Constitutive Expression of Hif2α Confers Acute O2 Sensitivity to Carotid Body Glomus Cells.

Acute oxygen (O2) sensing and adaptation to hypoxia are essential for physiological homeostasis. The prototypical acute O2 sensing organ is the carotid body, which contains chemosensory glomus cells expressing O2-sensitive K+ channels. Inhibition of these channels during hypoxia leads to cell depolarization, transmitter release, and activation of afferent sensory fibers terminating in the brain stem respiratory and autonomic centers. Focusing on recent data, here we discuss the special sensitivity of glomus cell mitochondria to changes in O2 tension due to Hif2α-dependent expression of several atypical mitochondrial electron transport chain subunits and enzymes. These are responsible for an accelerated oxidative metabolism and the strict dependence of mitochondrial complex IV activity on O2 availability. We report that ablation of Epas1 (the gene coding Hif2α) causes a selective downregulation of the atypical mitochondrial genes and a strong inhibition of glomus cell acute responsiveness to hypoxia. Our observations indicate that Hif2α expression is required for the characteristic metabolic profile of glomus cells and provide a mechanistic explanation for the acute O2 regulation of breathing.

Acute oxygen sensing by vascular smooth muscle cells.

An adequate supply of oxygen (O2) is essential for most life forms on earth, making the delivery of appropriate levels of O2 to tissues a fundamental physiological challenge. When O2 levels in the alveoli and/or blood are low, compensatory adaptive reflexes are produced that increase the uptake of O2 and its distribution to tissues within a few seconds. This paper analyzes the most important acute vasomotor responses to lack of O2 (hypoxia): hypoxic pulmonary vasoconstriction (HPV) and hypoxic vasodilation (HVD). HPV affects distal pulmonary (resistance) arteries, with its homeostatic role being to divert blood to well ventilated alveoli to thereby optimize the ventilation/perfusion ratio. HVD is produced in most systemic arteries, in particular in the skeletal muscle, coronary, and cerebral circulations, to increase blood supply to poorly oxygenated tissues. Although vasomotor responses to hypoxia are modulated by endothelial factors and autonomic innervation, it is well established that arterial smooth muscle cells contain an acute O2 sensing system capable of detecting changes in O2 tension and to signal membrane ion channels, which in turn regulate cytosolic Ca2+ levels and myocyte contraction. Here, we summarize current knowledge on the nature of O2 sensing and signaling systems underlying acute vasomotor responses to hypoxia. We also discuss similarities and differences existing in O2 sensors and effectors in the various arterial territories.

Mitochondrial Redox Signaling in O2-Sensing Chemoreceptor Cells.

Significance: Acute responses to hypoxia are essential for the survival of mammals. The carotid body (CB), the main arterial chemoreceptor, contains glomus cells with oxygen (O2)-sensitive K+ channels, which are inhibited during hypoxia to trigger adaptive cardiorespiratory reflexes. Recent Advances: In this review, recent advances in molecular mechanisms of acute O2 sensing in CB glomus cells are discussed, with a special focus on the signaling role of mitochondria through regulating cellular redox status. These advances have been achieved thanks to the use of genetically engineered redox-sensitive green fluorescent protein (roGFP) probes, which allowed us to monitor rapid changes in ROS production in real time in different subcellular compartments during hypoxia. This methodology was used in combination with conditional knockout mice models, pharmacological approaches, and transcriptomic studies. We have proposed a mitochondria-to-membrane signaling model of acute O2 sensing in which H2O2 released in the mitochondrial intermembrane space serves as a signaling molecule to inhibit K+ channels on the plasma membrane. Critical Issues: Changes in mitochondrial reactive oxygen species (ROS) production during acute hypoxia are highly compartmentalized in the submitochondrial regions. The use of redox-sensitive probes targeted to specific compartments is essential to fully understand the role of mitochondrial ROS in acute O2 sensing. Future Directions: Further studies are needed to specify the ROS and to characterize the target(s) of ROS in chemoreceptor cells during acute hypoxia. These data may also contribute to a more complete understanding of the implication of ROS in acute responses to hypoxia in O2-sensing cells in other organs. Antioxid. Redox Signal. 37, 274-289.

Molecular Mechanisms of Acute Oxygen Sensing by Arterial Chemoreceptor Cells. Role of Hif2α.

Carotid body glomus cells are multimodal arterial chemoreceptors able to sense and integrate changes in several physical and chemical parameters in the blood. These cells are also essential for O2 homeostasis. Glomus cells are prototypical peripheral O2 sensors necessary to detect hypoxemia and to elicit rapid compensatory responses (hyperventilation and sympathetic activation). The mechanisms underlying acute O2 sensing by glomus cells have been elusive. Using a combination of mouse genetics and single-cell optical and electrophysiological techniques, it has recently been shown that activation of glomus cells by hypoxia relies on the generation of mitochondrial signals (NADH and reactive oxygen species), which modulate membrane ion channels to induce depolarization, Ca2+ influx, and transmitter release. The special sensitivity of glomus cell mitochondria to changes in O2 tension is due to Hif2α-dependent expression of several atypical mitochondrial subunits, which are responsible for an accelerated oxidative metabolism and the strict dependence of mitochondrial complex IV activity on O2 availability. A mitochondrial-to-membrane signaling model of acute O2 sensing has been proposed, which explains existing data and provides a solid foundation for future experimental tests. This model has also unraveled new molecular targets for pharmacological modulation of carotid body activity potentially relevant in the treatment of highly prevalent medical conditions.

Acute O2 sensing through HIF2α-dependent expression of atypical cytochrome oxidase subunits in arterial chemoreceptors.

Acute cardiorespiratory responses to O2 deficiency are essential for physiological homeostasis. The prototypical acute O2-sensing organ is the carotid body, which contains glomus cells expressing K+ channels whose inhibition by hypoxia leads to transmitter release and activation of nerve fibers terminating in the brainstem respiratory center. The mechanism by which changes in O2 tension modulate ion channels has remained elusive. Glomus cells express genes encoding HIF2α (Epas1) and atypical mitochondrial subunits at high levels, and mitochondrial NADH and reactive oxygen species (ROS) accumulation during hypoxia provides the signal that regulates ion channels. We report that inactivation of Epas1 in adult mice resulted in selective abolition of glomus cell responsiveness to acute hypoxia and the hypoxic ventilatory response. Epas1 deficiency led to the decreased expression of atypical mitochondrial subunits in the carotid body, and genetic deletion of Cox4i2 mimicked the defective hypoxic responses of Epas1-null mice. These findings provide a mechanistic explanation for the acute O2 regulation of breathing, reveal an unanticipated role of HIF2α, and link acute and chronic adaptive responses to hypoxia.

α5-Integrin-mediated cellular signaling contributes to the myogenic response of cerebral resistance arteries.

The myogenic response of resistance arterioles and small arteries involving constriction in response to intraluminal pressure elevation and dilation on pressure reduction is fundamental to local blood flow regulation in the microcirculation. Integrins have garnered considerable attention in the context of initiating the myogenic response, but evidence indicative of mechanotransduction by integrin adhesions, for example established changes in tyrosine phosphorylation of key adhesion proteins, has not been obtained to substantiate this interpretation. Here, we evaluated the role of integrin adhesions and associated cellular signaling in the rat cerebral arterial myogenic response using function-blocking antibodies against α5ß1-integrins, pharmacological inhibitors of focal adhesion kinase (FAK) and Src family kinase (SFK), an ultra-high-sensitivity western blotting technique, site-specific phosphoprotein antibodies to quantify adhesion and contractile filament protein phosphorylation, and differential centrifugation to determine G-actin levels in rat cerebral arteries at varied intraluminal pressures. Pressure-dependent increases in the levels of phosphorylation of FAK (FAK-Y397, Y576/Y577), SFK (SFK-Y416; Y527 phosphorylation was reduced), vinculin-Y1065, paxillin-Y118 and phosphoinositide-specific phospholipase C-γ1 (PLCγ1)-Y783 were detected. Treatment with α5-integrin function-blocking antibodies, FAK inhibitor FI-14 or SFK inhibitor SU6656 suppressed the changes in adhesion protein phosphorylation, and prevented pressure-dependent phosphorylation of the myosin targeting subunit of myosin light chain phosphatase (MYPT1) at T855 and 20kDa myosin regulatory light chains (LC20) at S19, as well as actin polymerization that are necessary for myogenic constriction. We conclude that mechanotransduction by integrin adhesions and subsequent cellular signaling play a fundamental role in the cerebral arterial myogenic response.

PKC-mediated cerebral vasoconstriction: Role of myosin light chain phosphorylation versus actin cytoskeleton reorganization.

Defective protein kinase C (PKC) signaling has been suggested to contribute to abnormal vascular contraction in disease conditions including hypertension and diabetes. Our previous work on agonist and pressure-induced cerebral vasoconstriction implicated PKC as a major contributor to force production in a myosin light chain (LC20) phosphorylation-independent manner. Here, we used phorbol dibutyrate to selectively induce a PKC-dependent constriction in rat middle cerebral arteries and delineate the relative contribution of different contractile mechanisms involved. Specifically, we employed an ultra-sensitive 3-step western blotting approach to detect changes in the content of phosphoproteins that regulate myosin light chain phosphatase (MLCP) activity, thin filament activation, and actin cytoskeleton reorganization. Data indicate that PKC activation evoked a greater constriction at a similar level of LC20 phosphorylation achieved by 5-HT. PDBu-evoked constriction persisted in the presence of Gö6976, a selective inhibitor of Ca(2+)-dependent PKC, and in the absence of extracellular Ca(2+). Biochemical evidence indicates that either + or - extracellular Ca(2+), PDBu (i) inhibits MLCP activity via the phosphorylation of myosin targeting subunit of myosin phosphatase (MYPT1) and C-kinase potentiated protein phosphatase-1 inhibitor (CPI-17), (ii) increases the phosphorylation of paxillin and heat shock protein 27 (HSP27), and reduces G-actin content, and (iii) does not change the phospho-content of the thin filament proteins, calponin and caldesmon. PDBu-induced constriction was more sensitive to disruption of actin cytoskeleton compared to inhibition of cross-bridge cycling. In conclusion, this study provided evidence for the pivotal contribution of cytoskeletal actin polymerization in force generation following PKC activation in cerebral resistance arteries.

Cytoskeletal reorganization evoked by Rho-associated kinase- and protein kinase C-catalyzed phosphorylation of cofilin and heat shock protein 27, respectively, contributes to myogenic constriction of rat cerebral arteries.

Our understanding of the molecular events contributing to myogenic control of diameter in cerebral resistance arteries in response to changes in intravascular pressure, a fundamental mechanism regulating blood flow to the brain, is incomplete. Myosin light chain kinase and phosphatase activities are known to be increased and decreased, respectively, to augment phosphorylation of the 20-kDa regulatory light chain subunits (LC20) of myosin II, which permits cross-bridge cycling and force development. Here, we assessed the contribution of dynamic reorganization of the actin cytoskeleton and thin filament regulation to the myogenic response and serotonin-evoked constriction of pressurized rat middle cerebral arteries. Arterial diameter and the levels of phosphorylated LC(20), calponin, caldesmon, cofilin, and HSP27, as well as G-actin content, were determined. A decline in G-actin content was observed following pressurization from 10 mm Hg to between 40 and 120 mm Hg and in three conditions in which myogenic or agonist-evoked constriction occurred in the absence of a detectable change in LC20 phosphorylation. No changes in thin filament protein phosphorylation were evident. Pressurization reduced G-actin content and elevated the levels of cofilin and HSP27 phosphorylation. Inhibitors of Rho-associated kinase and PKC prevented the decline in G-actin; reduced cofilin and HSP27 phosphoprotein content, respectively; and blocked the myogenic response. Furthermore, phosphorylation modulators of HSP27 and cofilin induced significant changes in arterial diameter and G-actin content of myogenically active arteries. Taken together, our findings suggest that dynamic reorganization of the cytoskeleton involving increased actin polymerization in response to Rho-associated kinase and PKC signaling contributes significantly to force generation in myogenic constriction of cerebral resistance arteries.

Ca2+ sensitization due to myosin light chain phosphatase inhibition and cytoskeletal reorganization in the myogenic response of skeletal muscle resistance arteries.

Abstract The myogenic response of resistance arteries to intravascular pressure elevation is a fundamental physiological mechanism of crucial importance for blood pressure regulation and organ-specific control of blood flow. The importance of Ca(2+) entry via voltage-gated Ca(2+) channels leading to phosphorylation of the 20 kDa myosin regulatory light chains (LC20) in the myogenic response is well established. Recent studies, however, have suggested a role for Ca(2+) sensitization via activation of the RhoA/Rho-associated kinase (ROK) pathway in the myogenic response. The possibility that enhanced actin polymerization is also involved in myogenic vasoconstriction has been suggested. Here, we have used pressurized resistance arteries from rat gracilis and cremaster skeletal muscles to assess the contribution to myogenic constriction of Ca(2+) sensitization due to: (1) phosphorylation of the myosin targeting subunit of myosin light chain phosphatase (MYPT1) by ROK; (2) phosphorylation of the 17 kDa protein kinase C (PKC)-potentiated protein phosphatase 1 inhibitor protein (CPI-17) by PKC; and (3) dynamic reorganization of the actin cytoskeleton evoked by ROK and PKC. Arterial diameter, MYPT1, CPI-17 and LC20 phosphorylation, and G-actin content were determined at varied intraluminal pressures ± H1152, GF109203X or latrunculin B to suppress ROK, PKC and actin polymerization, respectively. The myogenic response was associated with an increase in MYPT1 and LC20 phosphorylation that was blocked by H1152. No change in phospho-CPI-17 content was detected although the PKC inhibitor, GF109203X, suppressed myogenic constriction. Basal LC20 phosphorylation at 10 mmHg was high at ∼40%, increased to a maximal level of ∼55% at 80 mmHg, and exhibited no additional change on further pressurization to 120 and 140 mmHg. Myogenic constriction at 80 mmHg was associated with a decline in G-actin content by ∼65% that was blocked by inhibition of ROK or PKC. Taken together, our findings indicate that two mechanisms of Ca(2+) sensitization (ROK-mediated phosphorylation of MYPT1-T855 with augmentation of LC20 phosphorylation, and a ROK- and PKC-evoked increase in actin polymerization) contribute to force generation in the myogenic response of skeletal muscle arterioles.

High blood pressure associates with the remodelling of inward rectifier K+ channels in mice mesenteric vascular smooth muscle cells.

The increased vascular tone that defines essential hypertension is associated with depolarization of vascular smooth muscle cells (VSMCs) and involves a change in the expression profile of ion channels promoting arterial contraction. As a major regulator of VSMC resting membrane potential (V(M)), K(+) channel activity is an important determinant of vascular tone and vessel diameter. However, hypertension-associated changes in the expression and/or modulation of K(+) channels are poorly defined, due to their large molecular diversity and their bed-specific pattern of expression. Moreover, the impact of these changes on the integrated vessel function and their contribution to the development of altered vascular tone under physiological conditions need to be confirmed. Hypertensive (BPH) and normotensive (BPN) mice strains obtained by phenotypic selection were used to explore whether changes in the functional expression of VSMC inward rectifier K(+) channels contribute to the more depolarized resting V(M) and the increased vascular reactivity of hypertensive arteries. We determined the expression levels of inward rectifier K(+) channel mRNA in several vascular beds from BPN and BPH animals, and their functional contribution to VSMC excitability and vascular tone in mesenteric arteries. We found a decrease in the expression of Kir2.1, Kir4.1, Kir6.x and SUR2 mRNA in BPH VSMCs, and a decreased functional contribution of both K(IR) and K(ATP) channels in isolated BPH VSMCs. However, only the effect of K(ATP) channel modulators was impaired when exploring vascular tone, suggesting that decreased functional expression of K(ATP) channels may be an important element in the remodelling of VSMCs in essential hypertension.

Characterization of ion channels involved in the proliferative response of femoral artery smooth muscle cells.

OBJECTIVE: Vascular smooth muscle cells (VSMCs) contribute significantly to occlusive vascular diseases by virtue of their ability to switch to a noncontractile, migratory, and proliferating phenotype. Although the participation of ion channels in this phenotypic modulation (PM) has been described previously, changes in their expression are poorly defined because of their large molecular diversity. We obtained a global portrait of ion channel expression in contractile versus proliferating mouse femoral artery VSMCs, and explored the functional contribution to the PM of the most relevant changes that we observed. METHODS AND RESULTS: High-throughput real-time polymerase chain reaction of 87 ion channel genes was performed in 2 experimental paradigms: an in vivo model of endoluminal lesion and an in vitro model of cultured VSMCs obtained from explants. mRNA expression changes showed a good correlation between the 2 proliferative models, with only 2 genes, Kv1.3 and Kvbeta2, increasing their expression on proliferation. The functional characterization demonstrates that Kv1.3 currents increased in proliferating VSMC and that their selective blockade inhibits migration and proliferation. CONCLUSIONS: These findings establish the involvement of Kv1.3 channels in the PM of VSMCs, providing a new therapeutical target for the treatment of intimal hyperplasia.

De novo expression of Kv6.3 contributes to changes in vascular smooth muscle cell excitability in a hypertensive mice strain.

Essential hypertension involves a gradual and sustained increase in total peripheral resistance, reflecting an increased vascular tone. This change associates with a depolarization of vascular myocytes, and relies on a change in the expression profile of voltage-dependent ion channels (mainly Ca(2+) and K(+) channels) that promotes arterial contraction. However, changes in expression and/or modulation of voltage-dependent K(+) channels (Kv channels) are poorly defined, due to their large molecular diversity and their vascular bed-specific expression. Here we endeavor to characterize the molecular and functional expression of Kv channels in vascular smooth muscle cells (VSMCs) and their regulation in essential hypertension, by using VSMCs from resistance (mesenteric) or conduit (aortic) arteries obtained from a hypertensive inbred mice strain, BPH, and the corresponding normotensive strain, BPN. Real-time PCR reveals a differential distribution of Kv channel subunits in the different vascular beds as well as arterial bed-specific changes under hypertension. In mesenteric arteries, the most conspicuous change was the de novo expression of Kv6.3 (Kcng3) mRNA in hypertensive animals. The functional relevance of this change was studied by using patch-clamp techniques. VSMCs from BPH arteries were more depolarized than BPN ones, and showed significantly larger capacitance values. Moreover, Kv current density in BPH VSMCs is decreased mainly due to the diminished contribution of the Kv2 component. The kinetic and pharmacological profile of Kv2 currents suggests that the expression of Kv6.3 could contribute to the natural development of hypertension.

Contribution of Kv channels to phenotypic remodeling of human uterine artery smooth muscle cells.

Vascular smooth muscle cells (VSMCs) perform diverse functions that can be classified into contractile and synthetic (or proliferating). All of these functions can be fulfilled by the same cell because of its capacity of phenotypic modulation in response to environmental changes. The resting membrane potential is a key determinant for both contractile and proliferating functions. Here, we have explored the expression of voltage-dependent K+ (Kv) channels in contractile (freshly dissociated) and proliferating (cultured) VSMCs obtained from human uterine arteries to establish their contribution to the functional properties of the cells and their possible participation in the phenotypic switch. We have studied the expression pattern (both at the mRNA and at the protein level) of Kvalpha subunits in both preparations as well as their functional contribution to the K+ currents of VSMCs. Our results indicate that phenotypic remodeling associates with a change in the expression and distribution of Kv channels. Whereas Kv currents in contractile VSMCs are mainly performed by Kv1 channels, Kv3.4 is the principal contributor to K+ currents in cultured VSMCs. Furthermore, selective blockade of Kv3.4 channels resulted in a reduced proliferation rate, suggesting a link between Kv channels expression and phenotypic remodeling.

Characterization of the Kv channels of mouse carotid body chemoreceptor cells and their role in oxygen sensing.

As there are wide interspecies variations in the molecular nature of the O(2)-sensitive Kv channels in arterial chemoreceptors, we have characterized the expression of these channels and their hypoxic sensitivity in the mouse carotid body (CB). CB chemoreceptor cells were obtained from a transgenic mouse expressing green fluorescent protein (GFP) under the control of tyrosine hydroxylase (TH) promoter. Immunocytochemical identification of TH in CB cell cultures reveals a good match with GFP-positive cells. Furthermore, these cells show an increase in [Ca(2+)](i) in response to low P(O(2)), demonstrating their ability to engender a physiological response. Whole-cell experiments demonstrated slow-inactivating K(+) currents with activation threshold around -30 mV and a bi-exponential kinetic of deactivation (tau of 6.24 +/- 0.52 and 32.85 +/- 4.14 ms). TEA sensitivity of the currents identified also two different components (IC(50) of 17.8 +/- 2.8 and 940.0 +/- 14.7 microm). Current amplitude decreased reversibly in response to hypoxia, which selectively affected the fast deactivating component. Hypoxic inhibition was also abolished in the presence of low (10-50 microm) concentrations of TEA, suggesting that O(2) interacts with the component of the current most sensitive to TEA. The kinetic and pharmacological profile of the currents suggested the presence of Kv2 and Kv3 channels as their molecular correlates, and we have identified several members of these two subfamilies by single-cell PCR and immunocytochemistry. This report represents the first functional and molecular characterization of Kv channels in mouse CB chemoreceptor cells, and strongly suggests that O(2)-sensitive Kv channels in this preparation belong to the Kv3 subfamily.