Thermostable proteins in the diapausing eggs of Brachionus manjavacas (Rotifera)

The Genome of the Marine Rotifer Brachionus manjavacas: Genome-Wide Identification of 310 G Protein-Coupled Receptor (GPCR) Genes

The marine rotifer Brachionus manjavacas is widely used in ecological, ecotoxicological, and ecophysiological studies. The reference genome of B. manjavacas is a good starting point to uncover the potential molecular mechanisms of responses to various environmental stressors. In this study, we assembled the whole-genome sequence (114.1 Mb total, N50 = 6.36 Mb) of B. manjavacas, consisting of 61 contigs with 18,527 annotated genes. To elucidate the potential ligand-receptor signaling pathways in marine Brachionus rotifers in response to environmental signals, we identified 310 G protein-coupled receptor (GPCR) genes in the B. manjavacas genome after comparing them with three other species, including the minute rotifer Proales similis, Drosophila melanogaster, and humans (Homo sapiens). The 310 full-length GPCR genes were categorized into five distinct classes: A (262), B (26), C (7), F (2), and other (13). Most GPCR gene families showed sporadic evolutionary processes, but some classes were highly conserved between species as shown in the minute rotifer P. similis. Overall, these results provide potential clues for in silico analysis of GPCR-based signaling pathways in the marine rotifer B. manjavacas and will expand our knowledge of ligand-receptor signaling pathways in response to various environmental signals in rotifers.

Thermotolerance experiments on active and desiccated states of Ramazzottius varieornatus emphasize that tardigrades are sensitive to high temperatures

Global warming is already having harmful effects on habitats worldwide and it is therefore important to gain an understanding of how rising temperatures may affect extant animals. Here, we investigate the tolerance to high temperatures of Ramazzottius varieornatus, a tardigrade frequently found in transient freshwater habitats. Using logistic modelling on activity we evaluate the effect of 24 hour temperature exposures on active tardigrades, with or without a short acclimation period, compared to exposures of desiccated tardigrades. We estimate that the 50% mortality temperature for non-acclimated active tardigrades is 37.1 °C, with a small but significant increase to 37.6 °C following acclimation. Desiccated specimens tolerate much higher temperatures, with an estimated 50% mortality temperature of 82.7 °C following 1 hour exposures, but with a significant decrease to 63.1 °C following 24 hour exposures. Our results show that metabolically active tardigrades are vulnerable to high temperatures, yet acclimatization could provide a tolerance increase. Desiccated specimens show a much higher resilience—exposure-time is, however, a limiting factor giving tardigrades a restricted window of high temperature tolerance. Tardigrades are renowned for their ability to tolerate extreme conditions, but their endurance towards high temperatures clearly has an upper limit—high temperatures thus seem to be their Achilles heel.

Dormancy in Embryos: Insight from Hydrated Encysted Embryos of an Aquatic Invertebrate

Numerous aquatic invertebrates remain dormant for decades in a hydrated state as encysted embryos. In search for functional pathways associated with this form of dormancy, we used label-free quantitative proteomics to compare the proteomes of hydrated encysted dormant embryos (resting eggs; RE) with nondormant embryos (amictic eggs; AM) of the rotifer Brachionus plicatilisA total of 2631 proteins were identified in rotifer eggs. About 62% proteins showed higher abundance in AM relative to RE (Fold Change>3; p = 0.05). Proteins belonging to numerous putative functional pathways showed dramatic changes during dormancy. Most striking were changes in the mitochondria indicating an impeded metabolism. A comparison between the abundance of proteins and their corresponding transcript levels, revealed higher concordance for RE than for AM. Surprisingly, numerous highly abundant dormancy related proteins show corresponding high mRNA levels in metabolically inactive RE. As these mRNAs and proteins degrade at the time of exit from dormancy they may serve as a source of nucleotides and amino acids during the exit from dormancy. Because proteome analyses point to a similarity in functional pathways of hydrated RE and desiccated life forms, REs were dried. Similar hatching and reproductive rates were found for wet and dried REs, suggesting analogous pathways for long-term survival in wet or dry forms. Analysis by KEGG pathways revealed a few general strategies for dormancy, proposing an explanation for the low transcriptional similarity among dormancies across species, despite the resemblance in physiological phenotypes.

Moderately lower temperatures greatly extend the lifespan of Brachionus manjavacas (Rotifera): Thermodynamics or gene regulation?

Environmental temperature greatly affects lifespan in a wide variety of animals, but the exact mechanisms underlying this effect are still largely unknown. A moderate temperature decrease from 22°C to 16°C extends the lifespan of the monogonont rotifer Brachionus manjavacas by up to 163%. Thermodynamic effects on metabolism contribute to this increase in longevity, but are not the only cause. When rotifers are exposed to 16°C for four days and then transfered to 22°C, they survive until day 13 at nearly identical rates as rotifers maintained at 16°C continuously. This persistence of the higher survival for nine days after transfer to 22°C suggests that low temperature exposure alters the expression of genes that affect the rate of aging. The relative persistence of the gene regulation effect suggests that it may play an even larger role in slowing aging than the thermodynamic effects. The life extending effects of these short-term low temperature treatments are largest when the exposure happens early in the life cycle, demonstrating the importance of early development. There is no advantage to lowering the temperature below 16°C to 11° or 5°C. Rotifers exposed to 16°C also displayed increased resistance to heat, starvation, oxidative and osmotic stress. Reproductive rates at 16°C were lower than those at 22°C, but because they reproduce longer, there is no significant change in the lifetime fecundity of females. To investigate which genes contribute to these effects, the expression of specific temperature sensing genes was knocked down using RNAi. Of 12 genes tested, RNAi knockdown of four eliminated the survival enhancing effects of the four-day cold treatment: TRP7, forkhead box C, Y-box factor, and ribosomal protein S6. This demonstrates that active gene regulation is an important factor in temperature mediated life extension, and that these particular genes play an integral role in these pathways. As a thermoresponsive sensor, TRP7 may be responsible for triggering the signaling cascade contributing to temperature mediated life extension. The TRP genes may also provide especially promising candidates for targeted gene manipulations or pharmacological interventions capable of mimicking the effects of low temperature exposure. These results support recent theories of aging that claim rate of aging is determined by an actively regulated genetic mechanism rather than an accumulation of molecular damage.

Stress granules form in Brachionus manjavacas (Rotifera) in response to a variety of stressors

Many eukaryotes share a common response to environmental stresses. The responses include reorganization of cellular organelles and proteins. Similar stress responses between divergent species suggest that these protective mechanisms may have evolved early and been retained from the earliest eukaryotic ancestors. Many eukaryotic cells have the capacity to sequester proteins and mRNAs into transient stress granules (SGs) that protect most cellular mRNAs (Anderson and Kedersha, 2008). Our observations extend the phylogenetic range of SGs from trypanosomatids, insects, yeast and mammalian cells, where they were first described, to a species of the lophotrochozoan animal phylum Rotifera. We focus on the distribution of three proteins known to be associated with both ribosomes and SG formation: eukaryotic initiation factors eIF3B, eIF4E and T-cell-restricted intracellular antigen 1. We found that these three proteins co-localize to SGs in rotifers in response to temperature stress, osmotic stress and nutrient deprivation as has been described in other eukaryotes. We have also found that the large ribosomal subunit fails to localize to the SGs in rotifers. Furthermore, the SGs in rotifers disperse once the environmental stress is removed as demonstrated in yeast and mammalian cells. These results are consistent with SG formation in trypanosomatids, insects, yeast and mammalian cells, further supporting the presence of this protective mechanism early in the evolution of eukaryotes.

Two novel heat-soluble protein families abundantly expressed in an anhydrobiotic tardigrade

Tardigrades are able to tolerate almost complete dehydration by reversibly switching to an ametabolic state. This ability is called anhydrobiosis. In the anhydrobiotic state, tardigrades can withstand various extreme environments including space, but their molecular basis remains largely unknown. Late embryogenesis abundant (LEA) proteins are heat-soluble proteins and can prevent protein-aggregation in dehydrated conditions in other anhydrobiotic organisms, but their relevance to tardigrade anhydrobiosis is not clarified. In this study, we focused on the heat-soluble property characteristic of LEA proteins and conducted heat-soluble proteomics using an anhydrobiotic tardigrade. Our heat-soluble proteomics identified five abundant heat-soluble proteins. All of them showed no sequence similarity with LEA proteins and formed two novel protein families with distinct subcellular localizations. We named them Cytoplasmic Abundant Heat Soluble (CAHS) and Secretory Abundant Heat Soluble (SAHS) protein families, according to their localization. Both protein families were conserved among tardigrades, but not found in other phyla. Although CAHS protein was intrinsically unstructured and SAHS protein was rich in β-structure in the hydrated condition, proteins in both families changed their conformation to an α-helical structure in water-deficient conditions as LEA proteins do. Two conserved repeats of 19-mer motifs in CAHS proteins were capable to form amphiphilic stripes in α-helices, suggesting their roles as molecular shield in water-deficient condition, though charge distribution pattern in α-helices were different between CAHS and LEA proteins. Tardigrades might have evolved novel protein families with a heat-soluble property and this study revealed a novel repertoire of major heat-soluble proteins in these anhydrobiotic animals.