Heat Stress Drives a Fertility Protein Into Worm Germ Cell Granules, Revealing a New Stress Response

When temperatures climb, the cells that safeguard a species’ future are among the most vulnerable in the body. Germ cells, which must remain functional to pass genetic material to the next generation, face a particular dilemma during heat stress: they need to protect their RNA molecules and protein-making machinery while still supporting the demanding process of reproduction. A new study published in PLOS Genetics by Diya Zang, Yifan Jing, and colleagues at the University of Science and Technology of China and collaborating institutions reveals how a conserved stress-response protein helps the germline of the tiny roundworm Caenorhabditis elegans cope with rising temperatures, and in doing so uncovers an unexpected partnership between two types of RNA-rich cellular structures.

The protein at the center of the study is GTBP-1, the worm equivalent of human G3BP1, a molecule long recognized as a master organizer of stress granules. Stress granules are dynamic, membrane-less assemblies of stalled messenger RNAs and proteins that form rapidly when cells encounter harsh conditions such as heat, oxidative damage, or nutrient deprivation. By temporarily sequestering mRNAs, these condensates pause protein production and shield RNA transcripts from degradation, buying the cell time to recover. In mammalian cells, G3BP1 acts as a molecular switch that triggers the phase separation driving stress granule assembly, making its worm counterpart an obvious candidate for investigating how germ cells weather thermal stress.

To probe GTBP-1’s role, the team used CRISPR/Cas9 to generate knockout worms lacking the gene entirely. The results were strikingly temperature-dependent. At cool temperatures of 15 and 20 degrees Celsius, gtbp-1 mutants actually produced more offspring than normal worms, suggesting the protein normally restrains reproduction under benign conditions. But when animals were raised at 25 degrees Celsius, the mutants became severely infertile. Microscopy showed that the germline tissue itself remained largely intact in the heat-stressed mutants, yet embryos were rarely seen, indicating that the fertility collapse stems from a failure in embryo production rather than a wholesale loss of reproductive tissue. GTBP-1, it appears, is dispensable or even slightly inhibitory when life is easy, but essential when the heat is on.

Fluorescent tagging revealed the protein’s dramatic behavioral shift. Under standard laboratory conditions, GFP-labeled GTBP-1 drifted diffusely through the cytoplasm of epidermal cells, germ cells, and early embryos. When worms were chronically cultured at 25 degrees Celsius, punctate GTBP-1 granules appeared in the germline of a subset of animals, though not in skin cells. Acute heat shock at 37 degrees Celsius for just 30 minutes produced even more pronounced granules across multiple tissues, with GTBP-1 condensing into structures at the perinuclear region surrounding germ cell nuclei, within the central rachis of the gonad, and in the cytoplasm of oocytes. The message was clear: heat stress rapidly reorganizes this protein from a dispersed state into concentrated assemblies precisely where reproductive RNA management happens.

The researchers then dissected which parts of the protein matter. GTBP-1 carries several conserved domains: an NTF2 domain, an intrinsically disordered region, an RNA recognition motif, and an arginine/glycine-rich region. Deleting the NTF2 domain nearly abolished GTBP-1’s recruitment into heat-induced granules, confirming that this module, already known to drive G3BP1-mediated condensation in mammals, is the engine of granule assembly. Deleting the disordered region or the RGG domain told a different story: granules still formed during heat stress but lingered far longer during recovery, indicating these domains govern the fluidity and reversibility of the condensates. Notably, all four domain-deletion variants showed reduced fertility at 25 degrees Celsius, and the severity of the reproductive defects did not simply track with granule defects, hinting that GTBP-1’s contributions to reproduction extend beyond granule formation, possibly through RNA-related functions of its binding domains.

Where exactly do these granules sit within the crowded landscape of germ cell structures? The C. elegans germline hosts an impressive array of perinuclear condensates, including P granules, Z granules, Mutator foci, SIMR foci, D granules, E granules, and P-bodies, each with distinct molecular markers and functions in small RNA metabolism and gene silencing. By crossing their GFP::GTBP-1 strain with strains carrying fluorescent markers for all seven compartments, the team quantified overlap after heat shock. The verdict was unambiguous: GTBP-1 showed its strongest correlation with the P-body marker CGH-1, with moderate associations to several other compartments and a notably weaker link to Z granules. The same GTBP-1 and CGH-1 pairing appeared in both mitotic and meiotic regions of the germline and persisted in early embryos through the two-cell, four-cell, and multicellular stages.

P-bodies are condensates dedicated to mRNA storage and decay, and their intimate relationship with stress granules has been documented in yeast and mammalian cells, where the two structures exchange messenger ribonucleoprotein components under stress. The worm study adds a germline dimension to this picture. When the researchers depleted core P-body components such as CGH-1, EDC-3, or IFET-1 by RNA interference, heat-induced GTBP-1 enrichment dropped significantly, both in the perinuclear region and in the rachis, while total GTBP-1 protein levels remained unchanged. In contrast, disrupting P, Z, Mutator, or E granule components did not prevent GTBP-1 from forming puncta. P-body organization, in other words, is specifically required for efficient GTBP-1 granule assembly during heat stress, positioning these mRNA-decay condensates as scaffolds or partners for the stress response in germ cells.

A candidate screen of 35 genes encoding stress granule proteins and predicted GTBP-1 interactors added further regulators to the network. Loss of PQN-59, a protein related to human UBAP2L, reduced both GTBP-1 granule enrichment and granule number, whereas depletion of the stress granule protein TIAR-1 had little effect. Two other hits stood out: LAF-1, a DEAD-box RNA helicase best known as a P granule component that drives phase separation, and SMO-1, the worm’s sole SUMO protein, implicating SUMOylation, a stress-elevated protein modification, in granule regulation. None of these depletions changed total GTBP-1 abundance, and none rescued the fertility of gtbp-1 mutants at 25 degrees Celsius, showing that granule disruption alone cannot restore reproduction.

The most surprising discovery came from a forward genetic screen of roughly 20,000 mutagenized genomes, hunting for mutations that restore fertility to heat-stressed gtbp-1 mutants. Only one suppressor emerged repeatedly: loss-of-function mutations in rsks-1, the worm homolog of ribosomal S6 kinase, a key effector of the mTOR signaling pathway that governs translation, growth, and aging. Knocking out rsks-1 by several independent means consistently rescued the brood size of gtbp-1 mutants at 25 degrees Celsius, while depleting a panel of well-known longevity genes did not, arguing that the effect is specific rather than a general slowdown of aging pathways. Heat-shock survival assays added nuance: rsks-1 mutants alone were more sensitive to lethal heat, but combining rsks-1 loss with gtbp-1 loss restored wild-type survival, suggesting the two proteins counterbalance each other in stress responses.

RSKS-1 also turned out to be a heat-responsive granule protein in its own right. Normally distributed throughout the cytoplasm and nucleus, it relocalized after heat shock into perinuclear granule-like structures in the germline and oocytes, and these structures overlapped substantially with CGH-1-positive P-bodies, yielding a mean Pearson’s correlation coefficient of about 0.67. Loss of RSKS-1 delayed the appearance of GTBP-1 granules and reduced their number during heat stress without changing how strongly GTBP-1 concentrated within individual granules, indicating that the kinase controls the kinetics and abundance of granule formation rather than granule composition. Together, the findings sketch a working model in which heat stress recruits GTBP-1, along with PQN-59, TIAR-1, and RSKS-1, into perinuclear P-body-associated condensates, linking mTOR-dependent translational control to the RNA granule remodeling that keeps fertility intact across a changing thermal world. As heat waves intensify and reproductive health under thermal stress becomes an ever more pressing question from agriculture to human fertility, this humble worm offers a molecular map of how germ cells fight back.

Subject of Research: Heat stress-induced GTBP-1 stress granule assembly and its association with P-bodies in the C. elegans germline

Article Title: Heat stress promotes GTBP-1 association with perinuclear P-bodies in the C. elegans germline

Article References: Heat stress promotes GTBP-1 association with perinuclear P-bodies in the C. elegans germline. (n.d.). https://doi.org/10.1371/journal.pgen.1012336

Image Credits: AI Generated

DOI: 10.1371/journal.pgen.1012336

Keywords: stress granules, GTBP-1, G3BP1, C. elegans, germline, heat stress, P-bodies, fertility, mTOR signaling, RSKS-1, phase separation, RNA granules