Perinatal loss of Nkx2-5 results in rapid conduction and contraction defects

Homeobox transcription factor Nkx2-5, highly expressed in heart, is a critical factor during early embryonic cardiac development. In this study, using tamoxifen-inducible Nkx2-5 knockout mice, we demonstrate the role of Nkx2-5 in conduction and contraction in neonates within 4 days after perinatal tamoxifen injection. Conduction defect was accompanied by reduction in ventricular expression of the cardiac voltage-gated Na+ channel pore-forming alpha-subunit (Na(v)1.5-alpha), the largest ion channel in the heart responsive for rapid depolarization of the action potential, which leads to increased intracellular Ca2+ for contraction (conduction-contraction coupling). In addition, expression of ryanodine receptor 2, through which Ca2+ is released from sarcoplasmic reticulum, was substantially reduced in Nkx2-5 knockout mice. These results indicate that Nkx2-5 function is critical not only during cardiac development but also in perinatal hearts, by regulating expression of several important gene products involved in conduction and contraction.

Macrophage migration inhibitory factor in hypothalamic paraventricular nucleus neurons decreases blood pressure in spontaneously hypertensive rats

Macrophage migration inhibitory factor (MIF) expression is increased by angiotensin II (Ang II) within paraventricular nucleus (PVN) neurons of normotensive rats and acts via its intrinsic thiol protein oxidoreductase (TPOR) to counterregulate the central nervous system-mediated pressor action of Ang II. Considering that the PVN-mediated actions of Ang II are enhanced in spontaneously hypertensive rats (SHRs) and contribute to the development of hypertension in these animals, we investigated this MIF regulatory mechanism in SHRs. Here, we have demonstrated that Ang II failed to increase MIF protein expression in the PVN of SHRs. Furthermore, although basal levels of MIF protein and mRNA were similar in the PVN of SHRs and normotensive rats, immunostaining revealed that MIF was either absent from or diminished in PVN neurons of SHRs. AAV2-mediated increases in MIF expression within PVN neurons of young (8 wk old) SHRs produced a chronic attenuation of hypertension and cardiac hypertrophy. However, similar AAV2-mediated transduction of [C60S]-MIF, which lacks TPOR activity, did not alter the development of hypertension or cardiac hypertrophy in SHRs. Collectively, these findings suggest that a lack of MIF expression within PVN neurons contributes to the development of hypertension and cardiac hypertrophy in SHRs.

Angiotensin II increases GABAB receptor expression in nucleus tractus solitarii of rats

Increasing evidence indicates that both the angiotensin II (ANG II) and gamma-aminobutyric acid (GABA) systems play a very important role in the regulation of blood pressure (BP). However, there is little information concerning the interactions between these two systems in the nucleus tractus solitarii (NTS). In the present study, we examined the effects of ANG II on GABAA and GABAB receptor (GAR and GBR) expression in the NTS of Sprague-Dawley rats. The direct effect of ANG II on GBR expression was determined in neurons cultured from NTS. Treatment of neuronal cultures with ANG II (100 nM, 5 h) induced a twofold increase in GBR1 expression, as detected with real-time RT-PCR and Western blots, but had no effect on GBR2 or GAR expression. In electrophysiological experiments, perfusion of neuronal cultures with the GBR agonist baclofen decreased neuronal firing rate by 39% and 63% in neurons treated with either PBS (control) or ANG II, respectively, indicating that chronic ANG II treatment significantly enhanced the neuronal response to GBR activation. In contrast, ANG II had no significant effect on the inhibitory action of the GAR agonist muscimol. In whole animal studies, intracerebroventricular infusion of ANG II induced a sustained increase in mean BP and an elevation of GBR1 mRNA and protein levels in the NTS. These results indicate that ANG II stimulates GBR expression in NTS neurons, and this could contribute to the central nervous system actions of ANG II that result in dampening of baroreflexes and elevated BP in the central actions of ANG II.

Immunostaining evidence for PI(4,5)P2 localization at the leading edge of chemoattractant-stimulated HL-60 cells

It is well known that in fMLP-stimulated neutrophils, phosphatidyl inositol 3,4,5-trisphosphate [PI(3,4,5)P3] localizes at the leading edge of the cells. However, no effort has been made to study the PI 4,5-bisphosphate [PI(4,5)P2] distribution in these cells. In fact, it has been suggested that PI(4,5)P2 is unlikely to localize, as its basal level is orders of magnitude higher than that of PI(3,4,5)P3. We developed an optimized immunostaining protocol for studying the endogenous distribution of PI(4,5)P2 in neutrophil-like HL-60 cells. We show that PI(4,5)P2 localizes sharply at the leading edge with an intensity gradient similar to that for PI(3,4,5)P3. The enzymes for the production of PI(4,5)P2, namely, PI5KIalpha and PI5KIgamma, were also found to localize at the leading edge, further supporting our finding that PI(4,5)P2 localizes at the leading edge. These results imply that complementary regulation of PI3K and phosphate and tensin homolog (PTEN) is not the sole or dominant mechanism of PI(3,4,5)P3 polarization in HL-60 cells.

Characterization of a functional (pro)renin receptor in rat brain neurons

(Pro)renin receptor (PRR), the newest member of the renin-angiotensin system (RAS), is turning out to be an important player in the regulation of the cardiovascular system. It plays a pivotal role in activation of the local RAS and stimulates signalling pathways involved in proliferative and hypertrophic mechanisms. However, the role of PRR in the brain remains unknown. Thus, our objective in this study was to determine whether a functional PRR is present in neurons within the brain. Neuronal co-cultures from the hypothalamus and brainstem areas of neonatal rat brain express PRR mRNA. Immunoreactivity for PRR was primarily localized on the neuronal cell soma and in discrete areas in the neurites. Treatment of neurons with renin, in the presence of 2 microm losartan, caused a time- and dose-dependent stimulation of phosphorylation of extracellular signal related kinase ERK1 (p44) and ERK2 (p42) isoforms of mitogen-activated protein kinase. Optimal stimulation of fourfold was observed within 2 min with 20 nm renin. Electrophysiological recordings showed that treatment of the neurons with renin, in the presence of 2 microm losartan, resulted in a steady and stable decrease in action potential frequency. A 46% decrease in action potential frequency was observed within 5 min of treatment and was attenuated by co-incubation with a PRR blocking peptide. These observations demonstrate that the PRR is present in neurons within the brain and that its activation by renin initiates the MAP kinase signalling pathway and inhibition of neuronal activity.

Halogenated derivatives of aromatic amino acids exhibit balanced antiglutamatergic actions: potential applications for the treatment of neurological and neuropsychiatric disorders

Glutamate, the major excitatory neurotransmitter, is critical for normal brain development and function. Both extremes of glutamate receptor activity are detrimental for the brain. Glutamate's role in excitotoxicity has driven the search for receptor antagonists as neuroprotective agents, most of which have failed to achieve clinical, i.e. efficacious and safe, neuroprotection. High selectivity and potency provide potential explanations for this failure. For example, targeting individual glutamate receptor subtypes leaves other pathways of glutamatergic excitotoxicity intact. Furthermore, potent depression of glutamate receptor activity causes clinical side effects, such as the symptoms of schizophrenia produced by NMDA receptor antagonists. To produce efficacious neuroprotection devoid of significant side effects, it may be necessary to normalize the function of all components of the glutamatergic system, instead of blocking a single type of glutamate receptors. Halogenated derivatives of aromatic amino acids modulate glutamatergic activity via multiple pre- and postsynaptic actions with moderate efficacy. In addition, these compounds may trap hydroxyl radicals and facilitate hydroxyl radical-impaired glutamate uptake. Their balanced polyvalent action may overcome the limitations of previously tested glutamatergic agents and provide a basis for their use in the treatment of neurological and neuropsychiatric disorders. The properties of this class of compounds and relevant patents are reviewed in this article.

Effects of angiotensin type 2 receptor overexpression in the rostral ventrolateral medulla on blood pressure and urine excretion in normal rats

Central angiotensin II plays a critical role in the regulation of cardiovascular function and autonomic activity, in part, via angiotensin type 1 receptors in the rostral ventrolateral medulla (RVLM). Increasing evidence indicates that angiotensin II can also act on angiotensin type 2 receptors (AT(2)Rs) to exert antagonistic effects. In the current study we determined the effects of overexpression of AT(2)R in the RVLM on sodium and water excretion and on blood pressure in conscious rats. The overexpression of AT(2)R was induced by bilateral microinjection of the AT(2)R adenovirus (Ad5-SYN-AT2R-IRES-EGFP, 2.5 x 10(6) infection units in 0.5 microL; Ad5-SYN-EGFP as the control, 2.5 x 10(6) infection units in 0.5 microL) into the RVLM of rats. Immunofluorescence staining showed that microinjection of AT(2)R adenovirus into the RVLM evoked local overexpression. Significant overexpression of AT(2)R in the RVLM began at 24 hours and was sustained up to 12 days after microinjection. Overexpression of AT(2)R in the RVLM significantly decreased the nocturnal arterial blood pressure and increased the 24-hour urine excretion at days 2, 3, and 4 after gene delivery compared with the control rats. These alterations were abolished by the microinfusion of captopril into the RVLM and were enhanced by angiotensin II infusion. Overexpression of AT(2)R in the RVLM also significantly decreased the urine concentration of noradrenaline and 24-hour noradrenaline excretion (1.1+/-0.5 microg in control rats and 2.4+/-0.5 microg in AT(2)R rats; P<0.05). These results suggest that overexpression of AT(2)R in the RVLM induced a diuresis that may be mediated, in part, by sympathoinhibition.

Basal and angiotensin II-inhibited neuronal delayed-rectifier K+ current are regulated by thioredoxin

In previous studies, we determined that macrophage migration inhibitory factor (MIF), acting intracellularly via its intrinsic thiol-protein oxidoreductase (TPOR) activity, stimulates basal neuronal delayed-rectifier K+ current ( IKv) and inhibits basal and angiotensin (ANG) II-induced increases in neuronal activity. These findings are the basis for our hypothesis that MIF is a negative regulator of ANG II actions in neurons. MIF has recently been recategorized as a member of the thioredoxin (Trx) superfamily of small proteins. In the present study we have examined whether Trx influences basal and ANG II-modulated IKv in an effort to determine whether the Trx superfamily can exert a general regulatory influence over neuronal activity and the actions of ANG II. Intracellular application of Trx (0.8–80 nM) into rat hypothalamic/brain stem neurons in culture increased neuronal IKv, as measured by voltage-clamp recordings. This effect of Trx was abolished in the presence of the TPOR inhibitor PMX 464 (800 nM). Furthermore, the mutant protein recombinant human C32S/C35S-Trx, which lacks TPOR activity, failed to alter neuronal IKv. Trx applied at a concentration (0.08 nM) that does not alter basal IKv abolished the inhibition of neuronal IKv produced by ANG II (100 nM). Given our observation that ANG II increases Trx levels in neuronal cultures, it is possible that Trx (like MIF) has a negative regulatory role over basal and ANG II-stimulated neuronal activity via modulation of IKv. Moreover, these data suggest that TPOR may be a general mechanism for negatively regulating neuronal activity.

Lack of macrophage migration inhibitory factor regulation is linked to the increased chronotropic action of angiotensin II in SHR neurons

Macrophage migration inhibitory factor acts via its intrinsic thiol-protein oxidoreductase activity to negatively regulate the neuronal chronotropic actions of angiotensin II in normotensive rat neurons. Because the chronotropic action of angiotensin II is potentiated in spontaneously hypertensive rat neurons, we investigated whether this negative regulatory mechanism is absent in these rats. Angiotensin II (100 nM) elicited an approximately 89% increase in neuronal firing in Wistar-Kyoto rat hypothalamus and brain stem cultured neurons and an increase in intracellular macrophage migration inhibitory factor levels in the same cells. The chronotropic action of angiotensin II was significantly greater (approximately 212% increase) in spontaneously hypertensive rat neurons, but angiotensin II failed to alter macrophage migration inhibitory factor expression in these cells. Intracellular application of recombinant macrophage migration inhibitory factor (0.8 nM) or its specific neuronal overexpression via Ad5-SYN-MIF (1x10(7) infectious units) significantly attenuated the chronotropic action of angiotensin II in spontaneously hypertensive rat neurons, similar to results from Wistar-Kyoto rat neurons. In contrast, C60S-macrophage migration inhibitory factor (0.8 nM), which lacks thiol-protein oxidoreductase activity, failed to alter the chronotropic action of angiotensin II in neurons from either rat strain. Thus, whereas macrophage migration inhibitory factor has the potential to depress the chronotropic action of angiotensin II in spontaneously hypertensive rat neurons, it is unlikely that this regulatory mechanism occurs, because angiotensin II does not increase the expression of this protein. The lack of this regulatory mechanism may contribute to the increased chronotropic action of angiotensin II in spontaneously hypertensive rat neurons.

Potentiation of the antihypertensive action of losartan by peripheral overexpression of the ANG II type 2 receptor

Our previous studies demonstrated that peripheral overexpression of angiotensin II (ANG II) type 2 receptors (AT(2)R) prevents hypertension-induced cardiac hypertrophy and remodeling without altering high blood pressure. This, coupled with the observations that AT(2)R play a role in the antihypertensive actions of ANG II type 1 receptor (AT(1)R) blockers (ARBs), led us to propose that peripheral overexpression of AT(2)R would improve the antihypertensive action of losartan (Los) in Sprague-Dawley (SD) rats made hypertensive via chronic infusion of ANG II. Here we utilized adenoviral vector-mediated AT(2)R gene transfer to test this hypothesis. A single intracardiac injection of adenoviral vector containing genomic AT(2)R (G-AT(2)R) DNA and enhanced green fluorescent protein (EGFP) gene controlled by cytomegalovirus (CMV) promoters (Ad-G-AT(2)R-EGFP; 5 x 10(9) infectious units) into adult SD rats produced robust AT(2)R overexpression in cardiovascular tissues (kidney, lung, heart, aorta, mesenteric artery, and renal artery) that persisted for 3-5 days postinjection. By 7 days post viral injection, the overexpressed AT(2)R are reduced toward basal values in certain tissues (lung, kidney, and heart) and are undetectable in others (kidney and blood vessels). In two separate protocols, we demonstrated that the hypotensive effect of Los (0.125, 0.5, and 1.0 mg/kg iv) was significantly greater in the AT(2)R-overexpressing animals (-40.7 +/- 4.3, -41.8 +/- 4.8, and -48.1 +/- 2.6 mmHg, respectively) compared with control vector (Ad-CMV-EGFP)-treated rats (-12.4 +/- 2.2, -20.2 +/- 3.4, and -27.3 +/- 3.4 mmHg, respectively). These results provide support for a depressor role of AT(2)R and the proposal that combined AT(2)R agonist and ARB treatment may be an improved therapeutic strategy for controlling hypertension.

Macrophage migration inhibitory factor in the PVN attenuates the central pressor and dipsogenic actions of angiotensin II

Macrophage migration inhibitory factor (MIF) acts intracellularly to counteract the angiotensin (ANG) II type 1 receptor (AT1-R)-mediated chronotropic effect of ANG II in hypothalamic neurons, an effect mediated by the thiol-protein oxidoreductase (TPOR) activity of the MIF molecule. Here we determined the in vivo actions of MIF in regulating the physiological actions of ANG II that are mediated via the paraventricular nucleus (PVN), an area that serves as a relay point in the central nervous system (CNS)-mediated effects of ANG II on cardiovascular functions and water intake. Intracerebroventricular (icv) injection of ANG II into normotensive rats selectively increased MIF protein levels in the PVN and produced significant pressor and drinking responses that were inhibited by PVN administration of the AT1-R antagonist losartan. Overexpression of MIF in PVN neurons via Ad-Syn-MIF gene transfer attenuated the pressor and drinking responses produced by icv-injected ANG II. Consistently, intracellular application of MIF or MIF-(50-65) (which harbors the TPOR activity of MIF) into PVN sympathetic regulatory neurons, blunted the electrophysiological actions of ANG II at these cells. These observations establish for the first time that MIF within the PVN, acting via TPOR, is an intracellular regulator of the central cardiovascular and dipsogenic effects of ANG II.

Macrophage Migration Inhibitory Factor Increases Neuronal Delayed Rectifier K+ Current

Macrophage migration inhibitory factor (MIF) has widespread actions in the immune, endocrine, and nervous systems. Previously, we reported that increases in the intracellular levels of MIF depress the firing of hypothalamus/brain stem neurons in culture, including the chronotropic actions of angiotensin II. The objective of this study was to investigate the effects of MIF on delayed rectifier K+ current ( IKv), one of the component currents whose activity contributes to neuronal firing. Intracellular perfusion of MIF (80 nM) into Sprague–Dawley rat neuronal cultures caused a significant increase in IKv, as measured by patch-clamp recordings. This effect was apparent by 3 min, and was maximal after 20–30 min. IKv current density (pA/pF) increased from 31.58 ± 2.36 in controls to 41.88 ± 3.76 in MIF-treated neurons (mean ± SE; n = 9; P < 0.01). MIF that had been inactivated by boiling did not alter IKv, and MIF-neutralizing antibodies abolished the action of recombinant MIF (rMIF). The stimulatory effect of MIF on IKv current density was mimicked by intracellular application of either P1S-MIF (80 nM) or the peptide MIF-(50–65) (0.8–8 μM), both of which harbor the thiol-protein oxidoreductase (TPOR) activity of the MIF molecule. Conversely, neither C60S-MIF (80 nM) nor the MIF homologue D-dopachrome tautomerase (80 nM), both of which lack TPOR activity, altered IKv. Finally, the increase in IKv produced by rMIF was abolished by the superoxide scavenger Tiron (1 mM). These studies indicate that the neuronal action of MIF includes a stimulatory action on IKv that may be mediated by a TPOR/superoxide-scavenging mechanism.

Impaired glutamatergic synaptic transmission in the PKU brain

This paper reviews recent results of our investigation of the mechanisms whereby hyperphenylalaninemia may cause brain dysfunction in classical phenylketonuria (PKU). Acute applications of L-Phe in rat and mouse hippocampal and cerebrocortical cultured neurons, at a range of concentrations found in PKU brain, significantly and reversibly depressed glutamatergic synaptic transmission by a combination of pre- and postsynaptic actions: (1) competition for the glycine-binding site of the N-methyl-D-aspartate (NMDA) receptors; (2) attenuation of neurotransmitter release; (3) competition for the glutamate-binding site of (RS)-amino-3-hydroxy-5-methyl-4-isoxazolepropioinic acid and kainate (AMPA/kainate) receptors. Unlike L-Phe, its non-tyrosine metabolites, phenylacetic acid, phenylpyruvic acid, and phenyllactic acid, did not produce antiglutamatergic effects. L-Phe did not affect inhibitory gamma-aminobutyric (GABA)-ergic transmission. Consistent with this specific pattern of effects caused by L-Phe in neuronal cultures, the expression of NMDA receptor NR2A and AMPA receptor Glu1 and Glu2/3 subunits in brain of hyperphenylalaninemic PKU mice (Pah(enu2) strain) was significantly increased, whereas expression of the NMDA receptor NR2B subunit was decreased. There was no change in GABA alpha1 subunit expression. Considering the important role of glutamatergic synaptic transmission in normal brain development and function, these L-Phe-induced changes in glutamatergic synaptic transmission in PKU brain may be a critical element of the neurological symptoms of PKU.

Intronic enhancement of angiotensin II type 2 receptor transgene expression in vitro and in vivo

The angiotensin II type 2 receptor (AT2R) can influence a variety of intracellular signaling molecules and cellular functions. However, its physiological functions and the roles of introns in the regulation of its expression have not been well determined. We first demonstrated that both intron 1 and intron 2 of AT2R could significantly enhance AT2R overexpression. Thus, we have provided some new prerequisites for further studies on the mechanisms that control AT2R gene expression. Next, we established a highly efficient method of delivering this receptor in vitro and in vivo using an AT2R recombinant adenoviral vector containing two introns of the AT2R. The vector may be useful in helping to uncover AT2R physiological functions and also for gene therapy related to AT2R. Moreover, we determined the important role of introns in gene expression cassettes and the inconsistency of expression between the targeted gene and the reporter gene.

A pH-dependent increase in neuronal glutamate efflux in vitro: Possible involvement of ASCT1

Efflux of glutamate from intracellular pools during hypoxia-ischemia has been postulated to be mediated by amino acid transporters and can lead to excitotoxicity. In addition, a decrease in pH seen during global hypoxia-ischemia may influence which transporter is responsible for this glutamate efflux. For example, the neutral amino acid transporter ASCT1 is an effective transporter of glutamate at low pH. We have examined the effects of pH, pH and temperature, and hypoxia on glutamate efflux in a rat primary neuronal cell culture model. We observed a marked increase of glutamate efflux as pH was decreased from 7.4 to 5.5. This pH-dependent efflux is likely due to a transporter-mediated process because it was seen in the presence of tetrodotoxin and was blunted by decreasing the temperature to either 35 degrees C or 33 degrees C. In addition, no increase in LDH was seen at pH 5.5 suggesting that increased glutamate levels were not due to cellular death. No change in glutamate levels was seen when the oxygen tension of the medium was lowered from 150 mm Hg to either 30 or 15 mm Hg. Given that EAAT transporters are inhibited by low pH, other transporters, such as ASCT1, may be responsible for this pH-dependent efflux of glutamate.

Macrophage migration inhibitory factor: an intracellular inhibitor of angiotensin II-induced increases in neuronal activity

Angiotensin II (Ang II) elicits Ang II type 1 receptor (AT1-R)-mediated increases in neuronal firing within the hypothalamus and brainstem that are ultimately responsible for physiological actions such as increased blood pressure and fluid intake. Although there is a growing literature on the intracellular mechanisms that mediate the actions of Ang II via AT1-R in neurons, little is known about the mechanisms that diminish or "switch-off" the neuronal chronotropic action of Ang II. In the present study, we identified macrophage migration inhibitory factor (MIF) as an intracellular inhibitor of the actions of Ang II in neurons. The evidence is as follows. First, Ang II, acting via AT1-R, increases the intracellular levels of MIF in neurons cultured from rat hypothalamus and brainstem. Second, elevation of intracellular MIF by Ang II prevents further chronotropic actions of this peptide. Third, intracellular application of exogenous recombinant MIF abolishes the Ang II-induced chronotropic action in neurons. Finally, intracellular application of the MIF peptide fragment MIF-(50-65), which harbors the thiol oxidoreductase property of the MIF molecule, mimics the inhibitory actions of MIF on Ang II-stimulated neuronal firing. Thus, this study is the first to demonstrate the existence of an intracellular negative regulator of Ang II-induced actions in neurons and indicates that MIF may act as a physiological brake for the chronotropic effects of Ang II in rat neurons.

Angiotensin II Type 2 Receptor–Mediated Gene Expression Profiling in Human Coronary Artery Endothelial Cells

Despite intensive investigation, the molecular mechanism by which the angiotensin II type 2 (AT 2 ) receptor exerts its cellular and physiological actions remains elusive. In the present study, we have used microarray expression analysis to identify genes whose expression was regulated by this receptor and to determine its cellular consequences. Lentiviral vector was used to express the AT 2 receptor in human coronary artery endothelial cells (HCAECs), followed by analysis of expression profiles. We observed ≈5224 genes regulated in an AT 2 receptor ligand-independent manner in HCAECs expressing the AT 2 receptor. In addition, 1235 genes were differentially expressed in response to the AT 2 receptor-specific ligand, CGP42112A. Validity of the expression profiles was demonstrated by real-time reverse-transcriptase polymerase chain reaction quantitation of 5 genes. Because some of these genes could be linked to the regulation of extracellular matrix association, we studied the effect of the AT 2 receptor on cell migration. Expression of the AT 2 receptor resulted in a 2-fold inhibition of HCAEC migration. Taken together, these observations demonstrate that the AT 2 receptor regulates expression of genes relevant to cell migration, protein processing, intracellular signaling, and DNA repair in both ligand-dependent and ligand-independent manners.