August 25, 2022
Perturbations in iron homeostasis affect aging, but how this happens has remained a bit of a black box. A new study by Patnaik et al. in Cell Reports illuminates this box by looking more closely at the transcription factors that are first to respond when iron becomes limiting.
Key among these are Atf1p and Atf2p, which activate the full suite of iron-mobilization genes, among which is TIS11/CTH2, which encodes an RNA-binding protein that targets specific messages for decay.
The targeted messages flagged for decay encode mitochondrial proteins, as these use iron but are not the most essential in the set. The most essential Fe-requiring enzymes are those involved in DNA synthesis and repair, such that slowing/shutting mitochondrial function is a response to iron deficiency. Intriguingly, mitochondrial function also happens to decline with age.
To find the specific mechanisms linking iron with aging, the authors used an unbiased analysis of genes involved in iron homeostasis to see which showed connection with aging. The strain with a tis11Δ mutation lived longer than any others, with a lifespan extended by 51.1%. In a broader sense, they found that genes involved in different aspects of response to iron deficiency also had different effects on fitness and aging.
Delving more deeply into the role of Tis11p/Cth2p in aging, the authors used RNA-seq and Ribo-seq to look at temporal changes in transcription versus translation in aging cells. They showed how, overall, aging leads to inhibition of translation—except for certain genes which are upregulated instead. Interestingly, most of the upregulated genes are in the Fe regulon that gets activated by the first responder Atf1p.
While the expression of TIS11/CTH2 increases both with aging and with iron deficiency, the deletion of the gene extends lifespan. Thus, multiple lines of evidence suggest Tis11p/Cth2p is a negative regulator of longevity. The key connection appears to be mitochondrial translation, where the function of Tis11p/Cth2p to inhibit translation of mitochondrial transcripts for repressing non-essential Fe-requiring enzymes serves to simultaneously repress overall mitochondrial respiration, which speeds aging.
As not all genes translationally upregulated in the tis11Δ mutant contained appropriate binding sites in the 3’ UTR, the authors looked further and found binding sequences for Puf3p, a protein known to bind and inhibit translation of mRNAs coding for mitochondrial ribosome proteins. Thus, Puf3p appears to be a critical partner for Tis11p/Cth2p in mediating downregulation of mitochondrial function. Further, they questioned the relationship with the Hap4p transcription factor, which regulates numerous components of the electron transport chain and whose overexpression extends lifespan. As the combination of a tis11Δ deletion with HAP4 overexpression had no additive effect in an epistasis experiment, they concluded that Tis11p-dependent repression acts through Hap4p.
The role of phosphorylation of Tis11p/Cth2p was examined by mutating N-terminal serine residues, which impairs degradation of the protein. Consistent with the converse result of extended lifespan in null mutants, the nondegradable version of the protein shortens lifespan.
Thus, the ease of the yeast model once more illuminates intricate connections between critical proteins, facilitating potential drug discovery around several new aging factors.
Categories: Research Spotlight
February 18, 2022
The telomerase ribonucleoprotein complex is the primary means by which yeast cells maintain telomeres. However, it turns out that cells lacking functional telomerase have a backup plan to restore telomere length by “alternative lengthening of telomeres” (ALT). ALT employs recombination via extrachromosomal telomere elements called C-circles. In a process for which the reasons remain unclear, C-circles get paired with eroded telomeres at the nuclear pore complex on the nuclear membrane. This pairing requires the SAGA/TREX2 complex and, once paired, the recombination between C-circles and telomeres appears to be effected by Rad59p, the paralog of Rad52p.
This interesting model is described in a recent paper in The EMBO Journal, in which Aguilera et al. adapt a method developed in human cancer studies to detect ALT and C-circles in yeast. In humans, ~10% of cancers depend on ALT for unchecked growth. In yeast, cells with ALT were able to be detected as survivors among telomerase mutant (est2∆) cells.
As other types of extrachromosomal DNA circles were previously reported to associate with the nuclear pore complex, the authors addressed the possibility that C-circles bind the NPC and demonstrated it clearly. They also showed the circles interact with the SAGA/TREX2 complex, which favors telomere recombination.
The novel finding that ALT in yeast so closely mirrors that of some human cancer cells is a boon to study of these cancers. The ability to develop ALT inhibitors in yeast would provide a new set of potential anticancer therapies, making this an ideal model system.
Categories: Research Spotlight