Birgit Gaßner

Inhibition of Na+−H+ exchange impairs receptor-mediated albumin endocytosis in renal proximal tubule-derived epithelial cells from opossum

1 Receptor‐mediated endocytosis is an important mechanism for transport of macromolecules and regulation of cell‐surface receptor expression. In renal proximal tubules, receptor‐mediated endocytosis mediates the reabsorption of filtered albumin. Acidification of the endocytic compartments is essential because it interferes with ligand‐receptor dissociation, vesicle trafficking, fusion events and coat formation. 2 Here we show that the activity of Na+‐H+ exchanger isoform 3 (NHE3) is important for proper receptor‐mediated endocytosis of albumin and endosomal pH homeostasis in a renal proximal tubular cell line (opossum kidney cells) which expresses NHE3 only. 3 Depending on their inhibitory potency with respect to NHE3 and their lipophilicity, the NHE inhibitors EIPA, amiloride and HOE694 differentially reduced albumin endocytosis. The hydrophilic inhibitor HOE642 had no effect. 4 Inhibition of NHE3 led to an alkalinization of early endosomes and to an acidification of the cytoplasm, indicating that Na+‐H+ exchange contributes to the acidification of the early endosomal compartment due to the existence of a sufficient Na+ gradient across the endosomal membrane. 5 Exclusive acidification of the cytoplasm with propionic acid or by removal of Na+ induced a significantly smaller reduction in endocytosis than that induced by inhibition of Na+‐H+ exchange. 6 Analysis of the inhibitory profiles indicates that in early endosomes and endocytic vesicles NHE3 is of major importance, whereas plasma membrane NHE3 plays a minor role. 7 Thus, NHE3‐mediated acidification along the first part of the endocytic pathway plays an important role in receptor‐mediated endocytosis. Furthermore, the involvement of NHE3 offers new ways to explain the regulation of receptor‐mediated endocytosis.

Atrial natriuretic peptide enhances microvascular albumin permeability by the caveolae-mediated transcellular pathway

Aims Cardiac atrial natriuretic peptide (ANP) participates in the maintenance of arterial blood pressure and intravascular volume homeostasis. The hypovolaemic effects of ANP result from coordinated actions in the kidney and systemic microcirculation. Hence, ANP, via its guanylyl cyclase-A (GC-A) receptor and intracellular cyclic GMP as second messenger, stimulates endothelial albumin permeability. Ultimately, this leads to a shift of plasma fluid into interstitial pools. Here we studied the role of caveolae-mediated transendothelial albumin transport in the hyperpermeability effects of ANP. Methods and results Intravital microscopy studies of the mouse cremaster microcirculation showed that ANP stimulates the extravasation of fluorescent albumin from post-capillary venules and causes arteriolar vasodilatation. The hyperpermeability effect was prevented in mice with conditional, endothelial deletion of GC-A (EC GC-A KO) or with deleted caveolin-1 (cav-1), the caveolae scaffold protein. In contrast, the vasodilating effect was preserved. Concomitantly, the acute hypovolaemic action of ANP was abolished in EC GC-A KO and Cav-1−/− mice. In cultured microvascular rat fat pad and mouse lung endothelial cells, ANP stimulated uptake and transendothelial transport of fluorescent albumin without altering endothelial electrical resistance. The stimulatory effect on albumin uptake was prevented in GC-A- or cav-1-deficient pulmonary endothelia. Finally, preparation of caveolin-enriched lipid rafts from mouse lung and western blotting showed that GC-A and cGMP-dependent protein kinase I partly co-localize with Cav-1 in caveolae microdomains. Conclusion ANP enhances transendothelial caveolae-mediated albumin transport via its GC-A receptor. This ANP-mediated cross-talk between the heart and the microcirculation is critically involved in the regulation of intravascular volume.

Homologous desensitization of guanylyl cyclase A, the receptor for atrial natriuretic peptide, is associated with a complex phosphorylation pattern

Atrial natriuretic peptide (ANP), via its guanylyl cyclase A (GC‐A) receptor and intracellular guanosine 3′,5′‐cyclic monophosphate production, is critically involved in the regulation of blood pressure. In patients with chronic heart failure, the plasma levels of ANP are increased, but the cardiovascular actions are severely blunted, indicating a receptor or postreceptor defect. Studies on metabolically labelled GC‐A‐overexpressing cells have indicated that GC‐A is extensively phosphorylated, and that ANP‐induced homologous desensitization of GC‐A correlates with receptor dephosphorylation, a mechanism which might contribute to a loss of function in vivo. In this study, tandem MS analysis of the GC‐A receptor, expressed in the human embryonic kidney cell line HEK293, revealed unambiguously that the intracellular domain of the receptor is phosphorylated at multiple residues: Ser487, Ser497, Thr500, Ser502, Ser506, Ser510 and Thr513. MS quantification based on multiple reaction monitoring demonstrated that ANP‐provoked desensitization was accompanied by a complex pattern of receptor phosphorylation and dephosphorylation. The population of completely phosphorylated GC‐A was diminished. However, intriguingly, the phosphorylation of GC‐A at Ser487 was selectively enhanced after exposure to ANP. The functional relevance of this observation was analysed by site‐directed mutagenesis. The substitution of Ser487 by glutamate (which mimics phosphorylation) blunted the activation of the GC‐A receptor by ANP, but prevented further desensitization. Our data corroborate previous studies suggesting that the responsiveness of GC‐A to ANP is regulated by phosphorylation. However, in addition to the dephosphorylation of the previously postulated sites (Ser497, Thr500, Ser502, Ser506, Ser510), homologous desensitization seems to involve the phosphorylation of GC‐A at Ser487, a newly identified site of phosphorylation. The identification and further characterization of the specific mechanisms involved in the downregulation of GC‐A responsiveness to ANP may have important pathophysiological implications.

Atrial Natriuretic Peptide Locally Counteracts the Deleterious Effects of Cardiomyocyte Mineralocorticoid Receptor Activation

Background—The endocrine balance between atrial natriuretic peptide (ANP) and the renin–angiotensin–aldosterone system is critical for the maintenance of arterial blood pressure and volume homeostasis. This study investigated whether a cardiac imbalance between ANP and aldosterone, toward increased mineralocorticoid receptor (MR) signaling, contributes to adverse left ventricular remodeling in response to pressure overload. Methods and Results—We used the MR-selective antagonist eplerenone to test the role of MRs in mediating pressure overload–induced dilatative cardiomyopathy of mice with abolished local, cardiac ANP activity. In response to 21 days of transverse aortic constriction, mice with cardiomyocyte-restricted inactivation (knockout) of the ANP receptor (guanylyl cyclase [GC]-A) or the downstream cGMP-dependent protein kinase I developed enhanced left ventricular hypertrophy and fibrosis together with contractile dysfunction. Treatment with eplerenone (100 mg/kg/d) attenuated left ventricular hypertrophy and fully prevented fibrosis, dilatation, and failure. Transverse aortic constriction induced the cardiac expression of profibrotic connective tissue growth factor and attenuated the expression of SERCA2a (sarcoplasmic reticulum Ca2+-ATPase) in knockout mice, but not in controls. These genotype-dependent molecular changes were similarly prevented by eplerenone. ANP attenuated the aldosterone-induced nuclear translocation of MRs via GC-A/cGMP-dependent protein kinase I in transfected HEK 293 (human embryonic kidney) cells. Coimmunoprecipitation and fluorescence resonance energy transfer experiments demonstrated that a population of MRs were membrane associated in close interaction with GC-A and cGMP-dependent protein kinase I and, moreover, that aldosterone caused a conformational change of this membrane MR/GC-A protein complex which was prevented by ANP. Conclusions—ANP counter-regulates cardiac MR activation in hypertensive heart disease. An imbalance in cardiac ANP/GC-A (inhibition) and aldosterone/MR signaling (augmentation) favors adverse cardiac remodeling in chronic pressure overload.

Atrial Natriuretic Peptide–Mediated Inhibition of Microcirculatory Endothelial Ca2+ and Permeability Response to Histamine Involves cGMP-Dependent Protein Kinase I and TRPC6 Channels

Objective—Histamine increases microvascular endothelial leakage by activation of complex calcium-dependent and -independent signaling pathways. Atrial natriuretic peptide (ANP) via its cGMP-forming guanylyl cyclase-A (GC-A) receptor counteracts this response. Here, we characterized the molecular mechanisms underlying this interaction, especially the role of cGMP-dependent protein kinase I (cGKI). Approach and Results—We combined intravital microscopy studies of the mouse cremaster microcirculation with experiments in cultured microvascular human dermal endothelial cells. In wild-type mice, ANP had no direct effect on the extravasation of fluorescent dextran from postcapillary venules, but strongly reduced the histamine-provoked vascular leakage. This anti-inflammatory effect of ANP was abolished in mice with endothelial-restricted inactivation of GC-A or cGKI. Histamine-induced increases in endothelial [Ca2+]i in vitro and of vascular leakage in vivo were markedly attenuated by the Ca2+-entry inhibitor SKF96365 and in mice with ablated transient receptor potential canonical (TRPC) 6 channels. Conversely, direct activation of TRPC6 with hyperforin replicated the hyperpermeability responses to histamine. ANP, via cGKI, stimulated the inhibitory phosphorylation of TRPC6 at position Thr69 and prevented the hyperpermeability responses to hyperforin. Moreover, inhibition of cGMP degradation by the phosphodiesterase 5 inhibitor sildenafil prevented the edematic actions of histamine in wild types but not in mice with endothelial GC-A or cGKI deletion. Conclusions—ANP attenuates the inflammatory actions of histamine via endothelial GC-A/cGMP/cGKI signaling and inhibitory phosphorylation of TRPC6 channels. The therapeutic potential of this novel regulatory pathway is indicated by the observation that sildenafil improves systemic endothelial barrier functions by enhancing the endothelial effects of endogenous ANP.

Endothelial actions of atrial natriuretic peptide prevent pulmonary hypertension in mice

The cardiac hormone atrial natriuretic peptide (ANP) regulates systemic and pulmonary arterial blood pressure by activation of its cyclic GMP-producing guanylyl cyclase-A (GC-A) receptor. In the lung, these hypotensive effects were mainly attributed to smooth muscle-mediated vasodilatation. It is unknown whether pulmonary endothelial cells participate in the homeostatic actions of ANP. Therefore, we analyzed GC-A/cGMP signalling in lung endothelial cells and the cause and functional impact of lung endothelial GC-A dysfunction. Western blot and cGMP determinations showed that cultured human and murine pulmonary endothelial cells exhibit prominent GC-A expression and activity which were markedly blunted by hypoxia, a condition known to trigger pulmonary hypertension (PH). To elucidate the consequences of impaired endothelial ANP signalling, we studied mice with genetic endothelial cell-restricted ablation of the GC-A receptor (EC GC-A KO). Notably, EC GC-A KO mice exhibit PH already under resting, normoxic conditions, with enhanced muscularization of small arteries and perivascular infiltration of inflammatory cells. These alterations were aggravated on exposure of mice to chronic hypoxia. Lung endothelial GC-A dysfunction was associated with enhanced expression of angiotensin converting enzyme (ACE) and increased pulmonary levels of Angiotensin II. Angiotensin II/AT1-blockade with losartan reversed pulmonary vascular remodelling and perivascular inflammation of EC GC-A KO mice, and prevented their increment by chronic hypoxia. This experimental study indicates that endothelial effects of ANP are critical to prevent pulmonary vascular remodelling and PH. Chronic endothelial ANP/GC-A dysfunction, e.g. provoked by hypoxia, is associated with activation of the ACE–angiotensin pathway in the lung and PH.

Dual actions of ANP on endothelial permeability

Atrial natriuretic peptide (ANP), via its cGMP-forming guanylyl cyclase-A (GC-A) receptor, is critically involved in the regulation of arterial blood pressure and intravascular volume. To elucidate the role of the endothelial effects of ANP, we generated mice with conditional, endothelium (EC)-restricted ablation of the GC-A gene. Our observations in these and control mice demonstrated that ANP, via GC-A, mildly stimulates systemic transendothelial albumin transport in the microvasculature of skeletal muscle and skin. These and other studies indicate that concerted renal diuretic/natriuretic and mild endothelial hyperpermeability actions of ANP are essential to adjust intravascular fluid volume. However, this notion apparently contradicts published in vitro and in vivo studies showing that ANP can strenghen the pulmonary barrier under inflammatory conditions, suggesting that the hormone either acts differently on pulmonary vs systemic endothelium, or it exerts opposite effects on quiescent endothelia (enhanced permeability) and an inflammation-activated endothelium (barrier stabilization). Endothelial hyperpermeability is characteristic of many systemic diseases, such as allergic responses, edema, and sepsis. In particular, histamine markedly and acutely enhances endothelial permeability. Activation of endothelial Gq-coupled H1 receptors activates phospholipase C and elevates intracellular [Ca2+], ultimately leading to actin-myosin contraction and paracellular leakiness. Here we combined studies in microvascular endothelial cells and intravital microscopy of vascular permeability in the m. cremaster microcirculation to study whether ANP counteracts not only pulmonary but also systemic inflammation, i.e. histamine-induced hyperpermeability. Our observations reveal that transient receptor potential canonical (TRPC) 6 channels mediate the inflammatory effects of histamine. Most importantly, they characterize a regulatory pathway by which histamine-induced activation of TRPC6 channels and subsequent calcium-dependent acute endothelial hyperpermeability are prevented by ANP/GC-A-induced, cGMP-dependent protein kinase I (cGKI) - mediated inhibitory phosphorylation of these channels.

The good and the ugly: ANP antagonizes the deleterious effects of aldosterone in hypertensive cardiac remodeling

Background Atrial natriuretic peptide (ANP), via its guanylyl cyclase A (GC-A) receptor and cyclic GMP formation, exerts cardiac antihypertrophic and antifibrotic actions. Conversely, aldosterone promotes pathological cardiac remodeling via the mineralocorticoid receptor (MR). To investigate whether local cardiac ANP/GC-A signaling counteracts the effects of aldosterone during experimental hypertensive cardiac remodeling. Results We studied the impact of the MR antagonist eplerenone (100 mg/kg/day) on cardiac remodeling after transverse aortic constriction (TAC) in mice with conditional, cardiomyocyte-restricted deletion of GC-A (CM GC-A KO) [1] or cGMP-dependent protein kinase I (CM cGK I KO) [2] and respective controls (n=10 in each group). Left ventricular (LV) hypertrophy and interstitial fibrosis were significantly exacerbated in both CM GC-A KO and cGK I KO mice after TAC. These histological changes were accompanied by decreased LV SERCA2a expression, increased CTGF and LV dilatation together with contractile dysfunction. Eplerenone had no effect on systemic blood pressure but fully prevented these pressure-overload induced cardiac morphological, molecular and functional alterations in both CM GC-A KO and CM cGK I KO mice. However, eplerenone did not inhibit TACinduced MAPK ERK1/2 phosphorylation/activation, suggesting that the non-genomic effects of aldosterone are not involved in exacerbated hypertensive cardiac remodeling of the KO mice. Intriguingly, in transfected HEK 293 cells, ANP inhibited the aldosterone-induced nuclear translocation of the MR. Conclusion ANP, via GC-A/cGMP/cGKI signaling in cardiac myocytes, attenuates hypertensive cardiac remodeling and dysfunction. These protective ANP effects seem to be mediated at least in part by counterregulation of the deleterious genomic (MR-mediated) cardiac actions of aldosterone.

Atrial natriuretic peptide increases the albumin permeability of mouse skeletal muscle microcirculation by the caveolae-mediated transcellular pathway

Background Cardiac atrial natriuretic peptide (ANP) is an essential physiological regulator of arterial blood pressure and intravascular volume. Unlike other “natriuretics”, which reduce interstitial fluid volume with little change in plasma volume, ANP has important extrarenal, endothelial actions that enable it to reduce plasma volume preferentially. Thus ANP, via its guanylyl cyclase-A (GC-A) receptor and cyclic GMP formation, enhances microvascular endothelial permeability to albumin, ultimately moving plasma fluid into interstitial pools. However, the cellular pathways mediating this effect are unknown.v Here we investigated the role of caveolae-mediated endothelial albumin transcytosis in the mechanism of increased permeability induced by ANP.

Guanylyl cyclase-A mediated endothelial actions of natriuretic peptides are critically involved in postischemic reperfusion

Background Published studies have revealed protective actions of high levels of natriuretic peptides on vascular regeneration in response to ischemia. BNP overexpressed systemically in transgenic mice or gene transfer of CNP into ischemic muscles effectively accelerates angiogenesis [1]. These studies suggest that pharmacological levels of exogenous NPs might be used as a strategy of therapeutic angiogenesis. However, they do not reveal whether endogenous NPs exert (patho-) physiological vasculoprotective actions. To investigate this question, we studied the impact of genetic ablation of guanylyl cyclase-A (GC-A), the receptor for ANP and BNP, on postocclusive hindlimb ischemia in mice.