AMP-activated protein kinase and the regulation of Ca2+ signalling in O2-sensing cells

SubSol-HIe is an AMPK-dependent hypoxia-responsive subnucleus of the nucleus tractus solitarius that coordinates the hypoxic ventilatory response and protects against apnoea in mice

Functional magnetic resonance imaging (fMRI) suggests that the hypoxic ventilatory response is facilitated by the AMP-activated protein kinase (AMPK), not at the carotid bodies, but within a subnucleus (Bregma -7.5 to -7.1 mm) of the nucleus tractus solitarius that exhibits right-sided bilateral asymmetry. Here, we map this subnucleus using cFos expression as a surrogate for neuronal activation and mice in which the genes encoding the AMPK-α1 (Prkaa1) and AMPK-α2 (Prkaa2) catalytic subunits were deleted in catecholaminergic cells by Cre expression via the tyrosine hydroxylase promoter. Comparative analysis of brainstem sections, relative to controls, revealed that AMPK-α1/α2 deletion inhibited, with right-sided bilateral asymmetry, cFos expression in and thus activation of a neuronal cluster that partially spanned three interconnected anatomical nuclei adjacent to the area postrema: SolDL (Bregma -7.44 mm to -7.48 mm), SolDM (Bregma -7.44 mm to -7.48 mm) and SubP (Bregma -7.48 mm to -7.56 mm). This approximates the volume identified by fMRI. Moreover, these nuclei are known to be in receipt of carotid body afferent inputs, and catecholaminergic neurons of SubP and SolDL innervate aspects of the ventrolateral medulla responsible for respiratory rhythmogenesis. Accordingly, AMPK-α1/α2 deletion attenuated hypoxia-evoked increases in minute ventilation (normalised to metabolism), reductions in expiration time, and increases sigh frequency, but increased apnoea frequency during hypoxia. The metabolic response to hypoxia in AMPK-α1/α2 knockout mice and the brainstem and spinal cord catecholamine levels were equivalent to controls. We conclude that within the brainstem an AMPK-dependent, hypoxia-responsive subnucleus partially spans SubP, SolDM and SolDL, namely SubSol-HIe, and is critical to coordination of active expiration, the hypoxic ventilatory response and defence against apnoea.

Of Mice and Men and Plethysmography Systems: Does LKB1 Determine the Set Point of Carotid Body Chemosensitivity and the Hypoxic Ventilatory Response?

Our recent studies suggest that the level of liver kinase B1 (LKB1) expression in some way determines carotid body afferent discharge during hypoxia and to a lesser extent during hypercapnia. In short, phosphorylation by LKB1 of an as yet unidentified target(s) determines a set point for carotid body chemosensitivity. LKB1 is the principal kinase that activates the AMP-activated protein kinase (AMPK) during metabolic stresses, but conditional deletion of AMPK in catecholaminergic cells, including therein carotid body type I cells, has little or no effect on carotid body responses to hypoxia or hypercapnia. With AMPK excluded, the most likely target of LKB1 is one or other of the 12 AMPK-related kinases, which are constitutively phosphorylated by LKB1 and, in general, regulate gene expression. By contrast, the hypoxic ventilatory response is attenuated by either LKB1 or AMPK deletion in catecholaminergic cells, precipitating hypoventilation and apnea during hypoxia rather than hyperventilation. Moreover, LKB1, but not AMPK, deficiency causes Cheyne-Stokes-like breathing. This chapter will explore further the possible mechanisms that determine these outcomes.

LKB1 is the gatekeeper of carotid body chemosensing and the hypoxic ventilatory response

The hypoxic ventilatory response (HVR) is critical to breathing and thus oxygen supply to the body and is primarily mediated by the carotid bodies. Here we reveal that carotid body afferent discharge during hypoxia and hypercapnia is determined by the expression of Liver Kinase B1 (LKB1), the principal kinase that activates the AMP-activated protein kinase (AMPK) during metabolic stresses. Conversely, conditional deletion in catecholaminergic cells of AMPK had no effect on carotid body responses to hypoxia or hypercapnia. By contrast, the HVR was attenuated by LKB1 and AMPK deletion. However, in LKB1 knockouts hypoxia evoked hypoventilation, apnoea and Cheyne-Stokes-like breathing, while only hypoventilation and apnoea were observed after AMPK deletion. We therefore identify LKB1 as an essential regulator of carotid body chemosensing and uncover a divergence in dependency on LKB1 and AMPK between the carotid body on one hand and the HVR on the other.

AMPK and the Challenge of Treating Hypoxic Pulmonary Hypertension

Hypoxic pulmonary hypertension (HPH) is characterized by sustained elevation of pulmonary artery pressure produced by vasoconstriction and hyperproliferative remodeling of the pulmonary artery and subsequent right ventricular hypertrophy (RVH). The search for therapeutic targets for cardiovascular pathophysiology has extended in many directions. However, studies focused on mitigating high-altitude pulmonary hypertension (HAPH) have been rare. Because AMP-activated protein kinase (AMPK) is involved in cardiovascular and metabolic pathology, AMPK is often studied as a potential therapeutic target. AMPK is best characterized as a sensor of cellular energy that can also restore cellular metabolic homeostasis. However, AMPK has been implicated in other pathways with vasculoprotective effects. Notably, cellular metabolic stress increases the intracellular ADP/ATP or AMP/ATP ratio, and AMPK activation restores ATP levels by activating energy-producing catabolic pathways and inhibiting energy-consuming anabolic pathways, such as cell growth and proliferation pathways, promoting cardiovascular protection. Thus, AMPK activation plays an important role in antiproliferative, antihypertrophic and antioxidant pathways in the pulmonary artery in HPH. However, AMPK plays contradictory roles in promoting HPH development. This review describes the main findings related to AMPK participation in HPH and its potential as a therapeutic target. It also extrapolates known AMPK functions to discuss the less-studied HAPH context.

Mechanism of Zinc Excitotoxicity: A Focus on AMPK

Over the last 20 years, it has been shown that complex signaling cascades are involved in zinc excitotoxicity. Free zinc rapidly induces PKC activation, which causes reactive oxygen species (ROS) production at least in part through NADPH oxidase. It also promotes neuronal nitric oxide synthase, thereby increasing nitric oxide (NO) production. Extracellular signal-regulated kinase activation and Egr-1 transcription factor activity were quickly induced by zinc, too. These concurrent actions of kinases consequently produce oxygen free radical, ROS, and NO, which may cause severe DNA damage. Following the excessive activity of poly(ADP-ribose) polymerase-1 depletes NAD⁺/ATP in the cells. Zinc excitotoxicity exhibits distinct characteristics of apoptosis, too. Activation of caspase-3 is induced by liver kinase B1 (LKB1)-AMP-activated kinase (AMPK)-Bim cascade signaling and induction of p75NTR receptors and p75NTR-associated Death Executor. Thus, zinc excitotoxicity is a mechanism of neuronal cell death showing various cell death patterns. In addition to the above signaling cascades, individual intracellular organelles also play a crucial role in zinc excitotoxicity. Mitochondria and lysosomes function as zinc reservoirs, and as such, are capable of regulating zinc concentration in the cytoplasm. However, when loaded with too much zinc, they may undergo mitochondrial permeability transition pore (mPTP) opening, and lysosomal membrane permeabilization (LMP), both of which are well-established mechanisms of cell death. Since zinc excitotoxicity has been reported to be associated with acute brain injuries, including stroke, trauma, and epilepsy, we performed to find the novel AMPK inhibitors as therapeutic agents for these diseases. Since we thought acute brain injury has complicated neuronal death pathways, we tried to see the neuroprotection against zinc excitotoxicity, calcium-overload excitotoxicity, oxidative damage, and apoptosis. We found that two chemicals showed significant neuroprotection against all cellular neurotoxic models we tested. Finally, we observed the reduction of infarct volume in a rat model of brain injury after middle cerebral artery occlusion (MCAO). In this review, we introduced the AMPK-mediated cell death mechanism and novel strategy for the development of stroke therapeutics. The hope is that this understanding would provide a rationale for acute brain injury and eventually find new therapeutics.

AMPK and the Need to Breathe and Feed: What's the Matter with Oxygen?

We live and to do so we must breathe and eat, so are we a combination of what we eat and breathe? Here, we will consider this question, and the role in this respect of the AMP-activated protein kinase (AMPK). Emerging evidence suggests that AMPK facilitates central and peripheral reflexes that coordinate breathing and oxygen supply, and contributes to the central regulation of feeding and food choice. We propose, therefore, that oxygen supply to the body is aligned with not only the quantity we eat, but also nutrient-based diet selection, and that the cell-specific expression pattern of AMPK subunit isoforms is critical to appropriate system alignment in this respect. Currently available information on how oxygen supply may be aligned with feeding and food choice, or vice versa, through our motivation to breathe and select particular nutrients is sparse, fragmented and lacks any integrated understanding. By addressing this, we aim to provide the foundations for a clinical perspective that reveals untapped potential, by highlighting how aberrant cell-specific changes in the expression of AMPK subunit isoforms could give rise, in part, to known associations between metabolic disease, such as obesity and type 2 diabetes, sleep-disordered breathing, pulmonary hypertension and acute respiratory distress syndrome.

Insights into the evolution of polymodal chemoreceptors

Respiratory chemoreceptors in vertebrates are specialized cells that detect chemical changes in the environment or arterial blood supply and initiate autonomic responses, such as hyperventilation or changes in heart rate, to improve O₂ uptake and delivery to tissues. These chemoreceptors are sensitive to changes in O₂, CO₂ and/or H⁺. In fish and mammals, respiratory chemoreceptors may be additionally sensitive to ammonia, hypoglycemia, and numerous other stimuli. Thus, chemoreceptors that affect respiration respond to different types of stimuli (or modalities) and are considered to be "polymodal". This review discusses the polymodal nature of respiratory chemoreceptors in vertebrates with a particular emphasis on chemoreceptors of the carotid body and pulmonary epithelium in mammals, and on neuroepithelial cells in water- and air-breathing fish. A major goal will be to examine the evidence for putative polymodal chemoreceptors in fish within the context of studies on mammalian models, for which polymodal chemoreceptors are well described, in order to improve our understanding of the evolution of polymodal chemoreceptors in vertebrates, and to aid in future studies that aim to identify putative receptors in air- and water-breathing fish.

AMPK-α1 or AMPK-α2 Deletion in Smooth Muscles Does Not Affect the Hypoxic Ventilatory Response or Systemic Arterial Blood Pressure Regulation During Hypoxia

The hypoxic ventilatory response (HVR) is markedly attenuated by AMPK-α1 deletion conditional on the expression of Cre-recombinase in tyrosine hydroxylase (TH) expressing cells, precipitating marked increases in apnea frequency and duration. It was concluded that ventilatory dysfunction caused by AMPK deficiency was driven by neurogenic mechanisms. However, TH is transiently expressed in other cell types during development, and it is evident that central respiratory depression can also be triggered by myogenic mechanisms that impact blood supply to the brain. We therefore assessed the effect on the HVR and systemic arterial blood pressure of AMPK deletion in vascular smooth muscles. There was no difference in minute ventilation during normoxia. However, increases in minute ventilation during severe hypoxia (8% O₂) were, if affected at all, augmented by AMPK-α1 and AMPK-α2 deletion in smooth muscles; despite the fact that hypoxia (8% O₂) evoked falls in arterial SpO₂ comparable with controls. Surprisingly, these mice exhibited no difference in systolic, diastolic or mean arterial blood pressure during normoxia or hypoxia. We conclude that neither AMPK-α1 nor AMPK-α2 are required in smooth muscle for the regulation of systemic arterial blood pressure during hypoxia, and that AMPK-α1 deficiency does not impact the HVR by myogenic mechanisms.

AMP-activated Protein Kinase Deficiency Blocks the Hypoxic Ventilatory Response and Thus Precipitates Hypoventilation and Apnea

RATIONALE

Modulation of breathing by hypoxia accommodates variations in oxygen demand and supply during, for example, sleep and ascent to altitude, but the precise molecular mechanisms of this phenomenon remain controversial. Among the genes influenced by natural selection in high-altitude populations is one for the adenosine monophosphate-activated protein kinase (AMPK) α1-catalytic subunit, which governs cell-autonomous adaptations during metabolic stress.

OBJECTIVES

We investigated whether AMPK-α1 and/or AMPK-α2 are required for the hypoxic ventilatory response and the mechanism of ventilatory dysfunctions arising from AMPK deficiency.

METHODS

We used plethysmography, electrophysiology, functional magnetic resonance imaging, and immediate early gene (c-fos) expression to assess the hypoxic ventilatory response of mice with conditional deletion of the AMPK-α1 and/or AMPK-α2 genes in catecholaminergic cells, which compose the hypoxia-responsive respiratory network from carotid body to brainstem.

MEASUREMENTS AND MAIN RESULTS

AMPK-α1 and AMPK-α2 deletion virtually abolished the hypoxic ventilatory response, and ventilatory depression during hypoxia was exacerbated under anesthesia. Rather than hyperventilating, mice lacking AMPK-α1 and AMPK-α2 exhibited hypoventilation and apnea during hypoxia, with the primary precipitant being loss of AMPK-α1 expression. However, the carotid bodies of AMPK-knockout mice remained exquisitely sensitive to hypoxia, contrary to the view that the hypoxic ventilatory response is determined solely by increased carotid body afferent input to the brainstem. Regardless, functional magnetic resonance imaging and c-fos expression revealed reduced activation by hypoxia of well-defined dorsal and ventral brainstem nuclei.

CONCLUSIONS

AMPK is required to coordinate the activation by hypoxia of brainstem respiratory networks, and deficiencies in AMPK expression precipitate hypoventilation and apnea, even when carotid body afferent input is normal.

The emerging role of AMPK in the regulation of breathing and oxygen supply

Regulation of breathing is critical to our capacity to accommodate deficits in oxygen availability and demand during, for example, sleep and ascent to altitude. It is generally accepted that a fall in arterial oxygen increases afferent discharge from the carotid bodies to the brainstem and thus delivers increased ventilatory drive, which restores oxygen supply and protects against hypoventilation and apnoea. However, the precise molecular mechanisms involved remain unclear. We recently identified as critical to this process the AMP-activated protein kinase (AMPK), which is key to the cell-autonomous regulation of metabolic homoeostasis. This observation is significant for many reasons, not least because recent studies suggest that the gene for the AMPK-α1 catalytic subunit has been subjected to natural selection in high-altitude populations. It would appear, therefore, that evolutionary pressures have led to AMPK being utilized to regulate oxygen delivery and thus energy supply to the body in the short, medium and longer term. Contrary to current consensus, however, our findings suggest that AMPK regulates ventilation at the level of the caudal brainstem, even when afferent input responses from the carotid body are normal. We therefore hypothesize that AMPK integrates local hypoxic stress at defined loci within the brainstem respiratory network with an index of peripheral hypoxic status, namely afferent chemosensory inputs. Allied to this, AMPK is critical to the control of hypoxic pulmonary vasoconstriction and thus ventilation-perfusion matching at the lungs and may also determine oxygen supply to the foetus by, for example, modulating utero-placental blood flow.

Experimental Observations on the Biological Significance of Hydrogen Sulfide in Carotid Body Chemoreception

The cascade of transduction of hypoxia and hypercapnia, the natural stimuli to chemoreceptor cells, is incompletely understood. A particular gap in that knowledge is the role played by second messengers, or in a most ample term, of modulators. A recently described modulator of chemoreceptor cell responses is the gaseous transmitter hydrogen sulfide, which has been proposed as a specific activator of the hypoxic responses in the carotid body, both at the level of the chemoreceptor cell response or at the level of the global output of the organ. Since sulfide behaves in this regard as cAMP, we explored the possibility that sulfide effects were mediated by the more classical messenger. Data indicate that exogenous and endogenous sulfide inhibits adenyl cyclase finding additionally that inhibition of adenylyl cyclase does not modify chemoreceptor cell responses elicited by sulfide. We have also observed that transient receptor potential cation channels A1 (TRPA1) are not regulated by sulfide in chemoreceptor cells.

Hypoxia, AMPK activation and uterine artery vasoreactivity

Genes near adenosine monophosphate-activated protein kinase-α1 (PRKAA1) have been implicated in the greater uterine artery (UtA) blood flow and relative protection from fetal growth restriction seen in altitude-adapted Andean populations. Adenosine monophosphate-activated protein kinase (AMPK) activation vasodilates multiple vessels but whether AMPK is present in UtA or placental tissue and influences UtA vasoreactivity during normal or hypoxic pregnancy remains unknown. We studied isolated UtA and placenta from near-term C57BL/6J mice housed in normoxia (n = 8) or hypoxia (10% oxygen, n = 7-9) from day 14 to day 19, and placentas from non-labouring sea level (n = 3) or 3100 m (n = 3) women. Hypoxia increased AMPK immunostaining in near-term murine UtA and placental tissue. RT-PCR products for AMPK-α1 and -α2 isoforms and liver kinase B1 (LKB1; the upstream kinase activating AMPK) were present in murine and human placenta, and hypoxia increased LKB1 and AMPK-α1 and -α2 expression in the high- compared with low-altitude human placentas. Pharmacological AMPK activation by A769662 caused phenylephrine pre-constricted UtA from normoxic or hypoxic pregnant mice to dilate and this dilatation was partially reversed by the NOS inhibitor l-NAME. Hypoxic pregnancy sufficient to restrict fetal growth markedly augmented the UtA vasodilator effect of AMPK activation in opposition to PE constriction as the result of both NO-dependent and NO-independent mechanisms. We conclude that AMPK is activated during hypoxic pregnancy and that AMPK activation vasodilates the UtA, especially in hypoxic pregnancy. AMPK activation may be playing an adaptive role by limiting cellular energy depletion and helping to maintain utero-placental blood flow in hypoxic pregnancy.

Modulation of the LKB1-AMPK Signalling Pathway Underpins Hypoxic Pulmonary Vasoconstriction and Pulmonary Hypertension

Perhaps the defining characteristic of pulmonary arteries is the process of hypoxic pulmonary vasoconstriction (HPV) which, under physiological conditions, supports ventilation-perfusion matching in the lung by diverting blood flow away from oxygen deprived areas of the lung to oxygen rich regions. However, when alveolar hypoxia is more widespread, either at altitude or with disease (e.g., cystic fibrosis), HPV may lead to hypoxic pulmonary hypertension. HPV is driven by the intrinsic response to hypoxia of pulmonary arterial smooth muscle and endothelial cells, which are acutely sensitive to relatively small changes in pO2 and have evolved to monitor oxygen supply and thus address ventilation-perfusion mismatch. There is now a consensus that the inhibition by hypoxia of mitochondrial oxidative phosphorylation represents a key step towards the induction of HPV, but the precise nature of the signalling pathway(s) engaged thereafter remains open to debate. We will consider the role of the AMP-activated protein kinase (AMPK) and liver kinase B1 (LKB1), an upstream kinase through which AMPK is intimately coupled to changes in oxygen supply via mitochondrial metabolism. A growing body of evidence, from our laboratory and others, suggests that modulation of the LKB1-AMPK signalling pathway underpins both hypoxic pulmonary vasoconstriction and the development of pulmonary hypertension.

Adenosine monophosphate-activated protein kinase is required for pulmonary artery smooth muscle cell survival and the development of hypoxic pulmonary hypertension

Human pulmonary artery smooth muscle cells (HPASMCs) express both adenosine monophosphate-activated protein kinase (AMPK) α1 and α2. We investigated the distinct roles of AMPK α1 and α2 in the survival of HPASMCs during hypoxia and hypoxia-induced pulmonary hypertension (PH). The exposure of HPASMCs to hypoxia (3% O2) increased AMPK activation and phosphorylation, and the inhibition of AMPK with Compound C during hypoxia decreased their viability and increased lactate dehydrogenase activity and apoptosis. Although the suppression of either AMPK α1 or α2 expression led to increased cell death, the suppression of AMPK α2 alone increased caspase-3 activity and apoptosis in HPASMCs exposed to hypoxia. It also resulted in the decreased expression of myeloid cell leukemia sequence 1 (MCL-1). The knockdown of MCL-1 or MCL-1 inhibitors increased caspase-3 activity and apoptosis in HPASMCs exposed to hypoxia. On the other hand, the suppression of AMPK α1 expression alone prevented hypoxia-mediated autophagy. The inhibition of autophagy induced cell death in HPASMCs. Our results suggest that AMPK α1 and AMPK α2 play differential roles in the survival of HPASMCs during hypoxia. The activation of AMPK α2 maintains the expression of MCL-1 and prevents apoptosis, whereas the activation of AMPK α1 stimulates autophagy, promoting HPASMC survival. Moreover, treatment with Compound C, which inhibits both isoforms of AMPK, prevented and partly reversed hypoxia-induced PH in mice. Taking these results together, our study suggests that AMPK plays a key role in the pathogenesis of pulmonary arterial hypertension, and AMPK may represent a novel therapeutic target for the treatment of pulmonary arterial hypertension.

AMP-activated kinase relaxes agonist induced contractions in the mouse aorta via effects on PKC signaling and inhibits NO-induced relaxation

Adenosine monophosphate activated kinase (AMPK), a regulator of cellular metabolism, has been shown to relax arterial smooth muscle via endothelium-dependent and independent mechanisms. We have examined the role of AMPK in different smooth muscles using the activating compound, 5-amino-4-imidazolecarboxamide riboside-1-β-d-ribofuranoside (AICAR). Isolated preparations of mouse aorta, saphenous artery, ileum and urinary bladder were compared. AICAR produced a reversible dose-dependent relaxation in aortic rings pre-incubated with AICAR and activated with phenylephrine. Less prominent relaxation was noted in the other tissues. This difference in sensitivity to AICAR was not due to differences in the expression levels of AMPK α1 mRNA. In the aorta, AICAR had a greater effect on contractions induced by phenylephrine, compared to high-K(+) induced contractions. Contractions of the aorta in response to the protein kinase C activator PDBu were prominently inhibited by AICAR. The AICAR relaxation observed in the aorta was not prevented by the NOS inhibitor L-NAME, Indomethacin or endothelium removal. Nitric oxide (NO) mediated relaxations in aortic preparations induced by acetylcholine or sodium nitroprusside (SNP) were attenuated by AICAR. In conclusion, AMPK induced relaxation of smooth muscle is tissue-dependent and most prominent in large elastic arteries. The smooth muscle relaxation is NO-independent and occurs downstream of PKC activation and is associated with attenuated relaxant responses to NO.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Ion channel regulation by the LKB1-AMPK signalling pathway: the key to carotid body activation by hypoxia and metabolic homeostasis at the whole body level

Our recent investigations provide further support for the proposal that, consequent to inhibition of mitochondrial oxidative phosphorylation, activation of AMP-activated protein kinase (AMPK) mediates carotid body excitation by hypoxia. Consistent with the effects of hypoxia, intracellular dialysis from a patch pipette of an active (thiophosphorylated) recombinant AMPK heterotrimer (α2β2γ1) or application of the AMPK activators AICAR and A769662: (1) Inhibited BK(Ca) currents and TASK K(+) currents in rat carotid body type I cells; (2) Inhibited whole-cell currents carried by KCa1.1 and TASK3, but not TASK1 channels expressed in HEK293 cells; (3) Triggered carotid body activation. Furthermore, preliminary studies using mice with conditional knockout in type I cells of the primary upstream kinase that activates AMPK in response to metabolic stresses, LKB1, appear to confirm our working hypothesis. Studies on mice with knockout of the catalytic α1 subunit and α2 subunits of AMPK, respectively, have proved equally consistent. Accumulating evidence therefore suggests that the LKB1-AMPK signalling pathway is necessary for hypoxia-response coupling by the carotid body, and serves to regulate oxygen and therefore energy supply at the whole body level.

Peripheral chemoreceptors: function and plasticity of the carotid body

The discovery of the sensory nature of the carotid body dates back to the beginning of the 20th century. Following these seminal discoveries, research into carotid body mechanisms moved forward progressively through the 20th century, with many descriptions of the ultrastructure of the organ and stimulus-response measurements at the level of the whole organ. The later part of 20th century witnessed the first descriptions of the cellular responses and electrophysiology of isolated and cultured type I and type II cells, and there now exist a number of testable hypotheses of chemotransduction. The goal of this article is to provide a comprehensive review of current concepts on sensory transduction and transmission of the hypoxic stimulus at the carotid body with an emphasis on integrating cellular mechanisms with the whole organ responses and highlighting the gaps or discrepancies in our knowledge. It is increasingly evident that in addition to hypoxia, the carotid body responds to a wide variety of blood-borne stimuli, including reduced glucose and immune-related cytokines and we therefore also consider the evidence for a polymodal function of the carotid body and its implications. It is clear that the sensory function of the carotid body exhibits considerable plasticity in response to the chronic perturbations in environmental O2 that is associated with many physiological and pathological conditions. The mechanisms and consequences of carotid body plasticity in health and disease are discussed in the final sections of this article.

Adenosine A₂a receptors and O₂ sensing in development

Reduced mitochondrial oxidative phosphorylation, via activation of adenylate kinase and the resulting exponential rise in the cellular AMP/ATP ratio, appears to be a critical factor underlying O₂ sensing in many chemoreceptive tissues in mammals. The elevated AMP/ATP ratio, in turn, activates key enzymes that are involved in physiologic adjustments that tend to balance ATP supply and demand. An example is the conversion of AMP to adenosine via 5'-nucleotidase and the resulting activation of adenosine A(₂A) receptors, which are involved in acute oxygen sensing by both carotid bodies and the brain. In fetal sheep, A(₂A) receptors associated with carotid bodies trigger hypoxic cardiovascular chemoreflexes, while central A(₂A) receptors mediate hypoxic inhibition of breathing and rapid eye movements. A(₂A) receptors are also involved in hypoxic regulation of fetal endocrine systems, metabolism, and vascular tone. In developing lambs, A(₂A) receptors play virtually no role in O₂ sensing by the carotid bodies, but brain A(₂A) receptors remain critically involved in the roll-off ventilatory response to hypoxia. In adult mammals, A(₂A) receptors have been implicated in O₂ sensing by carotid glomus cells, while central A(₂A) receptors likely blunt hypoxic hyperventilation. In conclusion, A(₂A) receptors are crucially involved in the transduction mechanisms of O₂ sensing in fetal carotid bodies and brains. Postnatally, central A(₂A) receptors remain key mediators of hypoxic respiratory depression, but they are less critical for O₂ sensing in carotid chemoreceptors, particularly in developing lambs.

Metabolic stress modulates motor patterning via AMP-activated protein kinase

Neural circuits are especially vulnerable to metabolic stress. The locust (Locusta migratoria) responds to anoxia by entering a coma during which neural and muscular systems shut down. During anoxic coma, arrest of the ventilatory central pattern generator is tightly correlated with an abrupt spreading depression (SD)-like increase in extracellular potassium concentration within the metathoracic neuropile. We examined the role of the AMP-activated protein kinase (AMPK), an evolutionarily conserved sensor of cellular energy status, in anoxia-induced ventilatory arrest and SD-like events in the locust. Perfusion of sodium azide (NaN(3); mitochondrial toxin) induced SD, temporary coma, and profound changes in the ventilatory motor pattern characterized as a rapid rhythm before coma and a slower rhythm following recovery. AMPK activation using 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) mimicked the motor pattern changes induced by NaN(3) but did not induce SD and coma. The effects of NaN(3) on the ventilatory rhythm were reversed by perfusion of compound-C (AMPK inhibitor) or glucose, and the effects of AICAR were also reversed by compound-C, confirming the modulatory roles of AMPK and energy status. Ouabain-induced recurring SD was suppressed by inhibition of AMPK and exacerbated by its activation. We show that the motor pattern changes induced by metabolic stress are not the result of SD alone, but that AMPK is necessary and sufficient for these changes and that AMPK activity strongly influences susceptibility to SD.

Hypoxic pulmonary vasoconstriction: mechanisms of oxygen-sensing

PURPOSE OF REVIEW

Hypoxic pulmonary vasoconstriction (HPV) is driven by the intrinsic response to hypoxia of pulmonary arterial smooth muscle and endothelial cells. These are representatives of a group of specialized O2-sensing cells, defined by their acute sensitivity to relatively small changes in pO2, which have evolved to modulate respiratory and circulatory function in order to maintain O2 supply within physiological limits. The aim of this article is to discuss recent investigations into the mechanism(s) of hypoxia-response coupling and, in light of these, provide a critical assessment of current working hypotheses.

RECENT FINDINGS

Upon exposure to hypoxia state-of-the-art technologies have now confirmed that mitochondrial oxidative phosphorylation is inhibited in all O2-sensing cells, including pulmonary arterial smooth muscle cells. Thereafter, evidence has been presented to indicate a role as principal effector for the 'gasotransmitters' carbon monoxide and hydrogen sulphide, reactive oxygen species or, in marked contrast, reduced cellular redox couples. Considering recent evidence in favour and against these proposals we suggest that an alternative mechanism may be key, namely the activation of adenosine monophosphate-activated protein kinase consequent to inhibition of mitochondrial oxidative phosphorylation.

SUMMARY

HPV supports ventilation-perfusion matching in the lung by diverting blood flow away from oxygen-deprived areas towards regions rich in O2. However, in diseases such as emphysema and cystic fibrosis, widespread HPV leads to hypoxic pulmonary hypertension and ultimately right heart failure. Determining the precise mechanism(s) that underpins hypoxia-response coupling will therefore advance understanding of the fundamental processes contributing to related pathophysiology and provide for improved therapeutics.

Mechanisms for acute oxygen sensing in the carotid body

Hypoxic chemotransduction in the carotid body requires release of excitatory transmitters from type I cells that activate afferent sensory neurones. Transmitter release is dependent on voltage-gated Ca2+ entry which is evoked by membrane depolarization. This excitatory response to hypoxia is initiated by inhibition of specific O2 sensitive K+ channels, of which several types have been reported. Here, we discuss mechanisms which have been put forward to account for hypoxic inhibition of type I cell K+ channels. Whilst evidence indicates that one O2 sensitive K+ channel, BKCa, may be regulated by gasotransmitters (CO and H2S) in an O2-dependent manner, other studies now indicate that activation of AMP-activated protein kinase (AMPK) accounts for inhibition of both BKCa and 'leak' O2 sensitive K+ channels, and perhaps also other O2 sensitive K+ channels reported in different species. We propose that type I cell AMPK activation occurs as a result of inhibition of mitochondrial oxidative phosphorylation, and does not require increased production of reactive oxygen species. Thus, AMPK activation provides the basis for unifying the 'membrane' and 'mitochondrial' hypotheses, previously regarded as disparate, to account for hypoxic chemotransduction.

Mitochondrial complex III: an essential component of universal oxygen sensing machinery?

Oxygen is necessary for the survival of mammalian cells. In order to maintain adequate cellular oxygenation, mammals have evolved multiple acute and long-term adaptive responses to hypoxia. These include hypoxic increases in erythropoiesis, pulmonary vasoconstriction and carotid body neurosecretion. Collectively, these responses help maintain oxygen homeostasis as oxygen levels remain scarce. There are multiple effectors proposed to underlie these diverse responses to hypoxia including PHD2, AMPK, NADPH oxidases, and mitochondrial complex III. Here I propose a model wherein complex III is integral to oxygen sensing in regulating diverse response to hypoxia.

The role of intracellular ion channels in regulating cytoplasmic calciumin pulmonary arterial mmooth muscle: which store and where?

The mobilisation of intracellular Ca(2+) stores plays a pivotal role in the regulation of arterial smooth muscle function, paradoxically during both contraction and relaxation. Moreover, different spatiotemporal Ca(2+) signalling patterns may trigger differential gene expression while mediating the same functional response. These facts alone serve to highlight the importance of the growing body of evidence in support of the view that different Ca(2+) storing organelles may be selected by the discrete or co-ordinated actions of multiple Ca(2+) mobilising messengers. In this respect, it is generally accepted that sarcoplasmic reticulum stores may be mobilised by the ubiquitous messenger inositol 1,4,5 trisphosphate. However, relatively little attention has been paid to the role of Ca(2+) mobilising pyridine nucleotides in arterial smooth muscle, namely cyclic adenosine diphosphate-ribose and nicotinic acid adenine dinucleotide phosphate. This review will, therefore, focus on the role of these novel Ca(2+) mobilising messengers in pulmonary arterial smooth muscle, with particular reference to hypoxic pulmonary vasoconstriction.

Ion channel regulation by AMPK: the route of hypoxia-response coupling in thecarotid body and pulmonary artery

Vital homeostatic mechanisms monitor O2 supply and adjust respiratory and circulatory function to meet demand. The pulmonary arteries and carotid bodies are key systems in this respect. Hypoxic pulmonary vasoconstriction (HPV) aids ventilation-perfusion matching in the lung by diverting blood flow from areas with an O2 deficit to those rich in O2, while a fall in arterial pO2 increases sensory afferent discharge from the carotid body to elicit corrective changes in breathing patterns. We discuss here the new concept that hypoxia, by inhibiting oxidative phosphorylation, activates AMP-activated protein kinase (AMPK) leading to consequent phosphorylation of target proteins, such as ion channels, which initiate pulmonary artery constriction and carotid body activation. Consistent with this view, AMPK knockout mice exhibit an impaired ventilatory response to hypoxia. Thus, AMPK may be sufficient and necessary for hypoxia-response coupling and may regulate O2 and thereby energy (ATP) supply at the whole body as well as the cellular level.

AMP-activated protein kinase and hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction is a vital homeostatic mechanism that aids ventilation-perfusion matching in the lung, for which the underlying mechanism(s) remains controversial. However, our most recent investigations strongly suggest that hypoxic pulmonary vasoconstriction is precipitated, at least in part, by the inhibition of mitochondrial oxidative phosphorylation by hypoxia, an increase in the AMP/ATP ratio and consequent activation of AMP-activated protein kinase (AMPK). Unfortunately, these studies lacked the definitive proof that can only be provided by selectively blocking AMPK-dependent signalling cascades. The aim of the present study was, therefore, to determine the effects of the AMPK inhibitor compound C upon: (1) phosphorylation in response to hypoxia of a classical AMPK substrate, acetyl CoA carboxylase, in rat pulmonary arterial smooth muscle and (2) hypoxic pulmonary vasoconstriction in rat isolated intrapulmonary arteries. Acetyl CoA carboxylase phosphorylation was increased approximately 3 fold in the presence of hypoxia (pO(2) = 16-21 mm Hg, 1 h) and 5-aminoimidazole-4-carboxamide riboside (AICAR; 1 mM; 4 h) and in a manner that was significantly attenuated by the AMPK antagonist compound C (40 microM). Most importantly, pre-incubation of intrapulmonary arteries with compound C (40 microM) inhibited phase II, but not phase I, of hypoxic pulmonary vasoconstriction. Likewise, compound C (40 microM) inhibited constriction by AICAR (1 mM). The results of the present study are consistent with the activation of AMPK being a key event in the initiation of the contractile response of pulmonary arteries to acute hypoxia.

5'-AMP-activated protein kinase signaling in Caenorhabditis elegans

5'-AMP-activated protein kinase (AMPK) has been called "the metabolic master switch" because of its central role in regulating fuel homeostasis. AMPK, a heterotrimeric serine/threonine protein kinase composed of alpha, beta, and gamma subunits, is activated by upstream kinases and by 5'-AMP in response to various nutritional and stress signals. Downstream effects include regulation of metabolism, protein synthesis, cell growth, and mediation of the actions of a number of hormones, including leptin. However, AMPK research represents a young and growing field; hence, there are many unanswered questions regarding the control and action of AMPK. This review presents evidence for the existence of AMPK signaling pathways in Caenorhabditis elegans, a genetically tractable model organism that has yet to be fully exploited to elucidate AMPK signaling mechanisms.

Stress preconditioning of spreading depression in the locust CNS

Cortical spreading depression (CSD) is closely associated with important pathologies including stroke, seizures and migraine. The mechanisms underlying SD in its various forms are still incompletely understood. Here we describe SD-like events in an invertebrate model, the ventilatory central pattern generator (CPG) of locusts. Using K(+) -sensitive microelectrodes, we measured extracellular K(+) concentration ([K(+)](o)) in the metathoracic neuropile of the CPG while monitoring CPG output electromyographically from muscle 161 in the second abdominal segment to investigate the role K(+) in failure of neural circuit operation induced by various stressors. Failure of ventilation in response to different stressors (hyperthermia, anoxia, ATP depletion, Na(+)/K(+) ATPase impairment, K(+) injection) was associated with a disturbance of CNS ion homeostasis that shares the characteristics of CSD and SD-like events in vertebrates. Hyperthermic failure was preconditioned by prior heat shock (3 h, 45 degrees C) and induced-thermotolerance was associated with an increase in the rate of clearance of extracellular K(+) that was not linked to changes in ATP levels or total Na(+)/K(+) ATPase activity. Our findings suggest that SD-like events in locusts are adaptive to terminate neural network operation and conserve energy during stress and that they can be preconditioned by experience. We propose that they share mechanisms with CSD in mammals suggesting a common evolutionary origin.

Hypoxic pulmonary vasoconstriction

HPV (hypoxic pulmonary vasoconstriction) is the critical and distinguishing characteristic of the arteries that feed the lung. In marked contrast, systemic arteries dilate in response to hypoxia to meet the metabolic demands of the tissues they supply. Physiologically, HPV contributes to ventilation-perfusion matching in the lung by diverting blood flow to oxygen-rich areas. However, when alveolar hypoxia is global, as in diseases such as emphysema and cystic fibrosis, HPV leads to HPH (hypoxic pulmonary hypertension) and right heart failure. HPV is driven by the intrinsic response to hypoxia of two different cell types, namely the pulmonary arterial smooth muscle and endothelial cells. These are representatives of a group of specialized cells, commonly referred to as oxygen-sensing cells, which are defined by their acute sensitivity to relatively small changes in PO(2) and have evolved to monitor oxygen supply and alter respiratory and circulatory function, as well as the capacity of the blood to transport oxygen. Upon exposure to hypoxia, mitochondrial oxidative phosphorylation is inhibited in all such cells and this, in part, mediates cell activation. In the case of pulmonary arteries, constriction is triggered via: (i) calcium release from the smooth muscle sarcoplasmic reticulum and consequent store-depletion-activated calcium entry into the smooth muscle cells and, (ii) the modulation of transmitter release from the pulmonary artery endothelium, which leads to further constriction of the smooth muscle by increasing the sensitivity of the contractile apparatus to calcium.

Chemoreception in the context of the general biology of ROS

Superoxide anion is the most important reactive oxygen species (ROS) primarily generated in cells. The main cellular constituents with capabilities to generate superoxide anion are NADPH oxidases and mitochondrial respiratory chain. The emphasis of our article is centered in critically examining hypotheses proposing that ROS generated by NADPH oxidase and mitochondria are key elements in O(2)-sensing and hypoxic responses generation in carotid body chemoreceptor cells. Available data indicate that chemoreceptor cells express a specific isoform of NADPH oxidase that is activated by hypoxia; generated ROS acting as negative modulators of the carotid body (CB) hypoxic responses. Literature is also consistent in supporting that poisoned respiratory chain can produce high amounts of ROS, making mitochondrial ROS potential triggers-modulators of the CB activation elicited by mitochondrial venoms. However, most data favour the notion that levels of hypoxia, capable of strongly activating chemoreceptor cells, would not increase the rate of ROS production in mitochondria, making mitochondrial ROS unlikely triggers of hypoxic responses in the CB. Finally, we review recent literature on heme oxygenases from two perspectives, as potential O(2)-sensors in chemoreceptor cells and as generators of bilirubin which is considered to be a ROS scavenger of major quantitative importance in mammalian cells.

Oxygen sensing in the carotid body and its relation to heart failure

This brief review first touches on the origins of the earth's oxygen. It then identifies and locates the principal oxygen sensor in vertebrates, the carotid body (CB). The CB is unique in that in human subjects, it is the only sensor of lower than normal levels in the partial pressure of oxygen (hypoxia, HH). Another oxygen sensor, the aortic bodies, are mostly vestigial in higher vertebrates. At least they play a much smaller role than the CB. In such an important role, the many reflexes in response to CB stimulation by HH are presented. After briefly reviewing what CB stimulation does, the next topic is to describe how the CB chemotransduces HH into neural signals to the brain. Several mechanisms are known, but critical steps in the mechanisms of chemosensation and chemotransduction are still under investigation. Finally, a brief glance at the operation of the CB in chronic heart failure patients is presented. Specifically, the role of nitric oxide, NO, is discussed.

Differential cerebral cortex transcriptomes of baboon neonates consuming moderate and high docosahexaenoic acid formulas

BACKGROUND

Docosahexaenoic acid (DHA, 22:6n-3) and arachidonic acid (ARA, 20:4n-6) are the major long chain polyunsaturated fatty acids (LCPUFA) of the central nervous system (CNS). These nutrients are present in most infant formulas at modest levels, intended to support visual and neural development. There are no investigations in primates of the biological consequences of dietary DHA at levels above those present in formulas but within normal breastmilk levels.

METHODS AND FINDINGS

Twelve baboons were divided into three formula groups: Control, with no DHA-ARA; "L", LCPUFA, with 0.33%DHA-0.67%ARA; "L3", LCPUFA, with 1.00%DHA-0.67%ARA. All the samples are from the precentral gyrus of cerebral cortex brain regions. At 12 weeks of age, changes in gene expression were detected in 1,108 of 54,000 probe sets (2.05%), with most showing <2-fold change. Gene ontology analysis assigns them to diverse biological functions, notably lipid metabolism and transport, G-protein and signal transduction, development, visual perception, cytoskeleton, peptidases, stress response, transcription regulation, and 400 transcripts having no defined function. PLA2G6, a phospholipase recently associated with infantile neuroaxonal dystrophy, was downregulated in both LCPUFA groups. ELOVL5, a PUFA elongase, was the only LCPUFA biosynthetic enzyme that was differentially expressed. Mitochondrial fatty acid carrier, CPT2, was among several genes associated with mitochondrial fatty acid oxidation to be downregulated by high DHA, while the mitochondrial proton carrier, UCP2, was upregulated. TIMM8A, also known as deafness/dystonia peptide 1, was among several differentially expressed neural development genes. LUM and TIMP3, associated with corneal structure and age-related macular degeneration, respectively, were among visual perception genes influenced by LCPUFA. TIA1, a silencer of COX2 gene translation, is upregulated by high DHA. Ingenuity pathway analysis identified a highly significant nervous system network, with epidermal growth factor receptor (EGFR) as the outstanding interaction partner.

CONCLUSIONS

These data indicate that LCPUFA concentrations within the normal range of human breastmilk induce global changes in gene expression across a wide array of processes, in addition to changes in visual and neural function normally associated with formula LCPUFA.

AMP-activated protein kinase in metabolic control and insulin signaling

The AMP-activated protein kinase (AMPK) system acts as a sensor of cellular energy status that is conserved in all eukaryotic cells. It is activated by increases in the cellular AMP:ATP ratio caused by metabolic stresses that either interfere with ATP production (eg, deprivation for glucose or oxygen) or that accelerate ATP consumption (eg, muscle contraction). Activation in response to increases in AMP involves phosphorylation by an upstream kinase, the tumor suppressor LKB1. In certain cells (eg, neurones, endothelial cells, and lymphocytes), AMPK can also be activated by a Ca(2+)-dependent and AMP-independent process involving phosphorylation by an alternate upstream kinase, CaMKKbeta. Once activated, AMPK switches on catabolic pathways that generate ATP, while switching off ATP-consuming processes such as biosynthesis and cell growth and proliferation. The AMPK complex contains 3 subunits, with the alpha subunit being catalytic, the beta subunit containing a glycogen-sensing domain, and the gamma subunits containing 2 regulatory sites that bind the activating and inhibitory nucleotides AMP and ATP. Although it may have evolved to respond to metabolic stress at the cellular level, hormones and cytokines such as insulin, leptin, and adiponectin can interact with the system, and it now appears to play a key role in maintaining energy balance at the whole body level. The AMPK system may be partly responsible for the health benefits of exercise and is the target for the antidiabetic drug metformin. It is a key player in the development of new treatments for obesity, type 2 diabetes, and the metabolic syndrome.

A search for genes that may confer divergent morphology and function in the carotid body between two strains of mice

The carotid body (CB) is the primary hypoxic chemosensory organ. Its hypoxic response appears to be genetically controlled. We have hypothesized that: 1) genes related to CB function are expressed less in the A/J mice (low responder to hypoxia) compared with DBA/2J mice (high responder to hypoxia); and 2) gene expression levels of morphogenic and trophic factors of the CB are significantly lower in the A/J mice than DBA/2J mice. This study utilizes microarray analysis to test these hypotheses. Three sets of CBs were harvested from both strains. RNA was isolated and used for global gene expression profiling (Affymetrix Mouse 430 v2.0 array). Statistically significant gene expression was determined as a minimum six counts of nine pairwise comparisons, a minimum 1.5-fold change, and P ≤ 0.05. Our results demonstrated that 793 genes were expressed less and that 568 genes were expressed more in the A/J strain vs. the DBA/2J strain. Analysis of individual genes indicates that genes encoding ion channels are differentially expressed between the two strains. Genes related to neurotransmitter metabolism, synaptic vesicles, and the development of neural crest-derived cells are expressed less in the A/J CB vs. the DBA/2J CB. Through pathway analysis, we have constructed a model that shows gene interactions and offers a roadmap to investigate CB development and hypoxic chemosensing/chemotransduction processes. Particularly, Gdnf, Bmp2, Kcnmb2, Tph1, Hif1a, and Arnt2 may contribute to the functional differences in the CB between the two strains. Bmp2, Phox2b, Dlx2, and Msx2 may be important for the morphological differences.

Hypoxia reduces KCa channel activity by inducing Ca2+ spark uncoupling in cerebral artery smooth muscle cells

Arterial smooth muscle cell large-conductance Ca(2+)-activated potassium (K(Ca)) channels have been implicated in modulating hypoxic dilation of systemic arteries, although this is controversial. K(Ca) channel activity in arterial smooth muscle cells is controlled by localized intracellular Ca(2+) transients, termed Ca(2+) sparks, but hypoxic regulation of Ca(2+) sparks and K(Ca) channel activation by Ca(2+) sparks has not been investigated. We report here that in voltage-clamped (-40 mV) cerebral artery smooth muscle cells, a reduction in dissolved O(2) partial pressure from 150 to 15 mmHg reversibly decreased Ca(2+) spark-induced transient K(Ca) current frequency and amplitude to 61% and 76% of control, respectively. In contrast, hypoxia did not alter Ca(2+) spark frequency, amplitude, global intracellular Ca(2+) concentration, or sarcoplasmic reticulum Ca(2+) load. Hypoxia reduced transient K(Ca) current frequency by decreasing the percentage of Ca(2+) sparks that activated a transient K(Ca) current from 89% to 63%. Hypoxia reduced transient K(Ca) current amplitude by attenuating the amplitude relationship between Ca(2+) sparks that remained coupled and the evoked transient K(Ca) currents. Consistent with these data, in inside-out patches at -40 mV hypoxia reduced K(Ca) channel apparent Ca(2+) sensitivity and increased the K(d) for Ca(2+) from approximately 17 to 32 microM, but did not alter single-channel amplitude. In summary, data indicate that hypoxia reduces K(Ca) channel apparent Ca(2+) sensitivity via a mechanism that is independent of cytosolic signaling messengers, and this leads to uncoupling of K(Ca) channels from Ca(2+) sparks. Transient K(Ca) current inhibition due to uncoupling would oppose hypoxic cerebrovascular dilation.