The glial cell response is an essential component of hypoxia-induced erythropoiesis in mice

Cardiomyocyte crosstalk with endothelium modulates cardiac structure, function, and ischemia-reperfusion injury susceptibility through erythropoietin

Erythropoietin (EPO) exerts non-canonical roles beyond erythropoiesis that are developmentally, structurally, and physiologically relevant for the heart as a paracrine factor. The role for paracrine EPO signalling and cellular crosstalk in the adult is uncertain. Here, we provided novel evidence showing cardiomyocyte restricted loss of function in Epo in adult mice induced hyper-compensatory increases in Epo expression by adjacent cardiac endothelial cells via HIF-2α independent mechanisms. These hearts showed concentric cellular hypertrophy, elevated contractility and relaxation, and greater resistance to ischemia-reperfusion injury. Voluntary exercise capacity compared to control hearts was improved independent of any changes to whole-body metabolism or blood O2 content or delivery (i.e., hematocrit). Our findings suggest cardiac EPO had a localized effect within the normoxic heart, which was regulated by cell-specific EPO-reciprocity between cardiomyocytes and endothelium. Within the heart, hyper-compensated endothelial Epo expression was accompanied by elevated Vegfr1 and Vegfb RNA, that upon pharmacological pan-inhibition of VEGF-VEGFR signaling, resulted in a paradoxical upregulation in whole-heart Epo. Thus, we provide the first evidence that a novel EPO-EPOR/VEGF-VEGFR axis exists to carefully mediate cardiac homeostasis via cardiomyocyte-endothelial EPO crosstalk.

Bilateral nephrectomy impairs cardiovascular function and cerebral perfusion in a rat model of acute hemodilutional anemia

Anemia and renal failure are independent risk factors for perioperative stroke, prompting us to assess the combined impact of acute hemodilutional anemia and bilateral nephrectomy (2Nx) on microvascular brain Po2 (PBro2) in a rat model. Changes in PBro2 (phosphorescence quenching) and cardiac output (CO, echocardiography) were measured in different groups of anesthetized Sprague-Dawley rats (1.5% isoflurane, n = 5-8/group) randomized to Sham 2Nx or 2Nx and subsequently exposed to acute hemodilutional anemia (50% estimated blood volume exchange with 6% hydroxyethyl starch) or time-based controls (no hemodilution). Outcomes were assessed by ANOVA with significance assigned at P < 0.05. At baseline, 2Nx rats demonstrated reduced CO (49.9 ± 9.4 vs. 66.3 ± 19.3 mL/min; P = 0.014) and PBro2 (21.1 ± 2.9 vs. 32.4 ± 3.1 mmHg; P < 0.001) relative to Sham 2Nx rats. Following hemodilution, 2Nx rats demonstrated a further decrease in PBro2 (15.0 ± 6.3 mmHg, P = 0.022). Hemodiluted 2Nx rats did not demonstrate a comparable increase in CO after hemodilution compared with Sham 2Nx (74.8 ± 22.4 vs. 108.9 ± 18.8 mL/min, P = 0.003) that likely contributed to the observed reduction in PBro2. This impaired CO response was associated with reduced fractional shortening (33 ± 9 vs. 51 ± 5%) and increased left ventricular end-systolic volume (156 ± 51 vs. 72 ± 15 µL, P < 0.001) suggestive of systolic dysfunction. By contrast, hemodiluted Sham 2Nx animals demonstrated a robust increase in CO and preserved PBro2. These data support the hypothesis that the kidney plays a central role in maintaining cerebral perfusion and initiating the adaptive increase in CO required to optimize PBro2 during acute anemia.NEW & NOTEWORTHY This study has demonstrated that bilateral nephrectomy acutely impaired cardiac output (CO) and microvascular brain Po2 (PBro2), at baseline. Following acute hemodilution, nephrectomy prevented the adaptive increase in CO associated with acute hemodilution leading to a further reduction in PBro2, accentuating the degree of cerebral tissue hypoxia. These data support a role for the kidney in maintaining PBro2 and initiating the increase in CO that optimized brain perfusion during acute anemia.

(H)IF applicable: promotion of neurogenesis by induced HIF-2 signalling after ischaemia

HIF-2 represents a tissue-specific isoform of the hypoxia-inducible factors (HIFs) which regulate oxygen homeostasis in the cell. In acute oxygen deficiency, HIF transcription factors ensure the timely restoration of adequate oxygen supply. Particularly in medical conditions such as stroke, which have a high mortality risk due to ischaemic brain damage, rapid recovery of oxygen supply is of extraordinary importance. Nevertheless, the endogenous mechanisms are often not sufficient to respond to severe hypoxic stress with restoring oxygenation and fail to protect the tissue. Herein, we analysed murine neurospheres without functioning HIF-2α and found that special importance in the differentiation of neurons can be attributed to HIF-2 in the brain. Other processes, such as cell migration and signal transduction of different signalling pathways, appear to be mediated to some extent via HIF-2 and illustrate the function of HIF-2 in brain remodelling. Without hypoxic stress, HIF-2 in the brain presumably focuses on the fine-tuning of the neural network. However, a therapeutically increase of HIF-2 has the potential to regenerate or replace destroyed brain tissue and help minimize the consequences of an ischaemic stroke.

HIF-1α in the Crosstalk Between Reactive Oxygen Species and Autophagy Process: A Review in Multiple Sclerosis

Cellular stress can lead to the production of reactive oxygen species (ROS) while autophagy, as a catabolic pathway, protects the cells against stress. Autophagy in its turn plays a pivotal role in the pathophysiology of multiple sclerosis (MS). In the current review, we first summarized the contribution of ROS and autophagy to MS pathogenesis. Then probable crosstalk between these two pathways through HIF-1α for the first time has been proposed with the hope of employing a better understanding of MS pathophysiology and probable therapeutic approaches.

Glial type specific regulation of CNS angiogenesis by HIFα-activated different signaling pathways

The mechanisms by which oligodendroglia modulate CNS angiogenesis remain elusive. Previous in vitro data suggest that oligodendroglia regulate CNS endothelial cell proliferation and blood vessel formation through hypoxia inducible factor alpha (HIFα)-activated Wnt (but not VEGF) signaling. Using in vivo genetic models, we show that HIFα in oligodendroglia is necessary and sufficient for angiogenesis independent of CNS regions. At the molecular level, HIFα stabilization in oligodendroglia does not perturb Wnt signaling but rather activates VEGF. At the functional level, genetically blocking oligodendroglia-derived VEGF but not Wnt significantly decreases oligodendroglial HIFα-regulated CNS angiogenesis. Blocking astroglia-derived Wnt signaling reduces astroglial HIFα-regulated CNS angiogenesis. Together, our in vivo data demonstrate that oligodendroglial HIFα regulates CNS angiogenesis through Wnt-independent and VEGF-dependent signaling. These findings suggest an alternative mechanistic understanding of CNS angiogenesis by postnatal glial cells and unveil a glial cell type-dependent HIFα-Wnt axis in regulating CNS vessel formation.

In the central nervous system, the maturation of glial cells is temporally and functionally coupled with that of the vascular network during postnatal development. Here the authors show that oligodendroglial HIFα regulates CNS angiogenesis through Wnt-independent and VEGF-dependent signaling, while astroglial HIFα participates through Wnt-dependent signaling.

The EPO-FGF23 Signaling Pathway in Erythroid Progenitor Cells: Opening a New Area of Research

We provide an overview of the evidence for an erythropoietin-fibroblast growth factor 23 (FGF23) signaling pathway directly influencing erythroid cells in the bone marrow. We outline its importance for red blood cell production, which might add, among others, to the understanding of bone marrow responses to endogenous erythropoietin in rare hereditary anemias. FGF23 is a hormone that is mainly known as the core regulator of phosphate and vitamin D metabolism and it has been recognized as an important regulator of bone mineralization. Osseous tissue has been regarded as the major source of FGF23. Interestingly, erythroid progenitor cells highly express FGF23 protein and carry the FGF receptor. This implies that erythroid progenitor cells could be a prime target in FGF23 biology. FGF23 is formed as an intact, biologically active protein (iFGF23) and proteolytic cleavage results in the formation of the presumed inactive C-terminal tail of FGF23 (cFGF23). FGF23-knockout or injection of an iFGF23 blocking peptide in mice results in increased erythropoiesis, reduced erythroid cell apoptosis and elevated renal and bone marrow erythropoietin mRNA expression with increased levels of circulating erythropoietin. By competitive inhibition, a relative increase in cFGF23 compared to iFGF23 results in reduced FGF23 receptor signaling and mimics the positive effects of FGF23-knockout or iFGF23 blocking peptide. Injection of recombinant erythropoietin increases FGF23 mRNA expression in the bone marrow with a concomitant increase in circulating FGF23 protein. However, erythropoietin also augments iFGF23 cleavage, thereby decreasing the iFGF23 to cFGF23 ratio. Therefore, the net result of erythropoietin is a reduction of iFGF23 to cFGF23 ratio, which inhibits the effects of iFGF23 on erythropoiesis and erythropoietin production. Elucidation of the EPO-FGF23 signaling pathway and its downstream signaling in hereditary anemias with chronic hemolysis or ineffective erythropoiesis adds to the understanding of the pathophysiology of these diseases and its complications; in addition, it provides promising new targets for treatment downstream of erythropoietin in the signaling cascade.

VEGF expands erythropoiesis via hypoxia-independent induction of erythropoietin in noncanonical perivascular stromal cells

Greenwald et al. show that VEGF can function as a potent hypoxia-independent inducer of erythropoietin in a population of VSMC-like perivascular stromal cells in a PDGFRβ-dependent manner, thereby expanding erythropoiesis by recruitment of additional cell types to the erythropoietin-producing cell pool.

Insufficient erythropoiesis due to increased demand is usually met by hypoxia-driven up-regulation of erythropoietin (Epo). Here, we uncovered vascular endothelial growth factor (VEGF) as a novel inducer of Epo capable of increasing circulating Epo under normoxic, nonanemic conditions in a previously unrecognized reservoir of Epo-producing cells (EPCs), leading to expansion of the erythroid progenitor pool and robust splenic erythropoiesis. Epo induction by VEGF occurs in kidney, liver, and spleen in a population of Gli1⁺SMA⁺PDGFRβ⁺ cells, a signature shared with vascular smooth muscle cells (VSMCs) derived from mesenchymal stem cell–like progenitors. Surprisingly, inhibition of PDGFRβ signaling, but not VEGF signaling, abrogated VEGF-induced Epo synthesis. We thus introduce VEGF as a new player in Epo induction and perivascular Gli1⁺SMA⁺PDGFRβ⁺ cells as a previously unrecognized EPC reservoir that could be harnessed for augmenting Epo synthesis in circumstances such as chronic kidney disease where production by canonical EPCs is compromised.

Astrocyte HIF-2α supports learning in a passive avoidance paradigm under hypoxic stress

BACKGROUND

The brain is extensively vascularized, useŝ20% of the body's oxygen, and is highly sensitive to changes in oxygen. While synaptic plasticity and memory are impaired in healthy individuals by exposure to mild hypoxia, aged individuals appear to be even more sensitive. Aging is associated with progressive failure in pulmonary and cardiovascular systems, exposing the aged to both chronic and superimposed acute hypoxia. The HIF proteins, the "master regulators" of the cellular response to hypoxia, are robustly expressed in neurons and astrocytes. Astrocytes support neurons and synaptic plasticity via complex metabolic and trophic mechanisms. The activity of HIF proteins in the brain is diminished with aging, and the increased exposure to chronic and acute hypoxia with aging combined with diminished HIF activity may impair synaptic plasticity.

PURPOSE

Herein, we test the hypothesis that astrocyte HIF supports synaptic plasticity and learning upon hypoxia.

MATERIALS AND METHODS

An Astrocyte-specific HIF loss-of-function model was employed, where knock-out of HIF-1α or HIF-2α in GFAP expressing cells was accomplished by cre-mediated recombination. Animals were tested for behavioral (open field and rotarod), learning (passive avoidance paradigm), and electrophysiological (long term potentiation) responses to mild hypoxic challenge.

RESULTS

In an astrocyte-specific HIF loss-of-function model followed by mild hypoxia, we identified that the depletion of HIF-2α resulted in an impaired passive avoidance learning performance. This was accompanied by an attenuated response to induction in long-term potentiation (LTP), suggesting that the hippocampal circuitry was perturbed upon hypoxic exposure following HIF-2α loss in astrocytes, and not due to hippocampal cell death. We investigated HIF-regulated trophic and metabolic target genes and found that they were not regulated by HIF-2α, suggesting that these specific targets may not be involved in mediating the phenotypes observed.

CONCLUSION

Together, these results point to a role for HIF-2α in the astrocyte's regulatory role in synaptic plasticity and learning under hypoxia and suggest that even mild, acute hypoxic challenges can impair cognitive performance in the aged population who harbor impaired HIF function.

Hypoxic stress upregulates Kir2.1 expression by a pathway including hypoxic-inducible factor-1α and dynamin2 in brain capillary endothelial cells

Brain capillary endothelial cells (BCECs) play a central role in maintenance of blood-brain barrier (BBB) function and, therefore, are essential for central nervous system homeostasis and integrity. Although brain ischemia damages BCECs and causes disruption of BBB, the related influence of hypoxia on BCECs is not well understood. Hypoxic stress can upregulate functional expression of specific K+ currents in endothelial cells, e.g., Kir2.1 channels without any alterations in the mRNA level, in t-BBEC117, a cell line derived from bovine BCECs. The hyperpolarization of membrane potential due to Kir2.1 channel upregulation significantly facilitates cell proliferation. In the present study, the mechanisms underlying the hypoxia-induced Kir2.1 upregulation was examined. We emphasize the involvement of dynamin2, a protein known to be involved in a number of surface expression pathways. Hypoxic culture upregulated dynamin2 expression in t-BBEC117 cells. The inhibition of dynamin2 by Dynasore canceled hypoxia-induced upregulation of Kir2.1 currents by reducing surface expression. On the contrary, Kir2.1 currents and proteins in t-BBEC117 cultured under normoxia were increased by overexpression of dynamin2, but not by dominant-negative dynamin2. Molecular imaging based on bimolecular fluorescence complementation, double-immunostaining, and coimmunoprecipitation assays revealed that dynamin2 can directly bind to the Kir2.1 channel. Moreover, hypoxic culture downregulated hypoxic-inducible factor-1α (HIF-1α) expression. Knockdown of HIF-1α increased dynamin2 expression in t-BBEC117 cells, in both normoxic and hypoxic culture conditions. In summary, our results demonstrated that hypoxia downregulates HIF-1α, increases dynamin2 expression, and facilitates Kir2.1 surface expression, resulting in hyperpolarization of membrane potential and subsequent increase in Ca2+ influx in BCECs.

Targeting Hypoxia-Inducible Factor-1α/Pyruvate Dehydrogenase Kinase 1 Axis by Dichloroacetate Suppresses Bleomycin-induced Pulmonary Fibrosis

Hypoxia has long been implicated in the pathogenesis of fibrotic diseases. Aberrantly activated myofibroblasts are the primary pathological driver of fibrotic progression, yet how various microenvironmental influences, such as hypoxia, contribute to their sustained activation and differentiation is poorly understood. As a defining feature of hypoxia is its impact on cellular metabolism, we sought to investigate how hypoxia-induced metabolic reprogramming affects myofibroblast differentiation and fibrotic progression, and to test the preclinical efficacy of targeting glycolytic metabolism for the treatment of pulmonary fibrosis. Bleomycin-induced pulmonary fibrotic progression was evaluated in two independent, fibroblast-specific, promoter-driven, hypoxia-inducible factor (Hif) 1A knockout mouse models and in glycolytic inhibitor, dichloroacetate-treated mice. Genetic and pharmacological approaches were used to explicate the role of metabolic reprogramming in myofibroblast differentiation. Hypoxia significantly enhanced transforming growth factor-β-induced myofibroblast differentiation through HIF-1α, whereas overexpression of the critical HIF-1α-mediated glycolytic switch, pyruvate dehydrogenase kinase 1 (PDK1) was sufficient to activate glycolysis and potentiate myofibroblast differentiation, even in the absence of HIF-1α. Inhibition of the HIF-1α/PDK1 axis by genomic deletion of Hif1A or pharmacological inhibition of PDK1 significantly attenuated bleomycin-induced pulmonary fibrosis. Our findings suggest that HIF-1α/PDK1-mediated glycolytic reprogramming is a critical metabolic alteration that acts to promote myofibroblast differentiation and fibrotic progression, and demonstrate that targeting glycolytic metabolism may prove to be a potential therapeutic strategy for the treatment of pulmonary fibrosis.

Erythropoietin Attenuates the Brain Edema Response after Experimental Traumatic Brain Injury

Erythropoietin (EPO) has neuroprotective effects in multiple central nervous system (CNS) injury models; however EPO's effects on traumatic brain edema are elusive. To explore EPO as an intervention in traumatic brain edema, male Sprague–Dawley (SD) rats were subjected to blunt, controlled traumatic brain injury (TBI). Animals were randomized to EPO 5000 IU/kg or saline (control group) intraperitoneally within 30 min after trauma and once daily for 4 consecutive days. Brain MRI, immunohistofluorescence, immunohistochemistry, and quantitative protein analysis were performed at days 1 and 4 post- trauma. EPO significantly prevented the loss of the tight junction protein zona occludens 1 (ZO-1) observed in control animals after trauma. The decrease of ZO-1 in the control group was associated with an immunoglobulin (Ig)G increase in the perilesional parenchyma, indicating blood–brain barrier (BBB) dysfunction and increased permeability. EPO treatment attenuated decrease in apparent diffusion coefficient (ADC) after trauma, suggesting a reduction of cytotoxic edema, and reduced the IgG leakage, indicating that EPO contributed to preserve BBB integrity and attenuated vasogenic edema. Animals treated with EPO demonstrated conserved levels of aquaporin 4 (AQP4) protein expression in the perilesional area, whereas control animals showed a reduction of AQP4. We show that post TBI administration of EPO decreases early cytotoxic brain edema and preserves structural and functional properties of the BBB, leading to attenuation of the vasogenic edema response. The data support that the mechanisms involve preservation of the tight junction protein ZO-1 and the water channel AQP4, and indicate that treatment with EPO may have beneficial effects on the brain edema response following TBI.

Prolyl-4-hydroxylase 2 and 3 coregulate murine erythropoietin in brain pericytes

A classic response to systemic hypoxia is the increased production of red blood cells due to hypoxia-inducible factor (HIF)-mediated induction of erythropoietin (EPO). EPO is a glycoprotein hormone that is essential for normal erythropoiesis and is predominantly synthesized by peritubular renal interstitial fibroblast-like cells, which express cellular markers characteristic of neuronal cells and pericytes. To investigate whether the ability to synthesize EPO is a general functional feature of pericytes, we used conditional gene targeting to examine the von Hippel-Lindau/prolyl-4-hydroxylase domain (PHD)/HIF axis in cell-expressing neural glial antigen 2, a known molecular marker of pericytes in multiple organs. We found that pericytes in the brain synthesized EPO in mice with genetic HIF activation and were capable of responding to systemic hypoxia with the induction of Epo. Using high-resolution multiplex in situ hybridization, we determined that brain pericytes represent an important cellular source of Epo in the hypoxic brain (up to 70% of all Epo-expressing cells). We furthermore determined that Epo transcription in brain pericytes was HIF-2 dependent and cocontrolled by PHD2 and PHD3, oxygen- and 2-oxoglutarate-dependent prolyl-4-hydroxylases that regulate HIF activity. In summary, our studies provide experimental evidence that pericytes in the brain have the ability to function as oxygen sensors and respond to hypoxia with EPO synthesis. Our findings furthermore suggest that the ability to synthesize EPO may represent a functional feature of pericytes in the brain and kidney.

Advances in understanding the mechanisms of erythropoiesis in homeostasis and disease

Anaemia or decreased blood haemoglobin is the most common blood disorder often characterized by reduced red blood cell (RBC) numbers. RBCs are produced from differentiation and commitment of haematopoietic stem cells to the erythroid lineage by a process called erythropoiesis. Coordination of erythropoietin receptor signalling with several erythroid transcription factors including GATA1 is essential for this process. A number of additional players that are critical for RBC production have been identified in recent years. Major technological advances, such as the development of RNA interference, genetically modified animals, including zebrafish, and imaging flow cytometry have led to these discoveries; the emergence of -omics approaches in combination with the optimization of ex vivo erythroid cultures have also produced a more comprehensive understanding of erythropoiesis. Here we summarize studies describing novel regulators of erythropoiesis that modulate erythroid cell production in the context of human erythroid disorders involving hypoxia, iron regulation, immune-related molecules, and the transcription factor FOXO3.

Anemia management after acute brain injury

Anemia is frequent among brain-injured patients, where it has been associated with an increased risk of poor outcome. The pathophysiology of anemia in this patient population remains multifactorial; moreover, whether anemia merely reflects a higher severity of the underlying disease or is a significant determinant of the neurological recovery of such patients remains unclear. Interestingly, the effects of red blood cell transfusions (RBCT) in moderately anemic patients remain controversial; although hemoglobin levels are increased, different studies observed only a modest and inconsistent improvement in cerebral oxygenation after RBCT and raised serious concerns about the risk of increased complications. Thus, considering this "blood transfusion anemia paradox", the optimal hemoglobin level to trigger RBCT in brain-injured patients has not been defined yet; also, there is insufficient evidence to provide strong recommendations regarding which hemoglobin level to target and which associated transfusion strategy (restrictive versus liberal) to select in this patient population. We summarize in this review article the more relevant studies evaluating the effects of anemia and RBCT in patients with an acute neurological condition; also, we propose some potential strategies to optimize transfusion management in such patients.

Type I interferons and microbial metabolites of tryptophan modulate astrocyte activity and central nervous system inflammation via the aryl hydrocarbon receptor

Astrocytes have important roles in the central nervous system (CNS) during health and disease. Through genome-wide analyses we detected a transcriptional response to type I interferons (IFN-Is) in astrocytes during experimental CNS autoimmunity and also in CNS lesions from patients with multiple sclerosis (MS). IFN-I signaling in astrocytes reduces inflammation and experimental autoimmune encephalomyelitis (EAE) disease scores via the ligand-activated transcription factor aryl hydrocarbon receptor (AHR) and the suppressor of cytokine signaling 2 (SOCS2). The anti-inflammatory effects of nasally administered interferon (IFN)-β are partly mediated by AHR. Dietary tryptophan is metabolized by the gut microbiota into AHR agonists that have an effect on astrocytes to limit CNS inflammation. EAE scores were increased following ampicillin treatment during the recovery phase, and CNS inflammation was reduced in antibiotic-treated mice by supplementation with the tryptophan metabolites indole, indoxyl-3-sulfate, indole-3-propionic acid and indole-3-aldehyde, or the bacterial enzyme tryptophanase. In individuals with MS, the circulating levels of AHR agonists were decreased. These findings suggest that IFN-Is produced in the CNS function in combination with metabolites derived from dietary tryptophan by the gut flora to activate AHR signaling in astrocytes and suppress CNS inflammation.

Distinct subpopulations of FOXD1 stroma-derived cells regulate renal erythropoietin

Renal peritubular interstitial fibroblast-like cells are critical for adult erythropoiesis, as they are the main source of erythropoietin (EPO). Hypoxia-inducible factor 2 (HIF-2) controls EPO synthesis in the kidney and liver and is regulated by prolyl-4-hydroxylase domain (PHD) dioxygenases PHD1, PHD2, and PHD3, which function as cellular oxygen sensors. Renal interstitial cells with EPO-producing capacity are poorly characterized, and the role of the PHD/HIF-2 axis in renal EPO-producing cell (REPC) plasticity is unclear. Here we targeted the PHD/HIF-2/EPO axis in FOXD1 stroma-derived renal interstitial cells and examined the role of individual PHDs in REPC pool size regulation and renal EPO output. Renal interstitial cells with EPO-producing capacity were entirely derived from FOXD1-expressing stroma, and Phd2 inactivation alone induced renal Epo in a limited number of renal interstitial cells. EPO induction was submaximal, as hypoxia or pharmacologic PHD inhibition further increased the REPC fraction among Phd2-/- renal interstitial cells. Moreover, Phd1 and Phd3 were differentially expressed in renal interstitium, and heterozygous deficiency for Phd1 and Phd3 increased REPC numbers in Phd2-/- mice. We propose that FOXD1 lineage renal interstitial cells consist of distinct subpopulations that differ in their responsiveness to Phd2 inactivation and thus regulation of HIF-2 activity and EPO production under hypoxia or conditions of pharmacologic or genetic PHD inactivation.

Serum erythropoietin and outcome after ischaemic stroke: a prospective study

OBJECTIVES

Erythropoietin (EPO), which is inversely associated with blood haemoglobin (Hb), exerts neuroprotective effects in experimental ischaemic stroke (IS). However, clinical treatment trials have so far been negative. Here, in patients with IS, we analysed whether serum EPO is associated with (1) initial stroke severity, (2) recovery and (3) functional outcome.

DESIGN

Prospective. Controls available at baseline.

SETTING

A Swedish hospital-initiated study with outpatient follow-up after 3 months.

PARTICIPANTS

Patients (n=600; 64% males, mean age 56 years, controls n=600) were included from the Sahlgrenska Academy Study on IS (SAHLSIS).

PRIMARY AND SECONDARY OUTCOME MEASURES

In addition to EPO and Hb, initial stroke severity was assessed by the Scandinavian Stroke Scale (SSS) and compared with SSS after 3 months (follow-up) as a measure of recovery. Functional outcome was evaluated using the modified Rankin Scale (mRS) at follow-up. Serum EPO and SSS were divided into quintiles in the multivariate regression analyses.

RESULTS

Serum EPO was 21% and 31% higher than in controls at the acute phase of IS and follow-up, respectively. In patients, acute serum EPO was 19.5% higher in severe versus mild IS. The highest acute EPO quintile adjusted for sex, age and Hb was associated with worse stroke severity quintile (OR 1.70, 95% CI 1.00 to 2.87), better stroke recovery quintile (OR 1.93, CI 1.09 to 3.41) and unfavourable mRS 3-6 (OR 2.59, CI 1.15 to 5.80). However, the fourth quintile of EPO increase (from acute to follow-up) was associated with favourable mRS 0-2 (OR 3.42, CI 1.46 to 8.03). Only the last association withstood full adjustment.

CONCLUSIONS

The crude associations between EPO and worse stroke severity and outcome lost significance after multivariate modelling. However, in patients in whom EPO increased, the association with favourable outcome remained after adjustment for multiple covariates.

Inducible glomerular erythropoietin production in the adult kidney

Hypoxia-inducible factor (HIF)-2-triggered erythropoietin production in renal interstitial fibroblast-like cells is the physiologically relevant source of erythropoietin for regulating erythropoiesis. During renal fibrosis, these cells transform into myofibroblasts and lose their ability to produce sufficient erythropoietin leading to anemia. To find if other cells for erythropoietin production might exist in the kidney we tested for the capability of nonepithelial glomerular cells to elaborate erythropoietin. Therefore, HIF transcription factors were stabilized by cell-specific deletion of the von Hippel-Lindau (VHL) gene. Inducible deletion of VHL in glomerular connexin40-expressing cells (endothelial, renin-expressing, and mesangial cells) markedly increased glomerular erythropoietin mRNA expression levels, plasma erythropoietin concentrations, and hematocrit values. These changes were mimicked by inducible cell-specific VHL deletion in renin-expressing and in mesangial cells but not in endothelial cells. The increases of erythropoietin production were absent, when VHL was co-deleted with HIF-2. The induction of glomerular erythropoietin expression was associated with the downregulation of juxtaglomerular renin expression, again in a HIF-2-dependent manner. Thus, VHL deletion in renin-expressing and in mesangial cells induces the capability to produce relevant amounts of erythropoietin and to suppress renin expression in the adult kidney if HIF-2 is stabilized.

Modulation of Sox10, HIF-1α, Survivin, and YAP by Minocycline in the Treatment of Neurodevelopmental Handicaps following Hypoxic Insult

Premature infants are at an increased risk of developing cognitive and motor handicaps due to chronic hypoxia. Although the current therapies have reduced the incidence of these handicaps, untoward side effects abound. Using a murine model of sublethal hypoxia, we demonstrated reduction in several transcription factors that modulate expression of genes known to be involved in several neural functions. We demonstrate the induction of these genes by minocycline, a tetracycline antibiotic with noncanonical functions, in both in vitro and in vivo studies. Specifically, there was induction of genes, including Sox10, Hif1a, Hif2a, Birc5, Yap1, Epo, Bdnf, Notch1 (cleaved), Pcna, Mag, Mobp, Plp1, synapsin, Adgra2, Pecam1, and reduction in activation of caspase 3, all known to affect proliferation, apoptosis, synaptic transmission, and nerve transmission. Minocycline treatment of mouse pups reared under sublethal hypoxic conditions resulted in improvement in open field testing parameters. These studies demonstrate beneficial effects of minocycline treatment following hypoxic insult, document up-regulation of several genes associated with improved cognitive function, and support the possibility of minocycline as a potential therapeutic target in the treatment of neurodevelopmental handicaps observed in the very premature newborn population. Additionally, these studies may aid in further interpretation of the effects of minocycline in the treatment trials and animal model studies of fragile X syndrome and multiple sclerosis.

Anaemia in kidney disease: harnessing hypoxia responses for therapy

Improved understanding of the oxygen-dependent regulation of erythropoiesis has provided new insights into the pathogenesis of anaemia associated with renal failure and has led to the development of novel therapeutic agents for its treatment. Hypoxia-inducible factor (HIF)-2 is a key regulator of erythropoiesis and iron metabolism. HIF-2 is activated by hypoxic conditions and controls the production of erythropoietin by renal peritubular interstitial fibroblast-like cells and hepatocytes. In anaemia associated with renal disease, erythropoiesis is suppressed due to inadequate erythropoietin production in the kidney, inflammation and iron deficiency; however, pharmacologic agents that activate the HIF axis could provide a physiologic approach to the treatment of renal anaemia by mimicking hypoxia responses that coordinate erythropoiesis with iron metabolism. This Review discusses the functional inter-relationships between erythropoietin, iron and inflammatory mediators under physiologic conditions and in relation to the pathogenesis of renal anaemia, as well as recent insights into the molecular and cellular basis of erythropoietin production in the kidney. It furthermore provides a detailed overview of current clinical experience with pharmacologic activators of HIF signalling as a novel comprehensive and physiologic approach to the treatment of anaemia.

Erythropoietin Synthesis in Renal Myofibroblasts Is Restored by Activation of Hypoxia Signaling

Erythropoietin (Epo) is produced by renal Epo-producing cells (REPs) in a hypoxia-inducible manner. The conversion of REPs into myofibroblasts and coincident loss of Epo-producing ability are the major cause of renal fibrosis and anemia. However, the hypoxic response of these transformed myofibroblasts remains unclear. Here, we used complementary in vivo transgenic and live imaging approaches to better understand the importance of hypoxia signaling in Epo production. Live imaging of REPs in transgenic mice expressing green fluorescent protein from a modified Epo-gene locus revealed that healthy REPs tightly associated with endothelium by wrapping processes around capillaries. However, this association was hampered in states of renal injury-induced inflammation previously shown to correlate with the transition to myofibroblast-transformed renal Epo-producing cells (MF-REPs). Furthermore, activation of hypoxia-inducible factors (HIFs) by genetic inactivation of HIF-prolyl hydroxylases (PHD1, PHD2, and PHD3) selectively in Epo-producing cells reactivated Epo production in MF-REPs. Loss of PHD2 in REPs restored Epo-gene expression in injured kidneys but caused polycythemia. Notably, combined deletions of PHD1 and PHD3 prevented loss of Epo expression without provoking polycythemia. Mice with PHD-deficient REPs also showed resistance to LPS-induced Epo repression in kidneys, suggesting that augmented HIF signaling counterbalances inflammatory stimuli in regulation of Epo production. Thus, augmentation of HIF signaling may be an attractive therapeutic strategy for treating renal anemia by reactivating Epo synthesis in MF-REPs.

Bevacizumab treatment for human glioblastoma. Can it induce cognitive impairment?

Recent results from 2 double-blind, placebo-controlled phase III trials (RTOG 0825) and (AVAglio) for first-line treatment of glioblastoma patients with the VEGF antibody bevacizumab, showed similar results, related to overall and progression-free survival. The RTOG 0825 trial indicated, opposed to the AVAglio trial, that patients treated with bevacizumab showed a decline in global neurocognitive function compared to untreated patients, -a decline that was most obvious after prolonged treatment. At present, there is a considerably controversy related to these observations. In the present work we point at the possibility that bevacizumab treatment of the normal brain can reduce synaptic plasticity in the hippocampus. We believe that such a phenomenon may partly explain the reduced cognitive function observed in patients in the RTOG 0825 trial. Since the same effects were not clearly defined in the AVAglio trial, further studies on putative neurocognitive effects after bevacizumab treatment are warranted.

Hypoxia Inducible Factor-1α in Astrocytes and/or Myeloid Cells Is Not Required for the Development of Autoimmune Demyelinating Disease

Despite numerous reports indicating HIF-1α expression in glia, neurons, and inflammatory cells in the CNS of MS patients, the cell-specific contribution of HIF-1α to disease pathogenesis remains unclear. Here we show that although HIF-1α is dramatically upregulated in astrocytes and myeloid cells in EAE, cell-specific depletion of HIF-1α in these two cell types surprisingly does not affect the development of neuroinflammatory disease.

Hypoxia-like tissue alterations, characterized by the upregulation of hypoxia-inducible factor-1α (HIF-1α), have been described in the normal appearing white matter and pre-demyelinating lesions of multiple sclerosis (MS) patients. As HIF-1α regulates the transcription of a wide set of genes involved in neuroprotection and neuroinflammation, HIF-1α expression may contribute to the pathogenesis of inflammatory demyelination. To test this hypothesis, we analyzed the effect of cell-specific genetic ablation or overexpression of HIF-1α on the onset and progression of experimental autoimmune encephalomyelitis (EAE), a mouse model for MS. HIF-1α was mainly expressed in astrocytes and microglia/macrophages in the mouse spinal cord at the peak of EAE. However, genetic ablation of HIF-1α in astrocytes and/or myeloid cells did not ameliorate clinical symptoms. Furthermore, conditional knock-out of Von Hippel Lindau, a negative regulator of HIF-1α stabilization, failed to exacerbate the clinical course of EAE. In accordance with clinical symptoms, genetic ablation or overexpression of HIF-1α did not change the extent of spinal cord inflammation and demyelination. Overall, our data indicate that despite dramatic upregulation of HIF-1α in astrocytes and myeloid cells in EAE, HIF-1α expression in these two cell types is not required for the development of inflammatory demyelination. Despite numerous reports indicating HIF-1α expression in glia, neurons, and inflammatory cells in the CNS of MS patients, the cell-specific contribution of HIF-1α to disease pathogenesis remains unclear. Here we show that although HIF-1α is dramatically upregulated in astrocytes and myeloid cells in EAE, cell-specific depletion of HIF-1α in these two cell types surprisingly does not affect the development of neuroinflammatory disease. Together with two recently published studies showing a role for oligodendrocyte-specific HIF-1α in myelination and T-cell-specific HIF-1α in EAE, our results demonstrate a tightly regulated cellular specificity for HIF-1α contribution in nervous system pathogenesis.

Kidney injury is independent of endothelial HIF-1α

Hypoxia-inducible transcription factors (HIFs) control cellular adaptation to low oxygen. In the kidney, activation of HIF is beneficial during injury; however, the specific contribution of HIF-1α in renal endothelial cells (EC) remains elusive. Since EC display tissue-specific heterogeneity, we investigated how HIF-1α affects key functions of glomerular EC in vitro and its contribution to renal development and pathophysiological adaptation to acute or chronic renal injury in vivo. Loss of HIF-1α in glomerular EC induces hypoxic cell death and reduces hypoxic adhesion of macrophages in vitro. In vivo, HIF-1α expression in EC in mouse kidneys is detectable but limited. Accordingly, EC-specific ablation of HIF-1α does not lead to developmental or phenotypical abnormalities in the kidney. Renal function and expression of adhesion molecules during acute ischemic kidney injury is independent of HIF-1α in EC. Likewise, inflammation and development of fibrosis after unilateral ureteric obstruction is not influenced by endothelial HIF-1α. Taken together, although HIF-1α exerts effects on glomerular EC in vitro, endothelial HIF-1α does not influence renal development and pathophysiological adaptation to kidney injury in vivo. This implies a profound difference of the hypoxic response of the renal vascular bed compared to other organs, such as the heart. This has implications for the development of pharmacological strategies targeting the endothelial hypoxic response pathways.

KEY MESSAGE

HIF-1α controls hypoxic survival and adhesion on endothelial cells (EC) in vitro. In vivo, HIF-1α expression in renal EC is low. Deletion of HIF-1α in EC does not affect kidney development and function in mice. Renal function after acute and chronic kidney injury is independent of HIF-1α in EC. Data suggest organ-specific regulation of HIF-1α function in EC.

Prolonged treatment with bevacizumab is associated with brain atrophy: a pilot study in patients with high-grade gliomas

Bevacizumab is widely used for treatment of high-grade gliomas and other malignancies. Because bevacizumab has been shown to be associated with neurocognitive decline, this study is designed to investigate whether prolonged treatment with bevacizumab is also associated with brain atrophy. We identified 12 high-grade glioma patients who received bevacizumab for 12 months at the first recurrence and 13 matched controls and blindly compared the volumes of the contralateral hemispheres and contralateral ventricle in these two groups at baseline and after 12 ± 2 months of the baseline scan by two independent analyses. The volumes of the contralateral hemispheres and ventricles did not differ significantly between the two groups at baseline. Whereas, in the control group the volumes of the contralateral hemisphere changed subtly from baseline to follow-up (p = 0.23), in the bevacizumab-treated group the volumes significantly decreased from baseline to follow-up (p = 0.03). There was significant increase in the contralateral ventricle volume from base line to follow-up scans in both the control group (p = 0.01) and in the bevacizumab group (p = 0.005). Both the absolute and the percentage changes of contralateral hemisphere volumes and contralateral ventricular volumes between the two patient groups were statistically significant (p < 0.05). Results of this study demonstrate prolonged treatment with bevacizumab is associated with atrophy of the contralateral brain hemisphere.

Regulation of obesity and insulin resistance by hypoxia-inducible factors

In obesity, dysregulated metabolism and aberrant expansion of adipose tissue lead to the development of tissue hypoxia that plays an important role in contributing to obesity-associated metabolic disorders. Recent studies utilizing adipocyte-specific hypoxia-inducible factor-α (HIF-α) gain- or loss-of-function animal models highlight the pivotal involvement of hypoxic responses in the pathogenesis of obesity-associated inflammation and insulin resistance. HIF-1α, a master transcription factor of oxygen homeostasis, induces inflammation and insulin resistance in obesity, whereas its isoform, HIF-2α, exerts opposing functions in these obesity-associated metabolic phenotypes. In this review, recent evidence elucidating functional implications of adipocyte HIFs in obesity and, more importantly, how these regulate obesity-associated inflammation, fibrosis, and insulin resistance will be discussed. Further, we propose that modulation of HIF-1 could be a potential novel therapeutic strategy for antidiabetic treatment.

Chronic hypoxia-inducible transcription factor-2 activation stably transforms juxtaglomerular renin cells into fibroblast-like cells in vivo

On the basis of previous observations that deletion of the von Hippel-Lindau protein (pVHL) in juxtaglomerular (JG) cells of the kidney suppresses renin and induces erythropoietin expression, this study aimed to characterize the events underlying this striking change of hormone expression. We found that renin cell-specific deletion of pVHL in mice leads to a phenotype switch in JG cells, from a cuboid and multiple vesicle-containing form into a flat and elongated form without vesicles. This shift of cell phenotype was accompanied by the disappearance of marker proteins for renin cells (e.g., aldo-keto reductase family 1, member 7 and connexin 40) and by the appearance of markers of fibroblast-like cells (e.g., collagen I, ecto-5'-nucleotidase, and PDGF receptor-β). Furthermore, hypoxia-inducible transcription factor-2α (HIF-2α) protein constitutively accumulated in these transformed cells. Codeletion of pVHL and HIF-2α in JG cells completely prevented the phenotypic changes. Similar to renin expression in normal JG cells, angiotensin II negatively regulated erythropoietin expression in the transformed cells. In summary, chronic activation of HIF-2 in renal JG cells leads to a reprogramming of the cells into fibroblast-like cells resembling native erythropoietin-producing cells located in the tubulointerstitium.

Hypoxia-inducible factors link iron homeostasis and erythropoiesis

Iron is required for efficient oxygen transport, and hypoxia signaling links erythropoiesis with iron homeostasis. Hypoxia induces a highly conserved signaling pathway in cells under conditions of low levels of O2. One component of this pathway, hypoxia-inducible factor (HIF), is a transcription factor that is highly active in hypoxic cells. The first HIF target gene characterized was EPO, which encodes erythropoietin-a glycoprotein hormone that controls erythropoiesis. In the past decade, there have been fundamental advances in our understanding of how hypoxia regulates iron levels to support erythropoiesis and maintain systemic iron homeostasis. We review the cell type-specific effects of hypoxia and HIFs in adaptive response to changes in oxygen and iron availability as well as potential uses of HIF modulators for patients with iron-related disorders.

HIF-1α activation results in actin cytoskeleton reorganization and modulation of Rac-1 signaling in endothelial cells

BACKGROUND

Hypoxia is a major driving force in vascularization and vascular remodeling. Pharmacological inhibition of prolyl hydroxylases (PHDs) leads to an oxygen-independent and long-lasting activation of hypoxia-inducible factors (HIFs). Whereas effects of HIF-stabilization on transcriptional responses have been thoroughly investigated in endothelial cells, the molecular details of cytoskeletal changes elicited by PHD-inhibition remain largely unknown. To investigate this important aspect of PHD-inhibition, we used a spheroid-on-matrix cell culture model.

RESULTS

Microvascular endothelial cells (glEND.2) were organized into spheroids. Migration of cells from the spheroids was quantified and analyzed by immunocytochemistry. The PHD inhibitor dimethyloxalyl glycine (DMOG) induced F-actin stress fiber formation in migrating cells, but only weakly affected microvascular endothelial cells firmly attached in a monolayer. Compared to control spheroids, the residual spheroids were larger upon PHD inhibition and contained more cells with tight VE-cadherin positive cell-cell contacts. Morphological alterations were dependent on stabilization of HIF-1α and not HIF-2α as shown in cells with stable knockdown of HIF-α isoforms. DMOG-treated endothelial cells exhibited a reduction of immunoreactive Rac-1 at the migrating front, concomitant with a diminished Rac-1 activity, whereas total Rac-1 protein remained unchanged. Two chemically distinct Rac-1 inhibitors mimicked the effects of DMOG in terms of F-actin fiber formation and orientation, as well as stabilization of residual spheroids. Furthermore, phosphorylation of p21-activated kinase PAK downstream of Rac-1 was reduced by DMOG in a HIF-1α-dependent manner. Stabilization of cell-cell contacts associated with decreased Rac-1 activity was also confirmed in human umbilical vein endothelial cells.

CONCLUSIONS

Our data demonstrates that PHD inhibition induces HIF-1α-dependent cytoskeletal remodeling in endothelial cells, which is mediated essentially by a reduction in Rac-1 signaling.

Deciphering the intracellular signaling of erythropoietin in neuronal cells

The search for potential drugs to treat neurodegenerative diseases has been intense in the last two decades. Among many candidates, erythropoietin (EPO) was identified as a potent protectant of neurons suffering from various adverse conditions. A wide array of literature indicates that endogenous or exogenous recombinant human erythropoietin and its variants activate cell signaling that initiates survival-promoting events in neurons and neuronal cells. This chapter gives an overview of the pro-survival signaling induced by endogenous and exogenous erythropoietin in vitro and in vivo and provides methods to further investigate the intracellular signaling. It is important to know that EPO is neuroprotective, but it will greatly enhance our chances to establish EPO as a new drug candidate if we know how EPO protects neurons.The descriptions below summarize our current knowledge in non-neuronal and neuronal signaling pathways induced by EPO. The signaling pathways involved in EPO are multiple; some are well known whereas others are still under intense investigation and few are observed in very specific cell types. It is important to note that neuronal signaling events triggered by EPO are still incomplete and require further research. Therefore, excellent review articles that explore specific EPO-signaling events are referenced.

MiR-17 partly promotes hematopoietic cell expansion through augmenting HIF-1α in osteoblasts

Background

Hematopoietic stem cell (HSC) regulation is highly dependent on interactions with the marrow microenvironment, of which osteogenic cells play a crucial role. While evidence is accumulating for an important role of intrinsic miR-17 in regulating HSCs and HPCs, whether miR-17 signaling pathways are also necessary in the cell-extrinsic control of hematopoiesis hereto remains poorly understood.

Methodology/Principal Findings

Using the immortalized clone with the characteristics of osteoblasts, FBMOB-hTERT, in vitro expansion, long-term culture initiating cell (LTC-IC) and non-obese diabetic/severe combined immunodeficient disease (NOD/SCID) mice repopulating cell (SRC) assay revealed that the ectopic expression of miR-17 partly promoted the ability of FBMOB-hTERT to support human cord blood (CB) CD34⁺ cell expansion and maintain their multipotency. It also seemed that osteoblastic miR-17 was prone to cause a specific expansion of the erythroid lineage. Conversely, deficient expression of miR-17 partly inhibited the hematopoietic supporting ability of FBMOB-hTERT. We further identified that HIF-1α is responsible for, at least in part, the promoted hematopoietic supporting ability of FBMOB-hTERT caused by miR-17. HIF-1α expression is markedly enhanced in miR-17 overexpressed FBMOB-hTERT upon interaction with CB CD34⁺ cells compared to other niche associated factors. More interestingly, the specific erythroid lineage expansion of CB CD34⁺ cells caused by osteoblastic miR-17 was abrogated by HIF-1α knock down.

Conclusion/Significance

Our data demonstrated that CB CD34⁺ cell expansion can be partly promoted by osteoblastic miR-17, and in particular, ectopic miR-17 can cause a specific expansion of the erythroid lineage through augmenting HIF-1α in osteoblasts.

Erythrocytosis and pulmonary hypertension in a mouse model of human HIF2A gain of function mutation

The central pathway for oxygen-dependent control of red cell mass is the prolyl hydroxylase domain protein (PHD):hypoxia inducible factor (HIF) pathway. PHD site specifically prolyl hydroxylates the transcription factor HIF-α, thereby targeting the latter for degradation. Under hypoxia, this modification is attenuated, allowing stabilized HIF-α to activate target genes, including that for erythropoietin (EPO). Studies employing genetically modified mice point to Hif-2α, one of two main Hif-α isoforms, as being the critical regulator of Epo in the adult mouse. More recently, erythrocytosis patients with heterozygous point mutations in the HIF2A gene have been identified; whether these mutations were polymorphisms unrelated to the phenotype could not be ruled out. In the present report, we characterize a mouse line bearing a G536W missense mutation in the Hif2a gene that corresponds to the first such human mutation identified (G537W). We obtained mice bearing both heterozygous and homozygous mutations at this locus. We find that these mice display, in a mutation dose-dependent manner, erythrocytosis and pulmonary hypertension with a high degree of penetrance. These findings firmly establish missense mutations in HIF-2α as a cause of erythrocytosis, highlight the importance of this HIF-α isoform in erythropoiesis, and point to physiologic consequences of HIF-2α dysregulation.

Deletion of von Hippel-Lindau protein converts renin-producing cells into erythropoietin-producing cells

States of low perfusion pressure of the kidney associate with hyperplasia or expansion of renin-producing cells, but it is unknown whether hypoxia-triggered genes contribute to these changes. Here, we stabilized hypoxia-inducible transcription factors (HIFs) in mice by conditionally deleting their negative regulator, Vhl, using the Cre/loxP system with renin-1d promoter-driven Cre expression. Vhl (−/−(REN)) mice were viable and had normal BP. Deletion of Vhl resulted in constitutive accumulation of HIF-2α in afferent arterioles and glomerular cells and HIF-1α in collecting duct cells of the adult kidney. The preglomerular vascular tree developed normally, but far fewer renin-expressing cells were present, with more than 70% of glomeruli not containing renin cells at the typical juxtaglomerular position. Moreover, these mice had an attenuated expansion of renin-producing cells in response to a low-salt diet combined with an ACE inhibitor. However, renin-producing cells of Vhl (−/−(REN)) mice expressed the erythropoietin gene, and they were markedly polycythemic. Taken together, these results suggest that hypoxia-inducible genes, regulated by VHL, are essential for normal development and physiologic adaptation of renin-producing cells. In addition, deletion of Vhl shifts the phenotype of juxtaglomerular cells from a renin- to erythropoietin-secreting cell type, presumably in response to HIF-2 accumulation.

Regulation of erythropoiesis by hypoxia-inducible factors

A classic physiologic response to systemic hypoxia is the increase in red blood cell production. Hypoxia-inducible factors (HIFs) orchestrate this response by inducing cell-type specific gene expression changes that result in increased erythropoietin (EPO) production in kidney and liver, in enhanced iron uptake and utilization and in adjustments of the bone marrow microenvironment that facilitate erythroid progenitor maturation and proliferation. In particular HIF-2 has emerged as the transcription factor that regulates EPO synthesis in the kidney and liver and plays a critical role in the regulation of intestinal iron uptake. Its key function in the hypoxic regulation of erythropoiesis is underscored by genetic studies in human populations that live at high-altitude and by mutational analysis of patients with familial erythrocytosis. This review provides a perspective on recent insights into HIF-controlled erythropoiesis and iron metabolism, and examines cell types that have EPO-producing capability. Furthermore, the review summarizes clinical syndromes associated with mutations in the O(2)-sensing pathway and the genetic changes that occur in high altitude natives. The therapeutic potential of pharmacologic HIF activation for the treatment of anemia is discussed.

HIF-1α is a protective factor in conditional PHD2-deficient mice suffering from severe HIF-2α-induced excessive erythropoiesis

Erythropoiesis must be tightly balanced to guarantee adequate oxygen delivery to all tissues in the body. This process relies predominantly on the hormone erythropoietin (EPO) and its transcription factor hypoxia inducible factor (HIF). Accumulating evidence suggests that oxygen-sensitive prolyl hydroxylases (PHDs) are important regulators of this entire system. Here, we describe a novel mouse line with conditional PHD2 inactivation (cKO P2) in renal EPO producing cells, neurons, and astrocytes that displayed excessive erythrocytosis because of severe overproduction of EPO, exclusively driven by HIF-2α. In contrast, HIF-1α served as a protective factor, ensuring survival of cKO P2 mice with HCT values up to 86%. Using different genetic approaches, we show that simultaneous inactivation of PHD2 and HIF-1α resulted in a drastic PHD3 reduction with consequent overexpression of HIF-2α-related genes, neurodegeneration, and lethality. Taken together, our results demonstrate for the first time that conditional loss of PHD2 in mice leads to HIF-2α-dependent erythrocytosis, whereas HIF-1α protects these mice, providing a platform for developing new treatments of EPO-related disorders, such as anemia.

Astrocyte dysfunction triggers neurodegeneration in a lysosomal storage disorder

The role of astrocytes in neurodegenerative processes is increasingly appreciated. Here we investigated the contribution of astrocytes to neurodegeneration in multiple sulfatase deficiency (MSD), a severe lysosomal storage disorder caused by mutations in the sulfatase modifying factor 1 (SUMF1) gene. Using Cre/Lox mouse models, we found that astrocyte-specific deletion of Sumf1 in vivo induced severe lysosomal storage and autophagy dysfunction with consequential cytoplasmic accumulation of autophagic substrates. Lysosomal storage in astrocytes was sufficient to induce degeneration of cortical neurons in vivo. Furthermore, in an ex vivo coculture assay, we observed that Sumf1(-/-) astrocytes failed to support the survival and function of wild-type cortical neurons, suggesting a non-cell autonomous mechanism for neurodegeneration. Compared with the astrocyte-specific deletion of Sumf1, the concomitant removal of Sumf1 in both neurons and glia in vivo induced a widespread neuronal loss and robust neuroinflammation. Finally, behavioral analysis of mice with astrocyte-specific deletion of Sumf1 compared with mice with Sumf1 deletion in both astrocytes and neurons allowed us to link a subset of neurological manifestations of MSD to astrocyte dysfunction. This study indicates that astrocytes are integral components of the neuropathology in MSD and that modulation of astrocyte function may impact disease course.

The HIF signaling pathway in osteoblasts directly modulates erythropoiesis through the production of EPO

Osteoblasts are an important component of the hematopoietic microenvironment in bone. However, the mechanisms by which osteoblasts control hematopoiesis remain unknown. We show that augmented HIF signaling in osteoprogenitors results in HSC niche expansion associated with selective expansion of the erythroid lineage. Increased red blood cell production occurred in an EPO-dependent manner with increased EPO expression in bone and suppressed EPO expression in the kidney. In contrast, inactivation of HIF in osteoprogenitors reduced EPO expression in bone. Importantly, augmented HIF activity in osteoprogenitors protected mice from stress-induced anemia. Pharmacologic or genetic inhibition of prolyl hydroxylases1/2/3 in osteoprogenitors elevated EPO expression in bone and increased hematocrit. These data reveal an unexpected role for osteoblasts in the production of EPO and modulation of erythropoiesis. Furthermore, these studies demonstrate a molecular role for osteoblastic PHD/VHL/HIF signaling that can be targeted to elevate both HSCs and erythroid progenitors in the local hematopoietic microenvironment.

Astrocyte pVHL and HIF-α isoforms are required for embryonic-to-adult vascular transition in the eye

The von Hippel–Lindau tumor suppressor and the hypoxia-inducible factor-αs are essential for the transition from embryonic hyaloidal vascular system to adult retinal vasculature in the mouse eye.

Successful transition from embryonic to adult circulation is critical for survival of mammalian organisms. This shift occurs in the central cardiovascular circulation and in the eye as oxygen tension increases. However, its regulation is not well understood. We have used combinatorial gene deletion and overexpression assays to assess the effect of astrocyte-targeted deletion of von Hippel–Lindau tumor suppressor (Vhl), hypoxia-inducible factor-αs (Hif-αs), and Vegf on the normal regression of the hyaloidal vessels, the fetal ocular circulation system. Astrocytic Vhl deletion induced accelerated hyaloidal regression and subsequent massive secondary outgrowth. Combinatorial gene deletion involving Vhl, Hif-αs, and Vegf genes revealed that HIF-2α/vascular endothelial growth factor signaling induces secondary outgrowth in Vhl mutants. Conversely, HIF-1α regulated macrophage migration inhibitory factor and promoted macrophage infiltration that accelerates hyaloidal vessel regression. The phenotype observed in Vhl mutants strongly resembles human persistent hyperplastic primary vitreous cases and may provide insights into vascular remodeling mechanisms in other systems.

Digoxin inhibits development of hypoxic pulmonary hypertension in mice

Chronic hypoxia is an inciting factor for the development of pulmonary arterial hypertension. The mechanisms involved in the development of hypoxic pulmonary hypertension (HPH) include hypoxia-inducible factor 1 (HIF-1)-dependent transactivation of genes controlling pulmonary arterial smooth muscle cell (PASMC) intracellular calcium concentration ([Ca(2+)](i)) and pH. Recently, digoxin was shown to inhibit HIF-1 transcriptional activity. In this study, we tested the hypothesis that digoxin could prevent and reverse the development of HPH. Mice were injected daily with saline or digoxin and exposed to room air or ambient hypoxia for 3 wk. Treatment with digoxin attenuated the development of right ventricle (RV) hypertrophy and prevented the pulmonary vascular remodeling and increases in PASMC [Ca(2+)](i), pH, and RV pressure that occur in mice exposed to chronic hypoxia. When started after pulmonary hypertension was established, digoxin attenuated the hypoxia-induced increases in RV pressure and PASMC pH and [Ca(2+)](i). These preclinical data support a role for HIF-1 inhibitors in the treatment of HPH.

Acute Vhl gene inactivation induces cardiac HIF-dependent erythropoietin gene expression

Von Hippel Lindau (Vhl) gene inactivation results in embryonic lethality. The consequences of its inactivation in adult mice, and of the ensuing activation of the hypoxia-inducible factors (HIFs), have been explored mainly in a tissue-specific manner. This mid-gestation lethality can be also circumvented by using a floxed Vhl allele in combination with an ubiquous tamoxifen-inducible recombinase Cre-ER^T2. Here, we characterize a widespread reduction in Vhl gene expression in Vhl^floxed-UBC-Cre-ER^T2 adult mice after dietary tamoxifen administration, a convenient route of administration that has yet to be fully characterized for global gene inactivation. Vhl gene inactivation rapidly resulted in a marked splenomegaly and skin erythema, accompanied by renal and hepatic induction of the erythropoietin (Epo) gene, indicative of the in vivo activation of the oxygen sensing HIF pathway. We show that acute Vhl gene inactivation also induced Epo gene expression in the heart, revealing cardiac tissue to be an extra-renal source of EPO. Indeed, primary cardiomyocytes and HL-1 cardiac cells both induce Epo gene expression when exposed to low O₂ tension in a HIF-dependent manner. Thus, as well as demonstrating the potential of dietary tamoxifen administration for gene inactivation studies in UBC-Cre-ER^T2 mouse lines, this data provides evidence of a cardiac oxygen-sensing VHL/HIF/EPO pathway in adult mice.

Pharmacological neuroprotection after perinatal hypoxic-ischemic brain injury

Perinatal hypoxia-ischemia (HI) is an important cause of neonatal brain injury. Recent progress in the search for neuroprotective compounds has provided us with several promising drugs to reduce perinatal HI-induced brain injury. In the early stage (first 6 hours after birth) therapies are concentrated on prevention of the production of reactive oxygen species or free radicals (xanthine-oxidase-, nitric oxide synthase-, and prostaglandin inhibition), anti-inflammatory effects (erythropoietin, melatonin, Xenon) and anti-apoptotic interventions (nuclear factor kappa B- and c-jun N-terminal kinase inhibition); in a later stage stimulation of neurotrophic properties in the neonatal brain (erythropoietin, growth factors) can be targeted to promote neuronal and oligodendrocyte regeneration. Combination of pharmacological means of treatment with moderate hypothermia, which is accepted now as a meaningful therapy, is probably the next step in clinical treatment to fight post-asphyxial brain damage. Further studies should be directed at a more rational use of therapies by determining the optimal time and dose to inhibit the different potentially destructive molecular pathways or to enhance endogenous repair while at the same time avoiding adverse effects of the drugs used.

Prophylactic neuroprotection against stroke: low-dose, prolonged treatment with deferoxamine or deferasirox establishes prolonged neuroprotection independent of HIF-1 function

Prophylactic neuroprotection against stroke could reduce stroke burden in thousands of patients at high risk of stroke, including those with recent transient ischemic attacks (TIAs). Prolyl hydroxylase inhibitors (PHIs), such as deferoxamine (DFO), reduce stroke volume when administered at high doses in the peristroke period, which is largely mediated by the hypoxia-inducible transcription factor (HIF-1). Yet, in vitro experiments suggest that PHIs may also induce neuroprotection independent of HIF-1. In this study, we examine chronic, prophylactic, low-dose treatment with DFO, or another iron chelator deferasirox (DFR), to determine whether they are neuroprotective with this paradigm and mediate their effects through a HIF-1-dependent mechanism. In fact, prophylactic administration of low-dose DFO or DFR significantly reduces stroke volume. Surprisingly, DFO remained neuroprotective in mice haploinsufficient for HIF-1 (HIF-1+/-) and transgenic mice with conditional loss of HIF-1 function in neurons and astrocytes. Similarly, DFR was neuroprotective in HIF-1+/- mice. Neither DFO nor DFR induced expression of HIF-1 targets. Thus, low-dose chronic administration of DFO or DFR induced a prolonged neuroprotective state independent of HIF-1 function. As DFR is an orally administered and well-tolerated medication in clinical use, it has promise for prophylaxis against stroke in patients at high risk of stroke.

Nitric oxide prevents axonal degeneration by inducing HIF-1-dependent expression of erythropoietin

Nitric oxide (NO) is a signaling molecule that can trigger adaptive (physiological) or maladaptive (pathological) responses to stress stimuli in a context-dependent manner. We have previously reported that NO may signal axonal injury to neighboring glial cells. In this study, we show that mice deficient in neuronal nitric oxide synthase (nNOS-/-) are more vulnerable than WT mice to toxin-induced peripheral neuropathy. The administration of NO donors to primary dorsal root ganglion cultures prevents axonal degeneration induced by acrylamide in a dose-dependent manner. We demonstrate that NO-induced axonal protection is dependent on hypoxia-inducible factor (HIF)-1-mediated transcription of erythropoietin (EPO) within glial (Schwann) cells present in the cultures. Transduction of Schwann cells with adenovirus AdCA5 encoding a constitutively active form of HIF-1α results in amelioration of acrylamide-induced axonal degeneration in an EPO-dependent manner. Mice that are partially deficient in HIF-1α (HIF-1α+/-) are also more susceptible than WT littermates to toxic neuropathy. Our results indicate that NO→HIF-1→EPO signaling represents an adaptive mechanism that protects against axonal degeneration.

Hypoxia-inducible factors and the response to hypoxic stress

Oxygen (O(2)) is an essential nutrient that serves as a key substrate in cellular metabolism and bioenergetics. In a variety of physiological and pathological states, organisms encounter insufficient O(2) availability, or hypoxia. In order to cope with this stress, evolutionarily conserved responses are engaged. In mammals, the primary transcriptional response to hypoxic stress is mediated by the hypoxia-inducible factors (HIFs). While canonically regulated by prolyl hydroxylase domain-containing enzymes (PHDs), the HIFα subunits are intricately responsive to numerous other factors, including factor-inhibiting HIF1α (FIH1), sirtuins, and metabolites. These transcription factors function in normal tissue homeostasis and impinge on critical aspects of disease progression and recovery. Insights from basic HIF biology are being translated into pharmaceuticals targeting the HIF pathway.

Regulation of erythropoietin production

The hormone erythropoietin (Epo) maintains red blood cell mass by promoting the survival, proliferation and differentiation of erythrocytic progenitors. Circulating Epo originates mainly from fibroblasts in the renal cortex. Epo production is controlled at the transcriptional level. Hypoxia attenuates the inhibition of the Epo promoter by GATA-2. More importantly, hypoxia promotes the availability of heterodimeric (α/β) hypoxia-inducible transcription factors (predominantly HIF-2) which stimulate the Epo enhancer. The HIFs are inactivated in normoxia by enzymatic hydroxylation of their α-subunits. Three HIF-α prolyl hydroxylases (PHD-1, -2 and -3) initiate proteasomal degradation of HIF-α, while an asparaginyl hydroxylase ('factor inhibiting HIF-1', FIH-1) inhibits the transactivation potential. The HIF-α hydroxylases contain Fe(2+) and require 2-oxoglutarate as co-factor. The in vivo response is dynamic, i.e. the concentration of circulating Epo increases initially greatly following an anaemic or hypoxaemic stimulus and then declines despite continued hypoxia. Epo and angiotensin II collaborate in the maintenance of the blood volume. Whether extra-renal sites (brain, skin) modulate renal Epo production is a matter of debate. Epo overproduction results in erythrocytosis. Epo deficiency is the primary cause of the anaemia in chronic kidney disease and a contributing factor in the anaemias of chronic inflammation and cancer. Here, recombinant analogues can substitute for the hormone.

Acute anemia elicits cognitive dysfunction and evidence of cerebral cellular hypoxia in older rats with systemic hypertension

BACKGROUND

Postoperative cognitive dysfunction occurs frequently after cardiac, major vascular, and major orthopedic surgery. Aging and hypertensive cerebrovascular disease are leading risk factors for this disorder. Acute anemia, common to major surgery, has been identified as a possible contributor to postoperative cognitive dysfunction. The effect of hypoxia upon cognition and the cellular and molecular processes involved in learning and memory has been well described. Cerebrovascular changes related to chronic hypertension may expose cells to increased hypoxia with anemia.

METHODS

Young to aged spontaneously hypertensive rats underwent testing for visuospatial memory and learning in the Morris water maze, measurement of cerebral tissue oxygenation via tissue oxygen probe, and measurement of hypoxia-sensitive genes and proteins, under conditions of sham and experimental isovolemic anemia.

RESULTS

Acute isovolemic anemia elicited evidence of aging-dependent visuospatial working memory and learning impairment. Isovolemic anemia did not result in cerebral tissue hypoxia, when measured via tissue oxygen probe. Evidence of cellular hypoxia was, however, identified in response to the anemic challenge, as hypoxia-sensitive genes and proteins were up-regulated. Importantly, cellular hypoxic gene responses were increased with anemia in an age-dependent manner in this model of aging with chronic hypertension.

CONCLUSIONS

In a translational model of chronic hypertension, clinically relevant levels of acute anemia were associated with an age-dependent visuospatial working memory and learning impairment that was matched by an age-dependent cellular sensitivity to anemic hypoxia. These data offer support for a possible link between anemic hypoxia and postoperative cognitive dysfunction in humans.

Targeting astrocytes for stroke therapy

Stroke remains a major health problem and is a leading cause of death and disability. Past research and neurotherapeutic clinical trials have targeted the molecular mechanisms of neuronal cell death during stroke, but this approach has uniformly failed to reduce stroke-induced damage or to improve functional recovery. Beyond the intrinsic molecular mechanisms inducing neuronal death during ischemia, survival and function of astrocytes is absolutely required for neuronal survival and for functional recovery after stroke. Many functions of astrocytes likely improve neuronal viability during stroke. For example, uptake of glutamate and release of neurotrophins enhances neuronal viability during ischemia. Under certain conditions, however, astrocyte function may compromise neuronal viability. For example, astrocytes may produce inflammatory cytokines or toxic mediators, or may release glutamate. The only clinical neurotherapeutic trial for stroke that specifically targeted astrocyte function focused on reducing release of S-100β from astrocytes, which becomes a neurotoxin when present at high levels. Recent work also suggests that astrocytes, beyond their influence on cell survival, also contribute to angiogenesis, neuronal plasticity, and functional recovery in the several days to weeks after stroke. If these delayed functions of astrocytes could be targeted for enhancing stroke recovery, it could contribute importantly to improving stroke recovery. This review focuses on both the positive and the negative influences of astrocytes during stroke, especially as they may be targeted for translation to human trials.

Astrocyte hypoxic response is essential for pathological but not developmental angiogenesis of the retina

Vascular/parenchymal crosstalk is increasingly recognized as important in the development and maintenance of healthy vascularized tissues. The retina is an excellent model in which to study the role of cell type-specific contributions to the process of blood vessel and neuronal growth. During retinal vascular development, glial cells such as astrocytes provide the template over which endothelial cells migrate to form the retinal vascular network, and hypoxia-regulated vascular endothelial growth factor (VEGF) has been demonstrated to play a critical role in this process as well as pathological neovascularization. To investigate the nature of cell-specific contributions to this process, we deleted VEGF and its upstream regulators, the hypoxia-inducible transcription factors HIF-1 alpha and HIF-2 alpha, and the negative regulator of HIF alpha, von Hippel-Lindau protein (VHL), in astrocytes. We found that loss of hypoxic response and VEGF production in astrocytes does not impair normal development of retinal vasculature, indicating that astrocyte-derived VEGF is not essential for this process. In contrast, using a model of oxygen-induced ischemic retinopathy, we show that astrocyte-derived VEGF is essential for hypoxia-induced neovascularization. Thus, we demonstrate that astrocytes in the retina have highly divergent roles during developmental, physiological angiogenesis, and ischemia-driven, pathological neovascularization.