The Asparagine Hydroxylase FIH: A Unique Oxygen Sensor

Significance: Limited oxygen availability (hypoxia) commonly occurs in a range of physiological and pathophysiological conditions, including embryonic development, physical exercise, inflammation, and ischemia. It is thus vital for cells and tissues to monitor their local oxygen availability to be able to adjust in case the oxygen supply is decreased. The cellular oxygen sensor factor inhibiting hypoxia-inducible factor (FIH) is the only known asparagine hydroxylase with hypoxia sensitivity. FIH uniquely combines oxygen and peroxide sensitivity, serving as an oxygen and oxidant sensor. Recent Advances: FIH was first discovered in the hypoxia-inducible factor (HIF) pathway as a modulator of HIF transactivation activity. Several other FIH substrates have now been identified outside the HIF pathway. Moreover, FIH enzymatic activity is highly promiscuous and not limited to asparagine hydroxylation. This includes the FIH-mediated catalysis of an oxygen-dependent stable (likely covalent) bond formation between FIH and selected substrate proteins (called oxomers [oxygen-dependent stable protein oligomers]). Critical Issues: The (patho-)physiological function of FIH is only beginning to be understood and appears to be complex. Selective pharmacologic inhibition of FIH over other oxygen sensors is possible, opening new avenues for therapeutic targeting of hypoxia-associated diseases, increasing the interest in its (patho-)physiological relevance. Future Directions: The contribution of FIH enzymatic activity to disease development and progression should be analyzed in more detail, including the assessment of underlying molecular mechanisms and relevant FIH substrate proteins. Also, the molecular mechanism(s) involved in the physiological functions of FIH remain(s) to be determined. Furthermore, the therapeutic potential of recently developed FIH-selective pharmacologic inhibitors will need detailed assessment. Antioxid. Redox Signal. 37, 913-935.

A Food Supplement with Antioxidative Santa Herba Extract Modulates Energy Metabolism and Contributes to Weight Management

The plant Santa herba (Eriodictyon californicum) contains high flavonoids, thus potentially exerting beneficial effects in context of obesity, often accompanied by inflammation and metabolic imbalance. The study assessed the impact of Santa herba on oxidative stress, energy metabolism, weight reduction, and eating behavior, combining in vitro models with clinical data. Santa herba binding of the adenosine receptor A2A (ADORA2A) was assessed using a radioligand binding assay. A Caenorhabditis elegans model was used to determine mobility boosting effects, and Santa herba oxygen radical absorbance capacity (ORAC) values were determined in comparison to antioxidative plants. Clinical data, that is, body weight and appetite-related parameters, were obtained from overweight and obese women receiving either Santa herba or placebo for 12 weeks. Results showed that Santa herba extract binds to ADORA2A, stimulates C. elegans motility (+7.5%) and locomotion, and yields high antioxidative capacities (ORAC: 819 trolox equivalent). Clinical data, obtained from 24 overweight and 25 obese women (mean: 47.5 years), demonstrated a reduced body weight (P = .042) and body fat (P = .044), and by trend reduced leptin levels (P = .065) in women with obesity after Santa herba consumption compared to placebo. In conclusion, Santa herba extract has energizing and antioxidative properties and may aid in weight management of people with obesity. ClinicalTrials.gov Identifier: NCT03853603.

HIF hydroxylase inhibitors decrease cellular oxygen consumption depending on their selectivity

Pharmacologic HIF hydroxylase inhibitors (HIs) are effective for the treatment of anemia in chronic kidney disease patients and may also be beneficial for the treatment of diseases such as chronic inflammation and ischemia-reperfusion injury. The selectivities of many HIs for HIF hydroxylases and possible off-target effects in cellulo are unclear, delaying the translation from preclinical studies to clinical trials. We developed a novel assay that discriminates between the inhibition of HIF-α prolyl-4-hydroxylase domain (PHD) enzymes and HIF-α asparagine hydroxylase factor inhibiting HIF (FIH). We characterized 15 clinical and preclinical HIs, categorizing them into pan-HIF-α hydroxylase (broad spectrum), PHD-selective, and FIH-selective inhibitors, and investigated their effects on HIF-dependent transcriptional regulation, erythropoietin production, and cellular energy metabolism. While energy homeostasis was generally maintained following HI treatment, the pan-HIs led to a stronger increase in pericellular pO₂ than the PHD/FIH-selective HIs. Combined knockdown of FIH and PHD-selective inhibition did not further increase pericellular pO₂ . Hence, the additional increase in pericellular pO₂ by pan- over PHD-selective HIs likely reflects HIF hydroxylase independent off-target effects. Overall, these analyses demonstrate that HIs can lead to oxygen redistribution within the cellular microenvironment, which should be considered as a possible contributor to HI effects in the treatment of hypoxia-associated diseases.

Oxygen-dependent bond formation with FIH regulates the activity of the client protein OTUB1

Protein:protein interactions are the basis of molecular communication and are usually of transient non-covalent nature, while covalent interactions other than ubiquitination are rare. For cellular adaptations, the cellular oxygen and peroxide sensor factor inhibiting HIF (FIH) confers oxygen and oxidant stress sensitivity to the hypoxia inducible factor (HIF) by asparagine hydroxylation. We investigated whether FIH contributes to hypoxia adaptation also through other mechanisms and identified a hypoxia sensitive, likely covalent, bond formation by FIH with several client proteins, including the deubiquitinase ovarian tumor domain containing ubiquitin aldehyde binding protein 1 (OTUB1). Biochemical analyses were consistent with a co-translational amide bond formation between FIH and OTUB1, occurring within mammalian and bacterial cells but not between separately purified proteins. Bond formation is catalysed by FIH and highly dependent on oxygen availability in the cellular microenvironment. Within cells, a heterotrimeric complex is formed, consisting of two FIH and one covalently linked OTUB1. Complexation of OTUB1 by FIH regulates OTUB1 deubiquitinase activity. Our findings reveal an alternative mechanism for hypoxia adaptation with remarkably high oxygen sensitivity, mediated through covalent protein-protein interactions catalysed by an asparagine modifying dioxygenase.

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FIH forms a (likely amide) bond with client proteins.

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Bond formation is highly hypoxia sensitive and occurs co-translationally.

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FIH forms a heterotrimer with the client protein OTUB1 (FIH₂OTUB1₁).

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Complex formation between OTUB1 and FIH regulates OTUB1 deubiquitinase activity.

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Bond formation by hydroxylases is an alternative mechanism for hypoxia adaptation.

Genetic Knockdown and Pharmacologic Inhibition of Hypoxia-Inducible Factor (HIF) Hydroxylases

Reduced oxygen supply that does not satisfy tissue and cellular demand (hypoxia) regularly occurs both in health and disease. Hence, the capacity for cellular oxygen sensing is of vital importance for each cell to be able to alter its energy metabolism and promote adaptation to hypoxia. The hypoxia-inducible factor (HIF) prolyl hydroxylases 1-3 (PHD1-3) and the asparagine hydroxylase factor-inhibiting HIF (FIH) are the primary cellular oxygen sensors, which confer cellular oxygen-dependent sensitivity upon HIF as well as other hypoxia-sensitive pathways, such as nuclear factor κB (NF-κB). Studying these enzymes allows us to understand the oxygen-dependent regulation of cellular processes and has led to the development of several putative novel therapeutics, which are currently in clinical trials for the treatment of anemia associated with kidney disease. Pharmacologic inhibition and genetic knockdown are commonly established techniques in protein biochemistry and are used to investigate the activity and function of proteins. Here, we describe specific protocols for the knockdown and inhibition of the HIF prolyl hydroxylases 1-3 (PHD1-3) and the asparagine hydroxylase factor-inhibiting HIF (FIH) using RNA interference (RNAi) and hydroxylase inhibitors, respectively. These techniques are essential tools for the analysis of the function of the HIF hydroxylases, allowing the investigation and discovery of novel functions and substrates of these enzymes.

Effectiveness of a standardized footbath protocol for prevention of digital dermatitis

A footbath is the most common herd-level approach to control digital dermatitis (DD) in intensive dairy farming. However, wide variation in footbath design and protocols suggests a gap between evidence-based management practices and on-farm implementation. The objective was to evaluate effectiveness of a standardized footbath protocol in decreasing prevalence of active DD lesions in lactating dairy cattle. The protocol was based on current scientific literature, including footbath design and management. The M-stage classification system was used to score DD lesions. Scores were also combined into a simplified scoring system: no lesions (M0), active lesions (M1, M2, and M4.1), and chronic lesions (M3 and M4). A controlled intervention trial was conducted on 9 farms over 22 wk. Each farm served as its own control with data collected for 10 wk before and 12 wk after intervention. A total of 1,978 lactating cattle were assessed biweekly for DD lesions and leg cleanliness in the milking parlor. Lactating cattle were also inspected in the trimming chute at 3 time points: start, intervention, and end of trial. Intervention consisted of implementing an automated footbath that measured 3 m long, 0.50 m wide, held a fluid depth of 0.15 m, along with a weekly footbath protocol using 5% CuSO₄ for 4 consecutive milkings, with footbath content replaced at a maximum of 200 cow passes. Multilevel logistic regression models for repeated measures were used to evaluate effects of the standardized footbath protocol in preventing active DD lesions. For the purpose of analysis, farms' within-herd prevalence of active DD lesions (at baseline) was assessed and categorized as low (<15%) or high (≥15%). Farms with low or high within-herd prevalence of active DD lesions at trial outset had a mean cow-level prevalence of active DD lesions of 8% (range, 2 to 13) and 31% (range, 18 to 43), respectively. At milking parlor inspections, apparent prevalence of active DD lesions decreased from the time of footbath intervention, but this effect interacted with the farms' baseline prevalence of active DD lesions. In that regard, on farms with high prevalence of active DD at baseline, apparent prevalence of active DD lesions decreased after intervention, whereas on farms with low prevalence of active DD at baseline, apparent prevalence of active DD lesions did not change. At the cow level, poor leg cleanliness was associated with higher prevalence of active DD lesions. At trimming chute inspections, prevalence of active DD lesions decreased from start to the end of the trial (22 and 14%, respectively); concurrently, prevalence of feet with no DD lesions (M0) increased (39 and 48%). We concluded that on farms with high DD prevalence, implementation of proper footbath design and improvement of footbathing management will decrease prevalence of active DD lesions and increase prevalence of feet without DD lesions. In addition, improving cow cleanliness will further result in control of active DD lesions.

Crosstalk between Microbiota-Derived Short-Chain Fatty Acids and Intestinal Epithelial HIF Augments Tissue Barrier Function

Interactions between the microbiota and distal gut are fundamental determinants of human health. Such interactions are concentrated at the colonic mucosa and provide energy for the host epithelium through the production of the short-chain fatty acid butyrate. We sought to determine the role of epithelial butyrate metabolism in establishing the austere oxygenation profile of the distal gut. Bacteria-derived butyrate affects epithelial O2 consumption and results in stabilization of hypoxia-inducible factor (HIF), a transcription factor coordinating barrier protection. Antibiotic-mediated depletion of the microbiota reduces colonic butyrate and HIF expression, both of which are restored by butyrate supplementation. Additionally, germ-free mice exhibit diminished retention of O2-sensitive dyes and decreased stabilized HIF. Furthermore, the influences of butyrate are lost in cells lacking HIF, thus linking butyrate metabolism to stabilized HIF and barrier function. This work highlights a mechanism where host-microbe interactions augment barrier function in the distal gut.

Distribution and morphology of immunoreactive gonadotropin-releasing hormone (GnRH) neurons in the basal forebrain of ponies

Recent reports have indicated that analysis of changes in the staining characteristics of gonadotropin-releasing hormone (GnRH) neurons and characterization of morphological plasticity of the related structural framework may help to elucidate the physiological mechanisms involved in neuroendocrine control of mammalian reproduction. Whether comparative studies will facilitate this process or simply elucidate species-specific mechanisms is not yet clear. The present study was performed in order to begin analysis of GnRH neurons in a seasonally breeding species that exhibits an unusually long ovulatory luteinizing hormone (LH) surge. To this end, light microscopy and image analysis were used to characterize distribution and morphology of GnRH neurons in 15 adult male and female ponies. Samples were collected in the middle of the normal ovulatory season. Unipolar, bipolar, and multipolar GnRH neurons were organized in a loosely defined continuum that extended from the medial septum to tuberoinfundibular areas in the medical basal hypothalamus (MBH). Most cells were bipolar, and the majority of neurons were located in the MBH. Fiber projections to the median eminence included presumptive pathways similar to those previously described in other species. Image analysis of cell size indicated that cells in the MBH were larger than those in preoptic areas and GnRH neurons in both of these locations were larger than neurons in rostral areas of the medial septum. Results from this experiment suggest that the large population of MBH GnRH neurons in the equine species is likely to be of primary importance to reproductive function, whereas cells in other areas are fewer and smaller. Further work is needed to characterize morphological characteristics that may be related to physiological fluctuations in reproductive function of the equine species.