Brain-specific knock-out of hypoxia-inducible factor-1alpha reduces rather than increases hypoxic-ischemic damage

Hypoxic adaptation during development: relation to pattern of neurological presentation and cognitive disability

Children with acute hypoxic-ischaemic events (e.g. stroke) and chronic neurological conditions associated with hypoxia frequently present to paediatric neurologists. Failure to adapt to hypoxia may be a common pathophysiological pathway linking a number of other conditions of childhood with cognitive deficit. There is evidence that congenital cardiac disease, asthma and sleep disordered breathing, for example, are associated with cognitive deficit, but little is known about the mechanism and whether there is any structural change. This review describes what is known about how the brain reacts and adapts to hypoxia, focusing on epilepsy and sickle cell disease (SCD). We prospectively recorded overnight oxyhaemoglobin saturation (SpO2) in 18 children with intractable epilepsy, six of whom were currently or recently in minor status (MS). Children with MS were more likely to have an abnormal sleep study defined as either mean baseline SpO2 <94% or >4 dips of >4% in SpO2/hour (p = .04). In our series of prospectively followed patients with SCD who subsequently developed acute neurological symptoms and signs, mean overnight SpO2 was lower in those with cerebrovascular disease on magnetic resonance angiography (Mann-Whitney, p = .01). Acute, intermittent and chronic hypoxia may have detrimental effects on the brain, the clinical manifestations perhaps depending on rapidity of presentation and prior exposure.

Hypoxia-inducible factors in the kidney

Tissue hypoxia not only occurs under pathological conditions but is also an important microenvironmental factor that is critical for normal embryonic development. Hypoxia-inducible factors HIF-1 and HIF-2 are oxygen-sensitive basic helix-loop-helix transcription factors, which regulate biological processes that facilitate both oxygen delivery and cellular adaptation to oxygen deprivation. HIFs consist of an oxygen-sensitive alpha-subunit, HIF-alpha, and a constitutively expressed beta-subunit, HIF-beta, and regulate the expression of genes that are involved in energy metabolism, angiogenesis, erythropoiesis and iron metabolism, cell proliferation, apoptosis, and other biological processes. Under conditions of normal Po(2), HIF-alpha is hydroxylated and targeted for rapid proteasomal degradation by the von Hippel-Lindau (VHL) E3-ubiquitin ligase. When cells experience hypoxia, HIF-alpha is stabilized and either dimerizes with HIF-beta in the nucleus to form transcriptionally active HIF, executing the canonical hypoxia response, or it physically interacts with unrelated proteins, thereby enabling convergence of HIF oxygen sensing with other signaling pathways. In the normal, fully developed kidney, HIF-1alpha is expressed in most cell types, whereas HIF-2alpha is mainly found in renal interstitial fibroblast-like cells and endothelial cells. This review summarizes some of the most recent advances in the HIF field and discusses their relevance to renal development, normal kidney function and disease.

Neuroprotective effect of hyperbaric oxygen in a rat model of subarachnoid hemorrhage

Acute brain ischemia after subarachnoid hemorrhage (SAH) induces oxidative stress in brain tissues. Up-regulated NADPH oxidase (NOX), a major enzymatic source of superoxide anion in the brain, may contribute to early brain injury after SAH. We evaluated the effects of hyperbaric oxygen (HBO) on protein expression of gp91(phox) catalytic subunit of NOX, lipid peroxidation as a marker of oxidative stress, and on neurological and neuropathological outcomes after SAH. Twenty-nine male Sprague-Dawley rats (300 to 350 g) were randomly allocated to control (sham operation), SAH (endovascular perforation), and SAH treated with HBO groups (2.8 ATA for 2 hours, at 1 hour after SAH). Cerebral blood flow was measured using laser Doppler flowmetry. Rats were sacrificed after 24 hours and brain tissues collected for histology (Nissl staining and gp91 (phox) immunohistochemistry) and biochemistry. Mortality and neurological scores were evaluated. Neuronal injury associated with enhanced gp91 (phox) immunostaining was observed in the cerebral cortex after SAH. The lipid peroxidation product, malondialdehyde, accumulated in the ipsilateral cerebral cortex. HBO treatment reduced expression of NOX, diminished lipid peroxidation, and reduced neuronal damage. HBO caused a drop in mortality and ameliorated functional deficits. HBO-induced neuroprotection after SAH may involve down-regulation of NOX and a subsequent reduction in oxidative stress.

Preconditional Activation of Hypoxia-Inducible Factors Ameliorates Ischemic Acute Renal Failure

Activation of hypoxia-inducible transcription factor (HIF) has been identified as an important mechanism of cellular adaptation to low oxygen. Normoxic degradation of HIF is mediated by oxygen-dependent hydroxylation of specific prolyl residues of the regulative alpha-subunits by HIF prolyl hydroxylases (PHD). It was hypothesized that inhibition of HIF degradation by either hypoxia or pharmacologic inhibition of PHD would confer protection against subsequent ischemic injury. For testing this hypothesis ischemic acute renal failure was induced in rats by 40 min of clamping of the left renal artery after right-sided nephrectomy. Before surgery, pretreatment with either carbon monoxide, leading to tissue hypoxia, or the novel PHD inhibitor FG-4487 was applied. No toxic effects of FG-4487 were observed. Both pretreatments strongly induced the accumulation of HIF-1alpha and HIF-2alpha in tubular and peritubular cells, respectively, as well as HIF target gene expression. The course of subsequent ischemic injury was significantly ameliorated by both strategies of preconditioning, as evident from a significant improvement of serum creatinine and serum urea after 24 and 72 h. Furthermore, tissue injury and apoptosis were less severe, which were quantified by application of a standardized histologic scoring system in a blinded manner. In conclusion, the data provide proof of principle that preconditional activation of the HIF system protects against ischemic injury. Inhibiting the activity of HIF hydroxylases therefore seems to have considerable clinical perspectives.

Expression of the gene encoding the pro-apoptotic BNIP3 protein and stimulation of hypoxia-inducible factor-1α (HIF-1α) protein following focal cerebral ischemia in rats

Hypoxia is a common cause of cell death and is implicated in many disease processes including stroke and chronic degenerative disorders. In response to hypoxia, cells express a variety of genes which allow adaptation to altered metabolic demands, decreased oxygen demands, and the removal of irreversibly damaged cells. Hypoxia-inducible factor-1 (HIF-1) is a transcription factor that regulates the adaptive response to hypoxia in cells. In this study, we reported an early, time-related, gradual up-regulation of HIF-1alpha, and a moderate increase in vascular endothelial growth factor (VEGF)- and erythropoietin (Epo)-levels following transient focal ischemia. Moreover, we demonstrated, for the first time a specific localization of the pro-apoptotic regulator BNIP3 in striatal and cortical neurons after transient focal ischemia in rats. Prolonged intranuclear BNIP3 immunoreactivity was associated with delayed neuronal death. Experiments showed protein increases on Western blots of brain tissue with peaks at 48h after ischemia. Epo responds to ischemia in an early stage, whereas VEGF and BNIP3 accumulate in cells at later times after ischemia. This suggests the possibility that BH3-only proteins might be one of the major downstream effectors of HIF-1alpha in hypoxic cell death. These findings open the possibility that the hypoxia-regulated pro-apoptotic protein BNIP3 enters the nucleus and could interact with other proteins involved in DNA structure, transcription or mRNA splicing after focal brain ischemia.

The VHL/HIF oxygen-sensing pathway and its relevance to kidney disease

Over the past decade major advances have been made in our understanding of the molecular machinery that mammalian cells use to sense and to adapt to a low-oxygen environment. A critical mediator of cellular adaptation to hypoxia is hypoxia-inducible factor (HIF), a basic helix-loop-helix transcription factor that consists of an oxygen-sensitive alpha-subunit, HIF-alpha and a constitutively expressed beta-subunit, HIF-beta. Under conditions of normal oxygen tension, the HIF-alpha subunit is hydroxylated by specific prolyl-hydroxylases and targeted for rapid proteasomal degradation by the von Hippel-Lindau (VHL) tumor suppressor, which is the substrate recognition component of an E3-ubiquitin ligase. In a hypoxic environment or in the absence of functional VHL tumor suppressor protein irrespective of oxygen concentration, HIF-alpha is not degraded and translocates to the nucleus, where it dimerizes with HIF-beta to form transcriptionally active HIF. As a transcription factor, HIF is involved in the regulation of many biological processes that facilitate both oxygen delivery and adaptation to oxygen deprivation by regulating genes that are involved in glucose uptake and energy metabolism, angiogenesis, erythropoiesis, cell proliferation and apoptosis, cell-cell and cell-matrix interactions, and barrier function. This review summarizes some of the most recent advances in the VHL/HIF field and discusses their relevance for pathogenesis and treatment of acute ischemic renal failure, renal fibrosis, and renal cancer.

Perinatal brain injury and regulation of transcription

PURPOSE OF REVIEW

Perinatal hypoxic-ischemic brain injury is a major cause of mortality and morbidity in infants. The understanding of transcription factor activation leading to prosurvival gene expression is important as it pertains to the development of new therapy. Here, we highlight the regulation of transcription factors that potentially could promote neuro-survival in the immature brain.

RECENT FINDINGS

cAMP response element binding protein (CREB), nuclear factor-kappaB (NF-kappaB) and hypoxia-inducible factor 1 (HIF-1) are developmentally regulated in the neural system, and are necessary for the induction of preconditioning against hypoxic-ischemia. CREB and NF-kappaB are also involved in the regulation of synaptic plasticity, and learning and memory. CREB phosphorylation is sufficient and necessary for survival in adult and immature neurons, and NF-kappaB activation in neurons could promote survival, whereas activation in glial cells enhances neuronal death. Although HIF-1 is necessary for hypoxic preconditioning, paradoxically, in the absence of preconditioning, this factor promotes ischemia-induced neuronal death. Erythropoietin, one of the HIF-1 targeted genes, is potently neuroprotective and may be beneficial in treating newborns with hypoxic-ischemic brain damage.

SUMMARY

Drugs that activate the specific signaling leading to the transcriptional activation of prosurvival genes may provide therapy for the treatment of perinatal hypoxic-ischemic brain injury. Investigation of the transcriptional mechanisms of neuro-survival is likely to reveal other novel transcription factors whose activation by small molecules or drugs will complement current medication in activating the salutary gene program.

From angiogenesis to neuropathology

Angiogenesis--the growth of new blood vessels--is a crucial force for shaping the nervous system and protecting it from disease. Recent advances have improved our understanding of how the brain and other tissues grow new blood vessels under normal and pathological conditions. Angiogenesis factors, especially vascular endothelial growth factor, are now known to have roles in the birth of new neurons (neurogenesis), the prevention or mitigation of neuronal injury (neuroprotection), and the pathogenesis of stroke, Alzheimer's disease and motor neuron disease. As our understanding of pathophysiology grows, these developments may point the way towards new molecular and cell-based therapies.

Physiological hypoxia promotes survival of cultured cortical neurons

Physiological oxygen (O2) tensions in brain tissues vary widely, from approximately 5 to 40 Torr (1-6%), encompassing levels of moderate hypoxia that have often been considered neurotoxic in vitro. The effects of such hypoxia were examined in embryonic murine cortical neurons cultured continuously from plating in an atmosphere of 1% O2. Remarkably, cortical neurons thrived in 1% O2, with survival at 7-14 days significantly greater than that of neurons cultured in ambient conditions (20% O2). Immunostaining for microtubule-associated protein-2 (MAP-2) and NeuN confirmed the neuronal identity of surviving cells, and demonstrated robust development of dendritic structures and MAP-2 expression in hypoxia. Survival of neurons in 20% O2 could be promoted by transfer of medium conditioned by neurons in 1% O2, or by pharmacological induction of hypoxia-inducible factor-1alpha (HIF-1alpha), suggesting a possible role for secreted factors under transcriptional regulation by HIF-1 in the trophic effects of hypoxia. Vascular endothelial growth factor (VEGF), a factor regulated by HIF-1, was strongly stimulated in neurons cultured in 1% O2. Treatment of neurons with exogenous VEGF partially improved survival in 20% O2, and inhibitors of VEGF action reduced survival of neurons in 1% O2. These data point to the dynamic role played by hypoxia, associated with HIF-1 up-regulation, in promoting survival of cortical neurons, in part through stimulation of VEGF expression and release.

Effects of aging and HIF-1α deficiency on permeability of hippocampal vessels

We examined age-related changes in the blood-brain barrier (BBB) of neural cell-specific hypoxia inducible factor-1alpha (HIF-1alpha) deficient mice, which showed hydrocephalus with neuronal cell loss, to investigate an effect of neural cell-specific HIF-1alpha deficiency or hydrocephalus on vascular function. Vascular permeability of horseradish peroxidase (HRP) and binding of cationized ferritin (CF) particles to the endothelial cell luminal surface, as a marker of glycocalyx, were investigated. The thickness of CF-labeled glycocalyx was significantly decreased in the cortex in mutant mice compared with that of control mice, although it was not paralleled by increased vascular permeability. In addition, strong staining for HRP was seen around vessels located along the hippocampal fissure in 24-month-old mutant mice. The reaction product of HRP appeared in an increasing number of the endothelial cell abluminal vesicles and within the thickened basal lamina of arterioles in the hippocampus, showing increased vascular permeability. There were no leaky vessels in 10-week-old mutant mice or 10-week-old and 24-month-old control mice. These findings suggest the necessity of two factors, aging and hydrocephalus, for BBB dysfunction in HIF-1alpha deficient mice.

Protection of HIF-1-deficient primary renal tubular epithelial cells from hypoxia-induced cell death is glucose dependent

Ischemic acute renal failure is a frequent clinical problem in hospitalized patients and is associated with significant mortality. Hypoxia-inducible factor 1 (HIF-1) mediates cellular adaptation to hypoxia by regulating biological processes important for cell survival, which include glycolysis, angiogenesis, erythropoiesis, apoptosis, and proliferation. To investigate the role of HIF-1 in hypoxia-induced renal epithelial cell death, we generated mice that allow inactivation of HIF-1α by tetracycline-inducible Cre-loxP-mediated recombination in primary renal proximal tubule cells (PRPTC), resulting in a suppression of HIF-1-mediated gene transcription during oxygen deprivation. In the absence of glucose, the onset and the degree of hypoxia-induced cell death in HIF-1-deficient PRPTC were comparable to wild-type cells. However, when glucose availability was limited, the onset of cell death was delayed in either PRPTC that were HIF-1 deficient or in wild-type PRPTC when glycolysis or glucose uptake was partially inhibited. Our findings suggest in an in vitro genetic model that 1) the generation of adequate energy levels for the maintenance of PRPTC viability under hypoxia does not require HIF-1 and 2) that HIF-1 regulates the timing of hypoxia-induced cell death and apoptosis onset through its effects on glucose consumption.

Two pathways of apoptosis are simultaneously induced in the embryonal brains of neural cell-specific HIF-1α-deficient mice

The aim of this study was to clarify the mechanism of apoptosis seen in the cortex of neural cell-specific hypoxia inducible factor-1alpha (HIF-1alpha)-deficient embryos. A previous study showed that the neural cells in the cortical area of the mutant embryos underwent apoptosis coincident with vascular regression. Through histological, immunohistochemical, and electron microscopic technique, two kinds of apoptotic cells were detected in the mutant embryonal cortex. Apoptotic cells of one type were clustered in small round structures, 10-20 mum in diameter, whereas the others, present in large numbers, were distributed in a group at the cortical plate located more to the outer side than the round structures. The histochemical and electron microscopic findings indicate that the former represented the appearance of macrophages, in which cellular fragments including vascular cells underwent oxidative stress-related, TNF receptor-mediated, caspase-2-induced apoptosis, while the latter showed c-Myc-related, caspase-3-activated apoptosis of the neural cells. These results suggest that two pathways of apoptosis are induced in neuronal and vascular cells of the cortex in the neural cell-specific HIF-1alpha-deficient mouse.

Hypoxia-inducible factor as a physiological regulator

Hypoxia-inducible factor (HIF) is a transcription complex which responds to changes in oxygen, providing cells with a master regulator that coordinates changes in gene transcription. HIF operates in all mammalian cell types and is ancient in evolutionary terms, being conserved in C. elegans and D. melanogaster. This review summarizes recent insights into the molecular events that link reduced oxygenation to HIF activation and emerging insights into the extensive role of HIF in a broad range of physiological processes.