecules. Journal of Neuroscience, 21, 6105-6114, 2001.

Okabe, S., Urushido, T., Konno, D., Okado, H., and K. Sobue. Rapid redistribution of the postsynaptic density protein PSD-Zip45 (Homer 1c) and its differential regulation by NMDA receptors and calcium channels. Journal of Neuroscience, 21,9561-9571,2001.

Ebihara, T., Kawabata, I., Usui, S., Sobue, K., and S. Okabe. Synchronized formation and remodeling of postsynaptic densities: long-term visualization of hippocampal neurons expressing postsynaptic density proteins tagged with GFP. Journal of Neuroscience, 23, 2170-2181, 2003.

Inoue A., and S. Okabe The dynamic organization of postsynaptic proteins: translocating molecules regulate synaptic function. Current Opinion in Neurobiology, 13, 332-340, 2003.

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