22 Nov 2006

Many researchers are acquainted with Arc/Arg3.1 as a marker of neuronal activity in the brain. The gene is induced by synaptic activity, and its messenger RNA quickly localizes to active synapses. The induction of Arc/Arg3.1 is so uniform and robust that measuring increases in its mRNA and protein has become a standard method for tracking neuronal activity in brain tissue.

But even as more and more people became familiar with Arc/Arg3.1, none could say they knew the protein well. While the induction of Arc/Arg3.1 is required for the sustained synaptic changes that occur during learning and memory, and the RNA and protein are certainly in the right place to affect synapse function, the actual role of Arc/Arg3.1 remained obscure. That has changed with the simultaneous publication of four papers in the November 9 Neuron. The papers present an extensive analysis of the Arc/Arg3.1 protein function in mouse neurons and knockout animals to show that the protein regulates synaptic plasticity via its effects on AMPA receptor trafficking. Behavioral studies with Arc/Arg3.1 knockout mice link AMPA receptor changes to the consolidation of synaptic plasticity by showing that the protein is required for long-term memory formation.

While the results may not directly bear on memory loss in Alzheimer disease, they are of interest because of the prominent role of Arc in the formation of enduring memories. The Arc/Arg3.1 gene was identified independently in 1995 by two groups. Paul Worley at Johns Hopkins School of Medicine in Baltimore, Maryland, named it Arc, while Dietmar Kuhl of the Berlin Free University in Germany dubbed it Arg3.1. They each pulled the gene out of screens for immediate early genes induced by intensive neuronal stimulation, and showed that the Arc/Arg3.1 transcript rapidly localized to the activated dendrites. The new papers all involve either one or both of these researchers, with collaborators scattered all over Europe and the United States.

Changes in the levels of the AMPA glutamate receptor underlie synaptic plasticity, and the first paper shows data on how Arc/Arg3.1 modulates these receptors. Co-first authors Shoaib Chowdhury and Jason Shepherd of the Worley lab show that Arc/Arg3.1 regulates AMPA receptor trafficking by binding to two proteins (endophilin 3 and dynamin 2) that are involved in the endocytosis of membrane vesicles. They show that overexpression of Arc/Arg3.1 in cultured hippocampal neurons selectively increases AMPAR endocytosis, leading to a 50 percent decrease in cell surface expression of the receptor. Knocking out Arc/Arg3.1 reduces endocytosis and increases the surface expression of AMPA receptors.

The effects of Arc/Arg3.1 on AMPA receptor levels and synaptic transmission are detailed in additional papers by Shepherd and co-lead author Gavin Rumbaugh from the Worley lab, and by Emiliano Rial Verde, working with Hollis Cline at the Cold Spring Harbor Laboratory in New York. The experiments, which feature both Arc/Arg3.1 overexpression systems and knockout animals, all favor a role for the protein in the dynamic control of AMPA receptors during the homeostatic maintenance of synaptic strength.

The final installment of the story comes from the Kuhl lab, where the first authors Niels Plath and Ora Ohana present a behavioral and electrophysiological characterization of Arc/Arg3.1 knockout mice. The animals show defects in learning and memory, failing to form long-term spatial, taste, or fear memories. They have enhanced early-phase LTP, but impaired late-phase responses. Another recently published study also showed that Arc/Arg3.1 is required for processing visual experiences (Wang et al., 2006), and Plath and Ohana see evidence for defects in that realm as well.

In an accompanying mini-review (which is anything but mini), Anastassios Tzingounis and Roger Nicoll take on the task of summarizing and critiquing the mountain of data in the four papers. In their conclusion, they write, “The discovery of Arc/Arg3.1 a decade ago raised great promise of being able to link gene expression to synaptic plasticity and behavior. However, until now the most exciting finding involving Arc/Arg3.1 was based on its ability to identify recently activated neurons. The recent papers…go a long way toward filling in the gap between our knowledge of the expression of Arc/Arg3.1 and its postulated role in behavior.” Tzingounis and Nicoll call the data linking Arc/Arg3.1 to synaptic plasticity “compelling.”—Pat McCaffrey

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References

Paper Citations

1. Wang KH, Majewska A, Schummers J, Farley B, Hu C, Sur M, Tonegawa S. In vivo two-photon imaging reveals a role of arc in enhancing orientation specificity in visual cortex. Cell. 2006 Jul 28;126(2):389-402. PubMed.

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