Erythropoietin and respiratory control at adulthood and during early postnatal life

Low serum erythropoietin levels are associated with fatal COVID-19 cases at 4,150 meters above sea level

Previous studies suggested that erythropoietin (EPO) may protect against severe COVID-19-induced injuries, ultimately preventing mortality. This hypothesis is based on the fact that, in addition to promoting the increase in red blood cells, EPO is an anti-inflammatory, anti-apoptotic and protective factor in several non-erythropoietic tissues. Furthermore, EPO promotes nitric oxide production in the hypoxic lung and stimulates ventilation by interacting with the respiratory centers of the brainstem. Given that EPO in the blood is increased at high-altitude, we evaluated the serum levels of EPO in critical patients with COVID-19 at “Hospital Agramont” in the city of El Alto (4150 masl) in Bolivia. A total of 16 patients, 15 men, one woman, with a mean age of 55.8 ± 8.49 years, admitted to the Intensive Care Unit were studied. All patients were permanent residents of El Alto, with no travel history below 3000 masl for at least one year. Blood samples were collected upon admission to the ICU. Serum EPO concentration was assessed using an ELISA kit, and a standard technique determined hemoglobin concentration. Only half of the observed patients survived the disease. Remarkably, fatal cases showed 2.5 times lower serum EPO than survivors (2.78 ± 0.8643 mU/mL vs 7.06 ± 2.713 mU/mL; p = 0.0096), and 1.24 times lower hemoglobin levels (13.96 ± 2.56 g/dL vs 17.41 ± 1.61 g/dL; p = 0.0159). While the number of cases evaluated in this work is low, our findings strongly warrant further investigation of EPO levels in COVID-19 patients at high and low altitudes. Our results also support the hypothesis that exogenous EPO administration could help critically ill COVID-19 patients overcome the disease.

Systemic control of immune cell development by integrated carbon dioxide and hypoxia chemosensation in Drosophila

Drosophila hemocytes are akin to mammalian myeloid blood cells that function in stress and innate immune-related responses. A multi-potent progenitor population responds to local signals and to systemic stress by expanding the number of functional blood cells. Here we show mechanisms that demonstrate an integration of environmental carbon dioxide (CO2) and oxygen (O2) inputs that initiate a cascade of signaling events, involving multiple organs, as a stress response when the levels of these two important respiratory gases fall below a threshold. The CO2 and hypoxia-sensing neurons interact at the synaptic level in the brain sending a systemic signal via the fat body to modulate differentiation of a specific class of immune cells. Our findings establish a link between environmental gas sensation and myeloid cell development in Drosophila. A similar relationship exists in humans, but the underlying mechanisms remain to be established.

Acute Effects of Systemic Erythropoietin Injections on Carotid Body Chemosensory Activity Following Hypoxic and Hypercapnic Stimulation

The carotid body (CB) chemoreceptors sense changes in arterial blood gases. Upon stimulation CB chemoreceptors cells release one or more transmitters to excite sensory nerve fibers of the carotid sinus nerve. While several neurotransmitters have been described to contribute to the CB chemosensory process less is known about modulatory molecules. Recent data suggest that erythropoietin (Epo) is involved in the control of ventilation, and it has been shown that Epo receptor is constitutively expressed in the CB chemoreceptors, suggesting a possible role for Epo in regulation of CB function. Therefore, in the present study we aimed to determine whether exogenous applications of Epo modulate the hypoxic and hypercapnic CB chemosensory responses. Carotid sinus nerve discharge was recorded in-situ from anesthetized adult male and female Sprague Dawley rats (350 g, n = 8) before and after systemic administration of Epo (2000 UI/kg). CB-chemosensitivity to hypoxia and hypercapnia was calculated by exposing the rat to F_iO₂ 5-15% and F_iCO₂ 10% gas mixtures, respectively. During baseline recordings at normoxia, we found no effects of Epo on CB activity both in male and female rats. In addition, Epo had no effect on maximal CB response to hypoxia in both male and female rats. Epo injections enhanced the maximum CB chemosensory response to hypercapnia in female rats (before vs. after Epo, 72.5 ± 7.1 Hz vs. 108.3 ± 6.9 Hz, p < 0.05). In contrast, Epo had no effect on maximum CB chemosensory response to hypercapnia in male rats but significantly increased the response recovery times (time required to return to baseline discharge following hypercapnic stimulus) from 2.1 ± 0.1 s to 8.2 ± 2.3 s (p < 0.05). Taken together, our results suggest that Epo has some modulatory effect on the CB chemosensory response to hypercapnia.

Increased ventilation in female erythropoietin-deficient mouse line is not progesterone and estrous stage-dependent

Previous studies suggest that chronic erythropoietin (Epo) deficiency in male mice does not alter normoxic/hypoxic ventilation. As effects of Epo are sex specific and as progesterone could be a respiratory stimulant, we evaluated the impact of Epo deficiency and its possible interaction with progesterone in ventilatory control in female mice during estrous cycle phases. Compared to wild type (WT) animals, Epo-TAg^h female mice exhibited higher ventilation in hypoxia. However, when data were separated into luteal and follicular phases of the estrous cycle, basal ventilation and hypoxic ventilation were not different in both mice strains. As progesterone is known to be a potent respiratory stimulant, additional experiments were performed to elucidate its role. Interestingly, after mifepristone treatment, HVR was not modified in WT and Epo-TAg^h mice, showing that the ventilatory stimulation observed in females was not directly mediated by progesterone. We conclude that Epo-TAg^h female mice show no estrous stage-dependent increase of ventilatory control and progesterone independent response to hypoxia.

Carbamylated erythropoietin enhances mice ventilatory responses to changes in O2 but not CO2 levels

Erythropoietin (EPO) has beneficial tissue-protective effects in several diseases but erythrocytosis may cause deleterious effects in EPO-treated patients. Thus carbamylated-EPO (C-EPO) and other derivatives retaining tissue-protective but lacking bone marrow-stimulating actions have been developed. Although EPO modulates ventilatory responses, the effects of C-EPO on ventilation have not been investigated. Here, basal breathing and respiratory chemoreflexes were measured by plethysmography after acute and chronic treatments with recombinant human C-EPO (rhC-EPO; 15,000 IU/kg during 5days) or saline (control group). Hematocrit, plasma and brainstem rhC-EPO levels were also quantified. Chronic rhC-EPO significantly elevated tissue rhC-EPO levels but not hematocrit. None of the drug regimen altered basal ventilation (normoxia). Chronic but not acute rhC-EPO enhanced hyperoxic ventilatory depression, and sustained the hypoxic ventilatory response mainly via a reduction of the roll-off phase. By contrast, rhC-EPO did not blunt the ventilatory response to hypercapnia. Thus, chronic C-EPO may be a promising therapy to improve breathing during hypoxia while minimizing adverse effects on cardiovascular function.

Time Domains of the Hypoxic Ventilatory Response and Their Molecular Basis

Ventilatory responses to hypoxia vary widely depending on the pattern and length of hypoxic exposure. Acute, prolonged, or intermittent hypoxic episodes can increase or decrease breathing for seconds to years, both during the hypoxic stimulus, and also after its removal. These myriad effects are the result of a complicated web of molecular interactions that underlie plasticity in the respiratory control reflex circuits and ultimately control the physiology of breathing in hypoxia. Since the time domains of the physiological hypoxic ventilatory response (HVR) were identified, considerable research effort has gone toward elucidating the underlying molecular mechanisms that mediate these varied responses. This research has begun to describe complicated and plastic interactions in the relay circuits between the peripheral chemoreceptors and the ventilatory control circuits within the central nervous system. Intriguingly, many of these molecular pathways seem to share key components between the different time domains, suggesting that varied physiological HVRs are the result of specific modifications to overlapping pathways. This review highlights what has been discovered regarding the cell and molecular level control of the time domains of the HVR, and highlights key areas where further research is required. Understanding the molecular control of ventilation in hypoxia has important implications for basic physiology and is emerging as an important component of several clinical fields. © 2016 American Physiological Society. Compr Physiol 6:1345-1385, 2016.

Endogenous brain erythropoietin is a potent sex-specific respiratory stimulant in adult and newborn mice

We tested the hypothesis that endogenous brain Epo is a respiratory stimulant. Adult (3 mo) and newborn (10 days) male and female mice received an intracisternal (cisterna magna) injection of soluble Epo receptor (sEpoR; competes with EpoR to bind Epo; 50 μg/ml) or vehicle (0.1% BSA in PBS). Twenty-four hours after injection, we used whole body plethysmography to record minute ventilation (V̇e) tidal volume (VT), respiratory frequency (fR), O2 consumption (V̇o2), and CO2 production (V̇co2) under normoxia and progressive exposure to hypoxia (12-10-6% O2; 10 min each). In adult male and female mice sEpoR decreased normoxic V̇e (-25%), due to a decrease of VT in males and fR in females. Moreover, sEpoR injection decreased the ventilatory response to 12% O2, assessed as V̇e/V̇o2 or V̇e/V̇co2, in male but not in female mice. In newborn male and female mice sEpoR decreased V̇e (-37% in males, -59% in females) and VT (-38% in males, -47% in females) in normoxia and fR in females. During hypoxia, sEpoR decreased V̇e/V̇o2 and V̇e/V̇co2 in mice of both sexes. Upon extreme hypoxia (6% O2), the newborn mice treated with sEpoR showed respiratory depression, signs of asphyxia (gasping) and a high mortality rate in males and females. We concluded that endogenous brain Epo is a potent respiratory stimulant under normoxia and hypoxia in adult and newborn mice. Because sex-specific effects are different in newborn male and female, sex steroids secreted at different ages mice appear to modulate the effects of Epo on respiratory regulation in normoxia and in response to hypoxia.

Haematological acclimation and re-acclimation to hypoxia in the mouse

Haematological responses throughout 4 w of initial acclimation (IA) and three paradigms of re-acclimation (RA) to hypoxia (FI(O₂)) were examined in female mice. We hypothesised that (i) haematological responses would be increased during re-exposure, resulting in greater O₂-carrying capacity in RA compared to IA; and (ii) further improvements would occur when abbreviating the de-acclimation period to 1 w (RA↓DA) or extending the IA period to 8 w (RA↑IA). The serum [EPO] response was blunted in all RA groups compared to IA but the resulting reticulocyte response was similar in all experimental groups. The [Hb] response was the same in RA and RA↓DA as in IA but was blunted in RA↑IA due to a reduction in mean corpuscular Hb. The sensitivity of EPO-producing cells appears blunted but the sensitivity of erythroid precursors to EPO is enhanced by recent hypoxic exposure. Erythropoietic regulation is altered during RA in a manner that is dependent on the paradigm of initial exposure.

Serum testosterone levels and excessive erythrocytosis during the process of adaptation to high altitudes

Populations living at high altitudes (HAs), particularly in the Peruvian Andes, are characterized by a mixture of subjects with erythrocytosis (16 g dl(-1)<haemoglobin (Hb)≤21 g dl(-1)) and others with excessive erythrocytosis (EE) (Hb>21 g dl(-1)). Elevated haemoglobin values (EE) are associated with chronic mountain sickness, a condition reflecting the lack of adaptation to HA. According to current data, native men from regions of HA are not adequately adapted to live at such altitudes if they have elevated serum testosterone levels. This seems to be due to an increased conversion of dehydroepiandrosterone sulphate (DHEAS) to testosterone. Men with erythrocytosis at HAs show higher serum androstenedione levels and a lower testosterone/androstenedione ratio than men with EE, suggesting reduced 17beta-hydroxysteroid dehydrogenase (17beta-HSD) activity. Lower 17beta-HSD activity via Δ4-steroid production in men with erythrocytosis at HA may protect against elevated serum testosterone levels, thus preventing EE. The higher conversion of DHEAS to testosterone in subjects with EE indicates increased 17beta-HSD activity via the Δ5-pathway. Currently, there are various situations in which people live (human biodiversity) with low or high haemoglobin levels at HA. Antiquity could be an important adaptation component for life at HA, and testosterone seems to participate in this process.

Evaluation of the therapeutic benefit of delayed administration of erythropoietin following early hypoxic-ischemic injury in rodents

Hypoxia-ischemia (HI) and associated brain injuries are seen in premature as well as term infants with birth complications. The resulting impairments involve deficits in many cognitive domains, including language development. Poor rapid auditory processing is hypothesized to be one possible underlying factor leading to subsequent language delays. Mild hypothermia treatment for HI injuries in term infants is widely used as an intervention but can be costly and time consuming. Data suggest that the effectiveness of hypothermia treatment following HI injury declines beyond 6 h following injury. Consequently, the availability of a therapeutic alternative without these limitations could allow doctors to treat HI-injured infants more effectively and thus reduce deleterious cognitive and language outcomes. Evidence from both human studies and animal models of neonatal HI suggests that erythropoietin (Epo), an endogenous cytokine hormone, may be a therapeutic agent that can ameliorate HI brain injury and preserve subsequent cognitive development and function. The current study sought to investigate the therapeutic effectiveness of Epo when administered immediately after HI injury, or delayed at intervals following the injury, in neonatal rodents. Rat pups received an induced HI injury on postnatal day 7, followed by an intraperitoneal injection of Epo (1,000 U/kg) immediately, 60 min, or 180 min following induction of injury. Subjects were tested on rapid auditory processing tasks in juvenile (P38-42) and adult periods (P80-85). Ventricular and cortical size was also measured from post mortem tissue. Results from the current study show a therapeutic benefit of Epo when given immediately following induction of HI injury, with diminished benefit from a 60-min-delayed injection of Epo and no protection following a 180-min-delayed injection. The current data thus show that the effectiveness of a single dose of Epo in ameliorating auditory processing deficits following HI injury decreases precipitously as treatment is delayed following injury. These data may have important implications for experimental human neonatal intervention with Epo.