Hypertension-linked decrease in the expression of brain gamma-adducin

γ-Adducin promotes process outgrowth and secretory protein exit from the Golgi apparatus

α, β, and γ adducins mediate F-actin remodeling of plasma membrane structures as heterotetramers. Here, we present two new functions of γ-adducin. (1) Overexpression of γ-adducin promoted formation of neurite-like processes in non-neuronal fibroblast COS7 cells. Conversely, overexpression of the C-terminal 38 amino acids of γ-adducin (γAdd(C38)) acting as a dominant negative inhibited formation of neurites/processes in Neuro2A cells and anterior pituitary AtT20 cells. (2) γ-Adducin appears to facilitate pro-opiomelanocortin (POMC) exit from the trans-Golgi network (TGN) by re-organizing the actin network around the Golgi complex. Filamentous actins (F-actins) which formed puncti around the Golgi complex in control cells were dispersed in AtT20 cells stably transfected with γAdd(C38). Furthermore, γAdd(C38)-transfectants showed significant accumulation of POMC/adrenocorticotropin (ACTH) in the Golgi complex and diminished POMC/ACTH vesicles in the cell processes. The C-terminal 38 amino acids of γ-adducin interacted with F-actins around the Golgi complex, to facilitate F-actin-mediated budding of POMC/ACTH vesicles from the TGN. Thus, we propose that γ-adducin, via its interaction with F-actins, plays a critical role in actin remodeling to facilitate process/neurite outgrowth, as well as budding of POMC/ACTH vesicles from the TGN via its interaction with peri-Golgi F-actins.

Arterial properties in relation to genetic variations in the adducin subunits in a white population

BACKGROUND

Adducin is a membrane skeleton protein, which consists of either alpha- and beta- or alpha- and gamma-subunits. We investigated whether arterial characteristics might be related to the genes encoding ADD1 (Gly460Trp-rs4961), ADD2 (C1797T-rs4984), and ADD3 (IVS11+386A>G-rs3731566).

METHODS

We randomly recruited 1,126 Flemish subjects (mean age, 43.8 years; 50.3% women). Using a wall-tracking ultrasound system, we measured the properties of the carotid, femoral, and brachial arteries. We studied multivariate-adjusted phenotype-genotype associations, using a population- and family-based approach.

RESULTS

In single-gene analyses, brachial diameter was 0.15 mm (P = 0.0022) larger, and brachial distensibility and cross-sectional compliance were 1.55 x 10(-3)/kPa (P = 0.013) and 0.017 mm(2)/kPa (P = 0.0029) lower in ADD3 AA than ADD3 GG homozygotes with an additive effect of the G allele. In multiple-gene analyses, the association of brachial diameter and distensibility with the ADD3 G allele occurred only in ADD1 GlyGly homozygotes. Otherwise, the associations between the arterial phenotypes in the three vascular beds and the ADD1 or ADD2 polymorphisms were not significant. In family-based analyses, the multivariate-adjusted heritability was 0.52, 0.38, and 0.30 for brachial diameter, distensibility, and cross-sectional compliance, respectively (P < 0.001). There was no evidence for population stratification (0.07 < or = P < or = 0.96). Transmission of the mutated ADD3 G allele was associated with smaller brachial diameter in 342 informative offspring (-0.12 +/- 0.04 mm; P = 0.0085) and in 209 offspring, who were ADD1 GlyGly homozygotes (-0.14 +/- 0.06 mm; P = 0.018).

CONCLUSIONS

In ADD1 GlyGly homozygotes, the properties of the brachial artery are related to the ADD3 (A386G) polymorphism, but the underlying mechanism needs further clarification.

The central role of the brain in salt-sensitive hypertension

PURPOSE OF REVIEW

To integrate recent studies showing that abnormal Na transport in the central nervous system plays a pivotal role in genetic models of salt-sensitive hypertension.

RECENT FINDINGS

Na transport-regulating mechanisms classically considered to reflect renal control of the blood pressure, i.e. aldosterone-mineralocorticoid receptors-epithelial sodium channels-Na/K-ATPase, have now been demonstrated to be present in the central nervous system contributing to regulation of cerebrospinal fluid [Na] by the choroid plexus and to neuronal responsiveness to cerebrospinal fluid/brain [Na]. Dysfunction of either or both can activate central nervous system pathways involving 'ouabain' and angiotensin type 1 receptor stimulation. The latter causes sympathetic hyperactivity and adrenal release of marinobufagenin - a digitalis-like inhibitor of the alpha1 Na/K-ATPase isoform - both contributing to hypertension on high salt intake. Conversely, specific central nervous system blockade of mineralocorticoid receptors or epithelial sodium channels prevents the development of hypertension on high salt intake, irrespective of the presence of a 'salt-sensitive kidney'. Variants in the coding regions of some of the genes involved in Na transport have been identified, but sodium sensitivity may be mainly determined by abnormal regulation of expression, pointing to primary abnormalities in regulation of transcription.

SUMMARY

Looking beyond the kidney is providing new insights into mechanisms contributing to salt-sensitive hypertension, which will help to dissect the genetic factors involved and to discover novel strategies to prevent and treat salt-sensitive hypertension.

Genetic variations of tubular sodium reabsorption leading to “primary” hypertension: from gene polymorphism to clinical symptoms

The definition of the most appropriate strategy to demonstrate causation of a given genetic-molecular mechanism in a complex multifactorial polygenic disease like hypertension is hampered by the underestimation of the complexity arising from the genetic and environmental interactions. To disentangle this complexity, we developed a strategy based on six steps: 1) isolation of a rodent model of hypertension (Milan hypertensive strain and Milan normotensive strain) that shares some pathophysiological abnormalities with human primary hypertension; 2) definition in the model of the sequence of events linking these abnormalities to a genetic molecular mechanism; 3) determination of the polymorphism of the three adducin genes discovered in the model both in rats and in humans; 4) comparison at biochemical and physiological levels between the rodent models and the hypertensive carriers of the “mutated” gene variants; 5) evaluation of the impact of the adducin genes in hypertension and its organ complications with association and linkage studies in humans, also considering the genetic and environmental interactions; and 6) development of a pharmacogenomic approach aimed at establishing the therapeutic benefit of a drug interfering with the sequence of events triggered by adducin and their effect's size. The bulk of data obtained demonstrates the importance of a multidisciplinary approach considering a variety of genetic and environmental interactions. Adducin functions within the cells as a heterodimer composed of a combination of three subunits. Each of these subunits is coded by genes mapping to different chromosomes. Therefore, the interaction among these genes, taken together with the interactions with other modulatory genes or with the environment, is indispensable to establish the adducin clinical impact. The hypothesis that adducin polymorphism favors the development of hypertension via an increased tubular sodium reabsorption is well supported by a series of consistent experimental and clinical data. Many mechanistic aspects, underlying the link between these genes and clinical symptoms, need to be clarified. The clinical effect size of adducin must be established also with the contribution of pharmacogenomics with a drug that selectively interferes with the sequence of events triggered by the mutated adducin.

Role of the adducin family genes in human essential hypertension

OBJECTIVE

In both humans and rats, polymorphisms of the alpha adducin (ADD1) gene are involved in renal sodium handling, essential hypertension and some of its organ complications. Adducin functions within cells as a heterodimer composed of various combinations of three subunits that are coded by three genes (ADD1, 2, 3) each located on a different chromosome.

DESIGN

These characteristics provide the biochemical basis for investigating epistatic interactions among these loci.

METHODS

We examined the three adducin gene polymorphisms and their association with ambulatory blood pressure (ABPM) and with plasma levels of renin activity (PRA), endogenous ouabain (EO), in 512 newly discovered and never-treated hypertensive patients.

RESULTS

Relative to carriers of the wild type (Gly/Gly) ADD1 gene, patients carrying the mutated Trp ADD1 allele had higher blood pressure (systolic blood pressure (SBP) 143.2 +/- 1.0 versus 140.6 +/- 0.6 mmHg P = 0.027 and diastolic blood pressure (DBP) 94.2 +/- 0.77 versus 92.3 +/- 0.5 mmHg, P = 0.03), lower PRA and EO, consistent with the hypothesis of the renal sodium retaining effect of the Trp allele. Polymorphisms in the ADD2 and ADD3 genes taken alone were not associated with these variables. However, the differences in SBP and DBP between the two ADD1 genotypes were greatest in carriers of the ADD3 G allele (around + 8 mmHg). The significance of the interaction between ADD1 and ADD3 ranged between P = 0.020 to P = 0.006 according to the genetic model applied.

CONCLUSIONS

The interaction of ADD1 and ADD3 gene variants in humans is statistically associated with variation in blood pressure, suggesting the presence of epistatic effects among these loci.

Adducin Polymorphism

Adducin is a heterodymeric cytoskeleton protein, the 3 subunits of which are encoded by genes ( ADD1 , ADD2 , ADD3 ) mapping to 3 different chromosomes. A long series of parallel studies in the Milan hypertensive rat strain model of hypertension and humans indicated that an altered adducin function may cause hypertension through an enhanced constitutive tubular sodium reabsorption. Six human linkage studies showed positive results when a DNA marker mapping to 30 kb from the ADD1 locus or single-nucleotide polymorphisms (SNPs) of 1 of the 3 adducin genes were considered either alone or in combination with each other or angiotensin-converting enzyme (ACE) D allele or salt intake. When DNA markers mapping at much larger distance from the ADD1 locus were used, negative results were found by 4 studies. Positive results were also obtained in 18 of 20 association studies that, in addition to blood pressure, investigated variables reflecting body sodium or the renin-angiotensin system. Mixed results regarded case-control studies or studies in predominantly normotensive populations that did not consider the above-mentioned variables. Four of 5 studies showed a selective beneficial effect of diuretics in carriers of the mutated ADD1. Twelve of 16 studies found that ADD1 polymorphism alone or in combination with that of ACE positively associates with stroke or coronary heart disease or renal or vascular dysfunctions. In conclusion, when context is taken into account, the impact of adducin in hypertension and its related disorders is clear.

Carbon monoxide and hypertension

The enzymatic action of heme oxygenase yields carbon monoxide, biliverdin and iron. Carbon monoxide is implicated in many physiological processes, including the regulation of vascular tissue contractility and apoptosis. By stimulating the soluble guanylyl cyclase (sGC)/cGMP pathway and activating K channels in vascular smooth muscle cells (SMCs), carbon monoxide relaxes vascular tissues under physiological conditions. Altered metabolism and functions of carbon monoxide have been linked to the pathogenesis and maintenance of hypertension. The expression and activity of heme oxygenase-1, sGC and cGMP in vascular SMCs are associated with different stages of development of hypertension in spontaneously hypertensive rats (SHRs). The importance of altered heme oxygenase-2 expression in vascular tissues in hypertension remains unclear. Increased vascular contractility, unbalanced cellular apoptosis and proliferation in the vascular wall, increased oxidative stress, and the altered interaction of carbon monoxide and nitric oxide are among the consequences of heme oxygenase/carbon monoxide system dysfunction in hypertension. Acute application of pharmacological inducers to upregulate the expression of heme oxygenase-1 or the use of gene delivery method to overexpress heme oxygenase-1 decreases blood pressure in young SHRs and other animal models of hypertension. These blood pressure-decreasing effects are annulled by metalloporphyrins. In adult SHRs, the heme oxygenase/carbon monoxide system appears to be normalized as a compensatory reaction. To date, acute manipulation of the expression of heme oxygenase-1 has not been successful in decreasing blood pressure in adult SHRs. In conclusion, abnormality of the heme oxygenase/carbon monoxide system has a critical role in the pathogenesis of hypertension, and novel therapeutic approaches should be pursued to achieve selective improvement in the function of this system in hypertension.

Effect of Add1 gene transfer on blood pressure in reciprocal congenic strains of Milan rats

Genetic variants of alpha adducin (ADD1) taken alone or in interaction with those of beta (ADD2) and gamma (ADD3) subunits have been associated with primary hypertension in humans and in Milan hypertensive (MHS) rats. In this study, we report the dissection of the individual contribution of each rat Add gene to blood pressure, by congenic substitution mapping. Congenic strains were developed by introgressing Add1, Add2, and Add3 genes (and chr14, chr4, and chr1 associated segments) of MHS in the Milan normotensive rat (MNS) genetic background (MNS.H-Add1, MNS.H-Add2, and MNS.H-Add3) and vice versa (MHS.N-Add1, MHS.N-Add2, and MHS.N-Add3). Systolic blood pressure (SBP) of MNS.H-Add1 rats was significantly higher (+10 mmHg) than that of MNS, whereas SBP of MHS.N-Add1 was significantly lower (-10 mmHg) than that of MHS. The differences account for 43% of the blood pressure differences between MHS and MNS. In contrast, SBPs of Add2 and Add3 congenic strains were not different from those of the correspondent recipient parental strain. The fine mapping of chr14 congenic segment supports the identity of blood pressure QTL with Add1 gene.

Decrease in Hypothalamic Gamma Adducin in Rat Models of Hypertension

We have previously shown that a decrease in hypothalamic gamma adducin (γ-adducin) is associated with hypertension in the spontaneously hypertensive rat (SHR). In view of many inherent issues with SHR, our objective in the present study was to provide proof of this concept with the use of 2 nongenetic rat models of hypertension. Subcutaneous angiotensin II (Ang II) infusion for 2 weeks (55 ng/kg per day) resulted in an increase in blood pressure (BP) of 18 mm Hg. This was associated with a 70% decrease in hypothalamic γ-adducin. Concomitant administration of losartan attenuated the development of hypertension and a decrease in γ-adducin. Deoxycorticosterone acetate salt-induced hypertension also caused a 70% decrease in hypothalamic γ-adducin. Finally, neuronal cultures from neonatal rat brains were incubated with 100 nmol/L Ang II for 4 hours to mimic the in vivo Ang II infusion rat model. This chronic incubation with Ang II resulted in a 60% decrease in the neuronal γ-adducin. Taken together, these observations strengthen our hypothesis that a decrease in hypothalamic γ-adducin is linked to hypertension.

Functional genomics as an emerging strategy for the investigation of central mechanisms in experimental hypertension

Centrally mediated increases in sympathetic nerve activity and attenuated arterial baroreflexes contribute to the pathogenesis of hypertension. Despite the characterization of cellular and physiological mechanisms that regulate blood pressure and alterations that contribute to hypertension, the genetic and molecular basis of this pathophysiology remains poorly understood. Strategies to identify genes that contribute to central pathophysiologic mechanisms in hypertension include integrative biochemistry and physiology as well as functional genomics. This article summarizes recent progress in applying functional genomics to elucidate the genetic basis of altered central blood pressure regulatory mechanisms in hypertension. We describe approaches others and we have undertaken to investigate gene expression profiles in hypertensive models in order to identify genes that contribute to the pathogenesis of hypertension. Finally, we provide the readers a roadmap for negotiating the route from experimental findings of gene expression profiling to translating their therapeutic potential. The combination of gene expression profiling and the phenotypic characterization of in vitro and in vivo loss or gain of function experiments for candidate genes have the potential to identify genes involved in the pathogenesis of hypertension and may present novel targets for therapy.

PI3-kinase inhibitors abolish the enhanced chronotropic effects of angiotensin II in spontaneously hypertensive rat brain neurons

Angiotensin II (Ang II), acting at Ang II type 1 receptors (AT1Rs), increases the firing rate of neurons from Wistar-Kyoto (WKY) rat brain via protein kinase C (PKC)- and calcium-calmodulin kinase II (CaMKII)-dependent mechanisms. The objectives of this study were twofold; first, to compare the Ang-II-stimulated increase in firing of neurons from WKY and spontaneous hypertensive rats (SHR) and second, to elucidate the signaling mechanisms involved. Action potentials were measured in neurons cultured from SHR and WKY rat brains using the whole cell configuration of the patch-clamp technique in the current-clamp mode. Ang II (100 nM) caused three- and sixfold increases in neuronal firing rate in WKY rat and SHR neurons, respectively; effects that were abolished by the AT1R antagonist Losartan (1 microM). Co-administration of calphostin C (10 microM, a PKC inhibitor) and KN-93 (10 microM, a CaMKII inhibitor) completely blocked this Ang II action in WKY rat neurons, while they caused only a approximately 50% attenuation in SHR neurons. The residual increase in firing rate produced by Ang II in SHR neurons was blocked by inhibitors of phosphatidylinositol 3 kinase (PI3-kinase), either LY 294002 (10 microM) or wortmannin (100 nM). These observations suggest that a PI3-kinase signaling pathway may be responsible for the enhanced chronotropic effect produced by Ang II in SHR neurons.

Brain renin-angiotensin system dysfunction in hypertension: recent advances and perspectives

This review focuses on the dysfunction of the intrinsic brain renin-angiotensin system (RAS) in the pathogenesis of hypertension. Hyperactivity of the brain RAS plays a critical role in mediating hypertension in both humans and animal models of hypertension, including the spontaneously hypertensive rat (SHR). The specific mechanisms by which increased brain RAS activity results in hypertension are not well understood but include increases in sympathetic vasomotor tone and impaired arterial baroreflex function. We discuss the contribution of endogenous angiotensin (Ang) II actions on presympathetic vasomotor rostral ventrolateral medulla neurons to enhance sympathetic activity and maintain hypertension. In addition, we discuss Ang II-induced attenuation of afferent baroreceptor feedback within the nucleus tractus solitarius and its relevance to the development of hypertension. We also outline the cellular and molecular mechanisms of Ang II signal transduction that may be critical for the initiation and establishment of hypertension. In particular, we present evidence for a phosphoinositide-3-kinase-dependent signaling pathway that appears to contribute to hypertension in the SHR, possibly via augmented Ang II-induced increases in neuronal firing rate and enhanced transcriptional noradrenaline neuromodulation. Finally, we outline future directions in utilizing our understanding of the brain RAS dysfunction in hypertension for the development of improved therapeutic intervention in hypertension.

Tissue-specific modulation of beta-adducin transcripts in Milan hypertensive rats

Genetic variants in Adducins, a family of cytoskeleton proteins (alpha, beta, and gamma) encoded by three genes, have been associated with primary hypertension in humans and in Milan hypertensive (MHS) rats. The present paper describes the identification of a rat beta 4 alternative splicing isoform differing from beta subunit for an in-frame insertion of 18 amino acids and showing a polymorphic site (R592W) between MHS and its normotensive control (MNS). Furthermore, we established a quantitative real-time PCR assay for analyzing the tissue expression of adducin gene family and determining whether any subunit transcript demonstrates altered expression during the development of MHS hypertension, especially in tissues relevant for the control of cardiovascular phenotypes (i.e., kidney, left ventricle, and large arteries). Among the three adducins only beta transcripts were modulated, in a tissue-specific manner, during the development of hypertension in MHS, compared to age-matched MNS controls. A 43% decrease in renal outer medulla was already present at the prehypertensive phase; a 70% decrease in femoral artery and 66% increase in left ventricle were observed after the development of hypertension. Surprisingly beta 4-Add, which is a minor component of total beta transcripts, is drastically reduced up to 88% in all MHS tissues. Alteration in beta-Add expression levels may account, at least in part, for the observed phenotypic changes in MHS hypertension.