Independent Losses of the Hypoxia-Inducible Factor (HIF) Pathway within Crustacea

The relativity analysis of hypoxia inducible factor-1α in pulmonary arterial hypertension (ascites syndrome) in broilers: a review

Ascites syndrome (AS) in broiler chickens, also known as pulmonary arterial hypertension (PAH), is a significant disease in the poultry industry. It is a nutritional metabolic disease that is closely associated with hypoxia-inducible factors and rapid growth. The rise in pulmonary artery pressure is a crucial characteristic of AS and is instrumental in its development. Hypoxia-inducible factor 1α (HIF-1α) is an active subunit of a key transcription factor in the oxygen-sensing pathway. HIF-1α plays a vital role in oxygen homeostasis and the development of pulmonary hypertension. Studying the effects of HIF-1α on pulmonary hypertension in humans or mammals, as well as ascites in broilers, can help us understand the pathogenesis of AS. Therefore, this review aims to (1) summarize the mechanism of HIF-1α in the development of pulmonary hypertension, (2) provide theoretical significance in explaining the mechanism of HIF-1α in the development of pulmonary arterial hypertension (ascites syndrome) in broilers, and (3) establish the correlation between HIF-1α and pulmonary arterial hypertension (ascites syndrome) in broilers. HIGHLIGHTSExplains the hypoxic mechanism of HIF-1α.Linking HIF-1α to pulmonary hypertension in broilers.Explains the role of microRNAs in pulmonary arterial hypertension in broilers.

Hypoxia in aquatic invertebrates: Occurrence and phenotypic and molecular responses

Global deoxygenation in aquatic systems is an increasing environmental problem, and substantial oxygen loss has been reported. Aquatic animals have been continuously exposed to hypoxic environments, so-called "dead zones," in which severe die-offs among organisms are driven by low-oxygen events. Multiple studies of hypoxia exposure have focused on in vivo endpoints, metabolism, oxidative stress, and immune responses in aquatic invertebrates such as molluscs, crustaceans, echinoderms, and cnidarians. They have shown that acute and chronic exposure to hypoxia induces significant decreases in locomotion, respiration, feeding, growth, and reproduction rates. Also, several studies have examined the molecular responses of aquatic invertebrates, such as anaerobic metabolism, reactive oxygen species induction, increased antioxidant enzymes, immune response mechanisms, regulation of hypoxia-inducible factor 1-alpha (HIF-1α) genes, and differently expressed hemoglobin/hemocyanin. The genetic basis of those molecular responses involves HIF-1α pathway genes, which are highly expressed in hypoxic conditions. However, the identification of HIF-1α-related genes and understanding of their applications in some aquatic invertebrates remain inadequate. Also, some species of crustaceans, rotifers, sponges, and ctenophores that lack HIF-1α are thought to have alternative defense mechanisms to cope with hypoxia, but those factors are still unclear. This review covers the formation of hypoxia in aquatic environments and the various adverse effects of hypoxia on aquatic invertebrates. The limitations of current hypoxia research and genetic information about the HIF-1α pathway are also discussed. Finally, this paper explains the underlying processes of the hypoxia response and presents an integrated program for research about the molecular mechanisms of hypoxic stresses in aquatic invertebrates.

An experimental study on tolerance to hypoxia in tardigrades

Introduction: Tardigrades are small aquatic invertebrates with well documented tolerance to several environmental stresses, including desiccation, low temperature, and radiation, and an ability to survive long periods in a cryptobiotic state under arrested metabolism. Many tardigrade populations live in habitats where temporary exposure to hypoxia is expected, e.g., benthic layers or substrates that regularly undergo desiccation, but tolerance to hypoxia has so far not been thoroughly investigated in tardigrades. Method: We studied the response to exposure for hypoxia (<1 ppm) during 1-24 h in two tardigrade species, Richtersius cf. coronifer and Hypsibius exemplaris. The animals were exposed to hypoxia in their hydrated active state. Results: Survival was high in both species after the shortest exposures to hypoxia but tended to decline with longer exposures, with almost complete failure to recover after 24 h in hypoxia. R. cf. coronifer tended to be more tolerant than H. exemplaris. When oxygen level was gradually reduced from 8 to 1 ppm, behavioral responses in terms of irregular body movements were first observed at 3-4 ppm. Discussion: The study shows that both limno-terrestrial and freshwater tardigrades are able to recover after exposure to severe hypoxia, but only exposure for relatively short periods of time. It also indicates that tardigrade species have different sensitivity and response patterns to exposure to hypoxia. These results will hopefully encourage more studies on how tardigrades are affected by and respond to hypoxic conditions.

Reduction of Paraoxonase Expression Followed by Inactivation across Independent Semiaquatic Mammals Suggests Stepwise Path to Pseudogenization

Convergent adaptation to the same environment by multiple lineages frequently involves rapid evolutionary change at the same genes, implicating these genes as important for environmental adaptation. Such adaptive molecular changes may yield either change or loss of protein function; loss of function can eliminate newly deleterious proteins or reduce energy necessary for protein production. We previously found a striking case of recurrent pseudogenization of the Paraoxonase 1 (Pon1) gene among aquatic mammal lineages-Pon1 became a pseudogene with genetic lesions, such as stop codons and frameshifts, at least four times independently in aquatic and semiaquatic mammals. Here, we assess the landscape and pace of pseudogenization by studying Pon1 sequences, expression levels, and enzymatic activity across four aquatic and semiaquatic mammal lineages: pinnipeds, cetaceans, otters, and beavers. We observe in beavers and pinnipeds an unexpected reduction in expression of Pon3, a paralog with similar expression patterns but different substrate preferences. Ultimately, in all lineages with aquatic/semiaquatic members, we find that preceding any coding-level pseudogenization events in Pon1, there is a drastic decrease in expression, followed by relaxed selection, thus allowing accumulation of disrupting mutations. The recurrent loss of Pon1 function in aquatic/semiaquatic lineages is consistent with a benefit to Pon1 functional loss in aquatic environments. Accordingly, we examine diving and dietary traits across pinniped species as potential driving forces of Pon1 functional loss. We find that loss is best associated with diving activity and likely results from changes in selective pressures associated with hypoxia and hypoxia-induced inflammation.

Myxozoans (Cnidaria) do not Retain Key Oxygen-Sensing and Homeostasis Toolkit Genes

For aerobic organisms, both the hypoxia-inducible factor pathway and the mitochondrial genomes are key players in regulating oxygen homeostasis. Recent work has suggested that these mechanisms are not as highly conserved as previously thought, prompting more surveys across animal taxonomic levels, which would permit testing of hypotheses about the ecological conditions facilitating evolutionary loss of such genes. The Phylum Cnidaria is known to harbor wide variation in mitochondrial chromosome morphology, including an extreme example, in the Myxozoa, of mitochondrial genome loss. Because myxozoans are obligate endoparasites, frequently encountering hypoxic environments, we hypothesize that variation in environmental oxygen availability could be a key determinant in the evolution of metabolic gene networks associated with oxygen-sensing, hypoxia-response, and energy production. Here, we surveyed genomes and transcriptomes across 46 cnidarian species for the presence of HIF pathway members, as well as for an assortment of hypoxia, mitochondrial, and stress-response toolkit genes. We find that presence of the HIF pathway, as well as number of genes associated with mitochondria, hypoxia, and stress response, do not vary in parallel to mitochondrial genome morphology. More interestingly, we uncover evidence that myxozoans have lost the canonical HIF pathway repression machinery, potentially altering HIF pathway functionality to work under the specific conditions of their parasitic lifestyles. In addition, relative to other cnidarians, myxozoans show loss of large proportions of genes associated with the mitochondrion and involved in response to hypoxia and general stress. Our results provide additional evidence that the HIF regulatory machinery is evolutionarily labile and that variations in the canonical system have evolved in many animal groups.

The evolutionarily conserved hif-1/bnip3 pathway promotes mitophagy and mitochondrial fission in crustacean testes during hypoxia

The hypoxia-inducible factor 1 (HIF-1) cascade is an ancient and strongly evolutionarily conserved signaling pathway that is involved in the hypoxic responses of most metazoans. Despite immense advances in the understanding of the HIF-1-mediated regulation of hypoxic responses in mammals, the contribution of the hif-1 cascade in the hypoxic adaptation of nonmodel invertebrates remains unclear. In this study, we used the oriental river prawn Macrobrachium nipponense for investigating the roles of hif-1-regulated mitophagy in crustacean testes under hypoxic conditions. We identified that the Bcl-2/adenovirus E1B 19-kDa interacting protein (bnip3) functions as a regulator of mitophagy in M. nipponense and demonstrated that hif-1α activates bnip3 by binding to the bnip3 promoter. Hif-1α knockdown suppressed the expression of multiple mitophagy-related genes, and prawns with hif-1α knockdown exhibited higher mortality under hypoxic conditions. We observed that the levels of BNIP3 were induced under hypoxic conditions and detected that bnip3 knockdown inhibited the mitochondrial translocation of dynamin-related protein 1 (drp1), which is associated with mitochondrial fission. Notably, bnip3 knockdown inhibited hypoxia-induced mitophagy and aggravated the deleterious effects of hypoxia-induced reactive oxygen species (ROS) production and apoptosis. The experimental studies demonstrated that hypoxia induced mitochondrial fission in M. nipponense via drp1. Altogether, the study elucidated the mechanism underlying hif-1/bnip3-mediated mitochondrial fission and mitophagy and demonstrated that this pathway protects crustaceans against ROS production and apoptosis induced by acute hypoxia.

Glucose transporter GLUT1 expression is important for oriental river prawn (Macrobrachium nipponense) hemocyte adaptation to hypoxic conditions

Crustaceans have an open vascular system in which hemocytes freely circulate in hemolymph. Hemocytes are rich in hemocyanin, a specific oxygen-transport protein in crustaceans; therefore, understanding the response of hemocytes to hypoxia is crucial. Although hemocytes take up glucose during hypoxia, the molecular mechanism of glucose uptake in crustaceans remains unclear. Herein, we identified two highly conserved glucose transporters (GLUT1 and GLUT2) in Macrobrachium nipponense (oriental river prawn) and analyzed their tissue-specific expression patterns. Our immunofluorescence assays showed that GLUT1 and GLUT2 are located on the cell membrane, with a strong GLUT1 signal in primary hemocytes under hypoxia. We found that during acute hypoxia, hypoxia-inducible factor-1α-related metabolic alterations result in decreased mitochondrial cytochrome c oxidase activity, implying a classic glycolytic mechanism. As a proof of concept, we replicated these findings in insect S2 cells. Acute hypoxia significantly induced hypoxia-inducible factor-1α, GLUT1, and pyruvate dehydrogenase kinase isozyme 1 expression in primary hemocytes, and hypoxia-induced increases in glucose uptake and lactate secretion were observed. GLUT1 knockdown induced intracellular reactive oxygen species generation and apoptosis in vitro and in vivo, resulting in increased prawn mortality and more apoptotic cells in their brains, implying a vital function of GLUT1 in hypoxia adaptation. Taken together, our results suggest a close relationship between hypoxia-mediated glycolysis and GLUT1 in hemocytes. These results demonstrated that in crustaceans, adaptation to hypoxia involves glucose metabolic plasticity.

The origin and distribution of the main oxygen sensing mechanism across metazoans

Oxygen sensing mechanisms are essential for metazoans, their origin and evolution in the context of oxygen in Earth history are of interest. To trace the evolution of a main oxygen sensing mechanism among metazoans, the hypoxia induced factor, HIF, we investigated the phylogenetic distribution and phylogeny of 11 of its components across 566 eukaryote genomes. The HIF based oxygen sensing machinery in eukaryotes can be traced as far back as 800 million years (Ma) ago, likely to the last metazoan common ancestor (LMCA), and arose at a time when the atmospheric oxygen content corresponded roughly to the Pasteur point, or roughly 1% of present atmospheric level (PAL). By the time of the Cambrian explosion (541–485 Ma) as oxygen levels started to approach those of the modern atmosphere, the HIF system with its key components HIF1α, HIF1β, PHD1, PHD4, FIH and VHL was well established across metazoan lineages. HIF1α is more widely distributed and therefore may have evolved earlier than HIF2α and HIF3α, and HIF1β and is more widely distributed than HIF2β in invertebrates. PHD1, PHD4, FIH, and VHL appear in all 13 metazoan phyla. The O₂ consuming enzymes of the pathway, PHDs and FIH, have a lower substrate affinity, K_m, for O₂ than terminal oxidases in the mitochondrial respiratory chain, in line with their function as an environmental signal to switch to anaerobic energy metabolic pathways. The ancient HIF system has been conserved and widespread during the period when metazoans evolved and diversified together with O₂ during Earth history.

Endurance of extremely prolonged nutrient prevention across kingdoms of life

Numerous observations demonstrate that microorganisms can survive very long periods of nutrient deprivation and starvation. Moreover, it is evident that prolonged periods of starvation are a feature of many habitats, and many cells in all kingdoms of life are found in prolonged starvation conditions. Bacteria exhibit a range of responses to long-term starvation. These include genetic adaptations such as the long-term stationary phase and the growth advantage in stationary phase phenotypes characterized by mutations in stress-signaling genes and elevated mutation rates. Here, we suggest using the term "endurance of prolonged nutrient prevention" (EPNP phase), to describe this phase, which was also recently described in eukaryotes. Here, we review this literature and describe the current knowledge about the adaptations to very long-term starvation conditions in bacteria and eukaryotes, its conceptual and structural conservation across all kingdoms of life, and point out possible directions that merit further research.

Genome-wide identification and characterization of basic helix-loop-helix genes in nine molluscs

The basic helix-loop-helix (bHLH) transcription factors form a large superfamily that plays an important role in numerous physiological processes, including development and response to environmental stresses. In this study, the distribution of bHLH genes in nine molluscs was systematically investigated (including five bivalves, three gastropods and one cephalopod). Finally, 53-85 bHLH genes were identified from each genome and classified into corresponding families by using phylogenetic analysis. The results of gene structure and conserved motif analysis illustrated the hereditary conservation of bHLH transcription factors during evolution but showed low similarity in group C. Through transcription profile analysis of C. gigas and T. granosa, we found a important role of bHLH genes in responding to multiple external challenges and development; meanwhile, they also exhibited tissue-specific expression. Interestingly, we were also surprised to find different bHLH genes from the same group generally possess similar patterns expression that tends to simultaneously present high or lower expression of multiple challenges and different tissues in this study. In summary, this study lays the foundation for further investigation of the biological functions and evolution of molluscan bHLH genes.

Oxygen sensing in crustaceans: functions and mechanisms

Animals that live in changing environments need to adjust their metabolism to maintain body functions, and sensing these changing conditions is essential for mediating the short- and long-term physiological and behavioral responses that make these adjustments. Previous research on nematodes and insects facing changing oxygen levels has shown that these animals rapidly respond using atypical soluble guanylyl cyclases (sGCs) as oxygen sensors connected to downstream cGMP pathways, and they respond more slowly using hypoxia-inducible transcription factors (HIFs) that are further modulated by oxygen-sensing prolyl hydroxylases (PHs). Crustaceans are known to respond in different ways to hypoxia, but the mechanisms responsible for sensing oxygen levels are more poorly understood than in nematodes and insects. Our paper reviews the functions of and mechanisms underlying oxygen sensing in crustaceans. Furthermore, using the oxygen sensing abilities of nematodes and insects as guides in analyzing available crustacean transcriptomes, we identified orthologues of atypical sGCs, HIFs, and PHs in crustaceans, including in their chemosensory organs and neurons. These molecules include atypical sGCs activated by hypoxia (Gyc-88E/GCY-31 and Gyc-89D/GCY-33) but not those activated by hyperoxia (GCY-35, GCY-36), as well as orthologues of HIF-α, HIF-β, and PH. We offer possible directions for future research on oxygen sensing by crustaceans.