FUS RNA Granules Not So Stressed Out?

First author Kyota Yasuda and senior author Stavroula Mili at the National Cancer Institute in Bethesda, Maryland, research RNA-protein complexes found in cellular protrusions such as axon growth cones. These complexes are characterized by the presence of the protein adenomatous polyposis coli (APC), and include several mRNAs (Mili et al., 2008). None of these RNAs are directly related to any disease, Mili told Alzforum. In the study, Yasuda and colleagues used several mRNA members of APC-ribonucleoprotein complexes as convenient markers for the structures:

  • Plakopilin 4 (Pkp4), thought to be involved in organizing cell-cell junctions.
  • Rab13, which participates in neurite outgrowth.
  • KN motif and ankyrin repeat domains 2 (Kank2). Kank proteins regulate actin polymerization and cell movement.
  • Discoidin domain receptor tyrosine kinase 2 (Ddr2), an enzyme that regulates cell migration and adhesion to the extracellular matrix.
  • Human immunodeficiency virus-1 Tat specific factor 1 (TAT-SF1), a transcription factor that may also participate in RNA splicing.

“Wherever you have protrusions, you have localization of this group of RNAs,” Mili said. These RNAs and others undergo translation in the APC complexes, though how important that translation is to the overall cellular levels of the individual proteins likely varies, since they may be translated within the cell interior as well. For proteins that are already abundant, the APC translation might add little, but it could make a big difference for low-level proteins, Mili said.

How did Mili and Yasuda get from these protrusion granules to ALS-linked FUS? To understand the APC complexes better, they used mass spectrometry to identify proteins that co-immunoprecipitated with APC from mouse fibroblasts. This led them to FUS, a mainly nuclear protein that translocates to the cytoplasm in disease states. “Most of the work on [normal FUS] has overwhelmingly focused on it as a nuclear actor,” commented Benjamin Wolozin of Boston University, who was not part of the study (see Sep 2013 news story). “Here, they are showing a physiological condition where you have FUS out in the cytoplasm.” Mili noted their finding in no way diminishes the importance of nuclear FUS, which makes up the majority of the protein.

To find out what FUS does in APC complexes, Yasuda and colleagues knocked down its expression in the fibroblasts. All of the typical APC complex RNAs still localized to cellular protrusions. However, Kank2 levels fell, indicating that FUS promotes translation of at least some of the APC mRNAs. Wolozin found this surprising, because RNA-binding proteins such as FUS typically silence translation, rather than turning it up.

Next, the researchers studied the effects of FUS mutations associated with ALS. Overexpressing these mutants redistributed the APC-RNA granules, moving them from protrusions to the cellular interior. FUS, APC proteins, and Ddr2 and Pkp4 mRNA accumulated in these interior granules. The granules also contained the stress granule marker T-cell intracellular antigen (TIA-1).

Because researchers who have studied these pathological FUS granules identified them as stress granules (see Alzforum webinar; Jul 2010 news story; Bosco et al., 2010), Mili and colleagues expected that they would be translationally silent. Not so—Kank2, Pkp4, and Ddr2 were produced at normal levels and the FUS-containing granules co-localized with a puromycin label for newly synthesized proteins (see image below). “This goes against the dogma in the field,” commented Bosco. However, Wolozin said he was not entirely surprised. Though scientists typically describe stress granules as translationally silent, he said, their true biology is more complicated and translation may occur in some cases.

FUS Stress Granules Translate. Granules containing mutant FUS (red) colocalize with puromycin, which labels nascent proteins (green) in mouse embryonic fibroblast cultures.

FUS Stress Granules Translate.

Granules containing mutant FUS (red) colocalize with puromycin, which labels nascent proteins (green) in mouse embryonic fibroblast cultures

Indeed, what this study suggests is that the FUS-containing granules are not traditional stress granules at all, both Mili and Bosco said. “I think we are going to start to appreciate that the properties and functions of these granules depend on the conditions under which they are formed,” Bosco said. For example, cellular stress such as heat shock might lead to a different type of RNA granule than overexpression of a disease-linked protein such as FUS. The criteria to identify a stress granule rely on markers like TIA-1, she noted, not on functional assays. “This paper might cause us to stop and rethink our criteria for saying something is a stress granule, versus some other type of RNA granule,” she said.

Proper characterization matters if one is to understand disease pathology. When examining hippocampal tissue from people who died of FTD, Yasuda saw abnormal granules containing both FUS and APC.

Mili and colleagues are now comparing FUS-containing granules to traditional stress granules. They are also looking at how mislocalization of FUS might affect neurons, since the current study mainly addressed fibroblasts.

Might other ALS and FTD genes contribute to translation in RNA granules, as well? Mili will compare FUS granules to TDP-43 granules. Another gene to consider is repeat-expanded C9ORF72, which was shown earlier this year to form translated dipeptide aggregates as well as mRNA foci (see Feb 2013 news story news story). Researchers have not yet reported where the repeats undergo translation. The RNA and dipeptides rarely coincide; typically a cell contains one or the other but not both. That has led some scientists to suggest that C9ORF72 mRNAs in foci are sequestered away from the translation machinery (see Nov 2013 news story).

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