14 July 2007. The ability of messenger RNAs to make the long trek from the nucleus of a neuron to far-flung dendrites for the local production of proteins plays a critical role in maintaining synaptic activity and plasticity. The mRNA for Arc protein (activity regulated, cytoskeletal-associated protein or Arg3.1), which is induced by neuronal activity, is one of the best studied of these peripatetic messengers. Recently Arc protein expression was shown to link neuronal activity and the synaptic strengthening process of LTP, which forms the structural underpinning of memory formation (see ARF related news story).
Some new basic science findings shed light on a novel pathway for regulation of Arc protein production in dendrites by the mRNA splicing factor eIF4AIII. A paper in the July 13 Cell, from Melissa Moore at Brandeis University in Waltham, Massachusetts, and collaborators shows that 4AIII is present on dendritic Arc mRNA, and that the protein appears to regulate mRNA stability and protein expression. Knocking out 4AIII expression in cultured neurons increased Arc protein levels, AMPA-type glutamate receptor levels, and synaptic transmission. Their results suggest that, by limiting the translation of Arc, and possibly other mRNAs, 4AIII contributes to specific timing and location of protein production in synapses, with functional consequences.
Though the results do not bear directly on AD, they reveal a new complexity in the regulation of dendritic mRNA translation and protein production, a process tied intimately to memory formation.
The eIF4AIII protein binds nuclear mRNA during pre-splicing, forming a core part of the exon junction complex. The EJC then travels into the cytoplasm with the mRNA, where it can affect the nucleic acid’s fate in two ways. If the EJC sits in an open reading frame, it appears to help recruit ribosomes, before being displaced during translation. However, if the EJC binds downstream of an open reading frame, it can cause destruction of the mRNA after the first round of translation in a type of translation-dependent decay.
In the current work, first author Corinna Giorgi and coworkers show that eIF4AIII has the latter effect on Arc mRNA, and possibly on other dendritically localized transcripts. In non-neuronal cells, 4AIII is found mainly in the nucleus, but in hippocampal and cultured neurons, the authors also find the protein in the cell body and dendrites. They find that 4AIII preferentially associates with dendritic messenger RNAs, including Arc mRNA.
The investigators suspected that Arc mRNA could be subject to translation-dependent mRNA decay, based on its unusual structure, which features an exon downstream of its natural stop codon. This is a favored binding spot for 4AIII-containing EJCs. In support of this idea, they showed that knockdown of 4AIII in PC12 cells increased Arc message and protein levels. Knockdown of another EJC core protein also increased Arc mRNA levels, lending more support to the idea that Arc is a target for translation-dependent decay.
In primary cortical neurons, 4AIII also regulated Arc protein, and had functional effects on synapse strength. In stimulated neurons, where Arc expression levels are high, the induced mRNA was associated with 4AIII. Just as in the PC12 cells, depletion of 4AIII with shRNA increased Arc message and protein. When synaptic activity was measured, cells that had lost 4AIII showed a doubling of miniature excitatory post-synaptic currents, presumably reflecting an increase in the number of AMPA receptors. Immunofluorescence studies confirmed this, revealing a marked increase in the AMPAR subunit GluR1 at apparent synaptic sites. Loss of 4AIII did not affect the localization of Arc message or protein to dendrites, suggesting that it was not a localization factor, but regulated message stability.
From these results, the investigators concluded that Arc is a natural target of translation-dependent decay, and that the role of 4AIII is as a “general brake on Arc protein expression.”
The exact role of Arc expression in modulating synaptic strength is unclear, however, in light of previous work where increasing Arc resulted in reduced cell surface AMPA receptors and synaptic strength (see ARF related news story). One explanation for these conflicting results could be that 4AIII regulates other dendritic mRNAs, which lead to complex effects on synaptic protein production and function. To probe this idea, the investigators employed bioinformatics to search for other messages that might be subject to regulation by virtue of having an EJC binding site downstream of a stop codon. They came up with 149 candidate genes, including several implicated in synapse function.
“Our data clearly indicate that 4AIII does play a critical role in maintaining synapse strength and suggest that local synthesis of proteins subject to regulation by 4AIII could contribute to synaptic plasticity,” the authors write.
Neurons surmount the logistical challenge of making the right proteins in the right place at the right time by regulating mRNA transcription, transport, and translation, and protein degradation. The 4AIII protein and its associated EJC appear to provide neurons with yet one more level of spatial and temporal control of protein expression, by acting as a location-specific regulator of mRNA stability. In this scheme, the Arc mRNA, or other similarly regulated messages, would leave the nucleus loaded with 4AIII, and remain stable on the journey out to the dendrite. Once there, translation would lead to rapid destruction of the message. This regulation allows for certain mRNAs to accumulate to high levels in cells, and to provide a transient burst of protein production in the area of the synapse.—Pat McCaffrey.
Giorgi C, Yeo GW, Stone ME, Katz DB, Burge C, Turrigiano G, Moore MJ. The EJC factor eIF4AIII modulates synaptic strength and neuronal protein expression. Cell. 2007 July 13; 130:179-191. Abstract