Molecular cloning of an adducin-like protein: evidence of a polymorphism in the normotensive and hypertensive rats of the Milan strain

The adducin saga: pleiotropic genomic targets for precision medicine in human hypertension-vascular, renal, and cognitive diseases

Hypertension is a leading risk factor for stroke, heart disease, chronic kidney disease, vascular cognitive impairment, and Alzheimer's disease. Previous genetic studies have nominated hundreds of genes linked to hypertension, and renal and cognitive diseases. Some have been advanced as candidate genes by showing that they can alter blood pressure or renal and cerebral vascular function in knockout animals; however, final validation of the causal variants and underlying mechanisms has remained elusive. This review chronicles 40 years of work, from the initial identification of adducin (ADD) as an ACTIN-binding protein suggested to increase blood pressure in Milan hypertensive rats, to the discovery of a mutation in ADD1 as a candidate gene for hypertension in rats that were subsequently linked to hypertension in man. More recently, a recessive K572Q mutation in ADD3 was identified in Fawn-Hooded Hypertensive (FHH) and Milan Normotensive (MNS) rats that develop renal disease, which is absent in resistant strains. ADD3 dimerizes with ADD1 to form functional ADD protein. The mutation in ADD3 disrupts a critical ACTIN-binding site necessary for its interactions with actin and spectrin to regulate the cytoskeleton. Studies using Add3 KO and transgenic strains, as well as a genetic complementation study in FHH and MNS rats, confirmed that the K572Q mutation in ADD3 plays a causal role in altering the myogenic response and autoregulation of renal and cerebral blood flow, resulting in increased susceptibility to hypertension-induced renal disease and cerebral vascular and cognitive dysfunction.

Genetics of Human Primary Hypertension: Focus on Hormonal Mechanisms

Increasingly, primary hypertension is being considered a syndrome and not a disease, with the individual causes (diseases) having a common sign-an elevated blood pressure. To determine these causes, genetic tools are increasingly employed. This review identified 62 proposed genes. However, only 21 of them met our inclusion criteria: (i) primary hypertension, (ii) two or more supporting cohorts from different publications or within a single publication or one supporting cohort with a confirmatory genetically modified animal study, and (iii) 600 or more subjects in the primary cohort; when including our exclusion criteria: (i) meta-analyses or reviews, (ii) secondary and monogenic hypertension, (iii) only hypertensive complications, (iv) genes related to blood pressure but not hypertension per se, (v) nonsupporting studies more common than supporting ones, and (vi) studies that did not perform a Bonferroni or similar multiassessment correction. These 21 genes were organized in a four-tiered structure: distant phenotype (hypertension); intermediate phenotype [salt-sensitive (18) or salt-resistant (0)]; subintermediate phenotypes under salt-sensitive hypertension [normal renin (4), low renin (8), and unclassified renin (6)]; and proximate phenotypes (specific genetically driven hypertensive subgroup). Many proximate hypertensive phenotypes had a substantial endocrine component. In conclusion, primary hypertension is a syndrome; many proposed genes are likely to be false positives; and deep phenotyping will be required to determine the utility of genetics in the treatment of hypertension. However, to date, the positive genes are associated with nearly 50% of primary hypertensives, suggesting that in the near term precise, mechanistically driven treatment and prevention strategies for the specific primary hypertension subgroups are feasible.

Personalized Therapy of Hypertension: the Past and the Future

During the past 20 years, the studies on genetics or pharmacogenomics of primary hypertension provided interesting results supporting the role of genetics, but no actionable finding ready to be translated into personalized medicine. Two types of approaches have been applied: a "hypothesis-driven" approach on the candidate genes, coding for proteins involved in the biochemical machinery underlying the regulation of BP, and an "unbiased hypothesis-free" approach with GWAS, based on the randomness principles of frequentist statistics. During the past 10-15 years, the application of the latter has overtaken the application of the former leading to an enlargement of the number of previously unknown candidate loci or genes but without any actionable result for the therapy of hypertension. In the present review, we summarize the results of our hypothesis-driven approach based on studies carried out in rats with genetic hypertension and in humans with essential hypertension at the pre-hypertensive and early hypertensive stages. These studies led to the identification of mutant adducin and endogenous ouabain as candidate genetic-molecular mechanisms in both species. Rostafuroxin has been developed for its ability to selectively correct Na(+) pump abnormalities sustained by the two abovementioned mechanisms and to selectively reduce BP in rats and in humans carrying the gene variants underlying the mutant adducin and endogenous ouabain (EO) effects. A clinical trial is ongoing to substantiate these findings. Future studies should apply both the candidate gene and GWAS approaches to fully exploit the potential of genetics in optimizing the personalized therapy.

Discovery and Validation of Hypermethylated Markers for Colorectal Cancer

Colorectal carcinoma (CRC) is one of the most prevalent malignant tumors worldwide. Screening and early diagnosis are critical for the clinical management of this disease. DNA methylation changes have been regarded as promising biomarkers for CRC diagnosis. Here, we map DNA methylation profiling on CRC in six CRCs and paired normal samples using a 450 K bead array. Further analysis confirms the methylation status of candidates in two data sets from the Gene Expression Omnibus. Receiver operating characteristic (ROC) curves are calculated to determine the diagnostic performances. We identify 1549 differentially methylated regions (DMRs) showing differences in methylation between CRC and normal tissue. Two genes (ADD2 and AKR1B1), related to the DMRs, are selected for further validation. ROC curves show that the areas under the curves of ADD2 and AKR1B1 are higher than that of SEPT9, which has been clinically used as a screening biomarker of CRC. Our data suggests that aberrant DNA methylation of ADD2 and AKR1B1 could be potential screening markers of CRC.

TDP-43 regulates β-adducin (Add2) transcript stability

TDP-43 is an RNA-binding protein involved in several steps of mRNA metabolism including transcription, splicing and stability. It is also involved in ALS and FTD, neurodegenerative diseases characterized by TDP-43 nuclear depletion. We previously identified TDP-43 as a binder of the downstream element (DSE) of the β-Adducin (Add2) brain-specific polyadenylation site (A4 PAS), suggesting its involvement in pre-mRNA 3' end processing. Here, by using chimeric minigenes, we showed that TDP-43 depletion in HeLa and HEK293 cells resulted in down-regulation of both the chimeric and endogenous Add2 transcripts. Despite having confirmed TDP-43-DSE in vitro interaction, we demonstrated that the in vivo effect was not mediated by the TDP-43-DSE interaction. In fact, substitution of the Add2 DSE with viral E-SV40 and L-SV40 DSEs, which are not TDP-43 targets, still resulted in decreased Add2 mRNA levels after TDP-43 downregulation. In addition, we failed to show interaction between TDP-43 and key polyadenylation factors, such as CstF-64 and CPSF160 and excluded TDP-43 involvement in pre-mRNA cleavage and regulation of polyA tail length. These evidences allowed us to exclude the pre-hypothesized role of TDP43 in modulating 3' end processing of Add2 pre-mRNA. Finally, we showed that TDP-43 regulates Add2 gene expression levels by increasing Add2 mRNA stability. Considering that Add2 in brain participates in synapse assembly, synaptic plasticity and their stability, and its genetic inactivation in mice leads to LTP, LTD, learning and motor-coordination deficits, we hypothesize that a possible loss of Add2 function by TDP-43 depletion may contribute to ALS and FTD disease states.

Characterization of the distal polyadenylation site of the ß-adducin (Add2) pre-mRNA

Most genes have multiple polyadenylation sites (PAS), which are often selected in a tissue-specific manner, altering protein products and affecting mRNA stability, subcellular localization and/or translability. Here we studied the polyadenylation mechanisms associated to the beta-adducin gene (Add2). We have previously shown that the Add2 gene has a very tight regulation of alternative polyadenylation, using proximal PAS in erythroid tissues, and a distal one in brain. Using chimeric minigenes and cell transfections we identified the core elements responsible for polyadenylation at the distal PAS. Deletion of either the hexanucleotide motif (Hm) or the downstream element (DSE) resulted in reduction of mature mRNA levels and activation of cryptic PAS, suggesting an important role for the DSE in polyadenylation of the distal Add2 PAS. Point mutation of the UG repeats present in the DSE, located immediately after the cleavage site, resulted in a reduction of processed mRNA and in the activation of the same cryptic site. RNA-EMSA showed that this region is active in forming RNA-protein complexes. Competition experiments showed that RNA lacking the DSE was not able to compete the RNA-protein complexes, supporting the hypothesis of an essential important role for the DSE. Next, using a RNA-pull down approach we identified some of the proteins bound to the DSE. Among these proteins we found PTB, TDP-43, FBP1 and FBP2, nucleolin, RNA helicase A and vigilin. All these proteins have a role in RNA metabolism, but only PTB has a reported function in polyadenylation. Additional experiments are needed to determine the precise functional role of these proteins in Add2 polyadenylation.

Upregulation of the Renin-Angiotensin-aldosterone-ouabain system in the brain is the core mechanism in the genesis of all types of hypertension

Basic research using animal models points to a causal role of the central nervous system in essential hypertension; however, since clinical research is technically difficult to perform, this connection has not been confirmed in humans. Recently, renal nerve ablation in humans proved to continuously decrease blood pressure in resistant hypertension. Furthermore, when electrical stimulation was continuously applied to the carotid baroreceptor nerve of human adults, their blood pressure lowered. These findings promoted the concept that the central nervous system may actually be involved in the pathogenesis of essential hypertension, which is closely associated with excess sodium intake. We have demonstrated that endogenous digitalis plays a key role in hypertension associated with excess sodium intake via sympathetic activation in rats. Increased sodium concentration inside the brain activates epithelial sodium channels and the renin-angiotensin-aldosterone system in the brain. Aldosterone releases ouabain from neurons in the paraventricular nucleus in the hypothalamus. Angiotensin II and aldosterone of peripheral origin reach the brain to augment sympathetic outflow. Collectively essential hypertension associated with excess sodium intake and obesity, renovascular hypertension, and primary aldosteronism and pseudoaldosteronism all seem to have a common cause originating from the central nervous system.

Identification of a region of rat chromosome 1 that impairs the myogenic response and autoregulation of cerebral blood flow in fawn-hooded hypertensive rats

This study examined the effects of transfer of a 2.4-Mbp region of rat chromosome 1 (RNO1) from Brown Norway (BN) into fawn-hooded hypertensive (FHH) rats on autoregulation (AR) of cerebral blood flow (CBF) and the myogenic response of middle cerebral arteries (MCAs). AR of CBF was poor in FHH and FHH.1(BN) AR(-) congenic strains that excluded the critical 2.4-Mbp region. In contrast, AR was restored in FHH.1(BN) AR(+) congenic strains that included this region. The diameter of MCAs of FHH rats increased from 140 ± 14 to 157 ± 18 μm when transmural pressure was increased from 40 to 140 mmHg, but it decreased from 137 ± 5 to 94 ± 7 μm in FHH.1(BN) AR(+) congenic strains. Transient occlusion of MCAs reduced CBF by 80% in all strains. However, the hyperemic response following ischemia was significantly greater in FHH and AR(-) rats than that seen in AR(+) congenic strains (AR(-), 173 ± 11% vs. AR(+), 124 ± 5%). Infarct size and edema formation were also significantly greater in an AR(-) strain (38.6 ± 2.6 and 12.1 ± 2%) than in AR(+) congenic strains (27.6 ± 1.8 and 6.5 ± 0.9%). These results indicate that there is a gene in the 2.4-Mbp region of RNO1 that alters the development of myogenic tone in cerebral arteries. Transfer of this region from BN to FHH rats restores AR of CBF and vascular reactivity and reduces cerebral injury after transient occlusion and reperfusion of the MCA.

The central mechanism underlying hypertension: a review of the roles of sodium ions, epithelial sodium channels, the renin-angiotensin-aldosterone system, oxidative stress and endogenous digitalis in the brain

The central nervous system has a key role in regulating the circulatory system by modulating the sympathetic and parasympathetic nervous systems, pituitary hormone release, and the baroreceptor reflex. Digoxin- and ouabain-like immunoreactive materials were found >20 years ago in the hypothalamic nuclei. These factors appeared to localize to the paraventricular and supraoptic nuclei and the nerve fibers at the circumventricular organs and supposed to affect electrolyte balance and blood pressure. The turnover rate of these materials increases with increasing sodium intake. As intracerebroventricular injection of ouabain increases blood pressure via sympathetic activation, an endogenous digitalis-like factor (EDLF) was thought to regulate cardiovascular system-related functions in the brain, particularly after sodium loading. Experiments conducted mainly in rats revealed that the mechanism of action of ouabain in the brain involves sodium ions, epithelial sodium channels (ENaCs) and the renin-angiotensin-aldosterone system (RAAS), all of which are affected by sodium loading. Rats fed a high-sodium diet develop elevated sodium levels in their cerebrospinal fluid, which activates ENaCs. Activated ENaCs and/or increased intracellular sodium in neurons activate the RAAS; this releases EDLF in the brain, activating the sympathetic nervous system. The RAAS promotes oxidative stress in the brain, further activating the RAAS and augmenting sympathetic outflow. Angiotensin II and aldosterone of peripheral origin act in the brain to activate this cascade, increasing sympathetic outflow and leading to hypertension. Thus, the brain Na(+)-ENaC-RAAS-EDLF axis activates sympathetic outflow and has a crucial role in essential and secondary hypertension. This report provides an overview of the central mechanism underlying hypertension and discusses the use of antihypertensive agents.

Comparison between essential hypertension and pregnancy-induced hypertension: a genetic perspective

Essential hypertension (EH) accounts for 80-90% of hypertension, and pregnancy-induced hypertension (PIH) is responsible for hypertension during pregnancy. Both considered multifactorial disorders. While both have many features in common, conditions and causes of these diseases have yet to be clarified. Since both diseases are associated with hypertension, the genetic backgrounds may contain common features. The present study reviewed similarities and differences between women with EH and PIH, with a particular focus on their genetic backgrounds.

Brain-specific promoter and polyadenylation sites of the beta-adducin pre-mRNA generate an unusually long 3'-UTR

Adducins are a family of membrane skeleton proteins composed of α-, β- and γ-subunits that promote actin and spectrin association in erythrocytes. The α- and γ-subunits are expressed ubiquitously, while the β-subunit is found in brain and erythropoietic tissues. The brain β-adducin protein is similar in size to that of spleen, but the mRNA transcript is a brain-specific one that has not been yet characterized, having an estimated length of 8–9 kb instead of the 3–4 kb of spleen mRNA. Here, we show the molecular basis for these differences by determining the structure of the brain-specific β-adducin transcript in rats, mice and humans. We identified a brain-specific promoter in rodents that, apparently, was not conserved in humans. In addition, we present evidence that the brain-mRNAs are formed by a common mechanism consisting in the tissue-specific use of alternative polyadenylation sites generating unusually long 3′-untranslated region of up to 6.6 kb. This hypothesis is supported by the presence of highly-conserved regions flanking the brain-specific polyadenylation site that suggest the involvement of these sequences in the translational regulation, stability and/or subcellular localization of the β-adducin transcript in the brain.

Expression analysis of the human adducin gene family and evidence of ADD2 beta4 multiple splicing variants

Adducin is a cytoskeleton heterodimeric protein. Its subunits are encoded by three related genes (ADD1, ADD2, and ADD3) which show alternative spliced variants. Adducin polymorphisms are involved in blood pressure regulation in humans and rats. We have analyzed mRNA distribution of ADD gene family in human tissues and cells with Real-Time TaqMan RT-PCR. Whereas ADD1 is ubiquitously distributed, ADD3 is more expressed in kidney medulla and cortex than in fetal kidney, while in adult liver it is less abundant than in fetal liver. ADD2 beta1 and beta4 variants show the same pattern of distribution with the highest expression in brain, fetal liver, and kidney. Conventional RT-PCR identified new beta4 variants. Beta4a is characterized by an in-frame insertion of 21 nucleotides upstream exon 15 predicting a 7 amino acids longer protein with a similar C-terminus region. It is coexpressed with beta1 and beta4 in several tissues. Fetal kidney shows further beta4b, beta4c and beta4d variants containing internal exon deletions that enormously modify the predicted NH(2) and central regions. Our findings could help one to understand the functional role of adducin variants in specific tissues and cells.

alpha-Adducin Gly460Trp polymorphism is associated with low renin hypertension in younger subjects in the Ohasama study

BACKGROUND

The Gly460Trp polymorphism of the alpha-adducin gene (ADD-1 ) has been examined as a candidate gene for essential hypertension with salt sensitivity in the Caucasian population. However, we failed to detect a positive association between the Gly460Trp polymorphism of ADD-1 and hypertension in a small series of Japanese subjects.

OBJECTIVE

To examine the precise association between the Gly460Trp polymorphism of ADD-1 and blood pressure (BP), we carried out an association study using a Japanese population: the Ohasama Study.

DESIGN

Subjects (n = 1490) were recruited from participants in the Ohasama Study, which is a cohort in a rural community of northern Japan.

METHODS

DNA was extracted from the buffy coat of the participants who gave informed consent for genetic analysis, and the Gly460Trp polymorphism of ADD-1 was determined by the TaqMan polymerase chain reaction method. Various BP values (casual BP, ambulatory BP and home BP) were measured in the Ohasama study. We used the mean values of these BP measurements for analysis.

RESULTS

The frequencies of genotypes in the Ohasama population were 23% Gly/Gly, 49% Gly/Trp, and 28% Trp/Trp. In the baseline characteristics, age, sex, body mass index, frequency of diabetes and hyperlipidemia were significantly different between hypertensive or normotensive subjects. In total subjects, all BP values were not different among genotypes. In the younger subjects ( 60 years old) with low plasma renin activity (< 1.0 ng/ml per h), however, ambulatory BP and home BP were significantly higher in the subjects with the Gly/Trp or Trp/Trp genotypes of ADD-1 polymorphism than in those with the Gly/Gly genotype. In the same population, the frequency of the Gly/Trp or Trp/Trp genotypes of was significantly higher in hypertensives than in normotensives (83 versus 72%, chi1(2) = 4.04, P<0.05; odds ratio, 2.12; 95% confidence interval, 1.02-4.68).

CONCLUSIONS

These findings suggest the possibility that the Gly460Trp polymorphism of ADD-1 is associated with low renin hypertension.

Hypertension in beta-adducin-deficient mice

Polymorphic variants of the cytoskeletal protein adducin have been associated with hypertension in humans and rats. However, the direct role of this protein in modulating arterial blood pressure has never been demonstrated. To assess the effect of beta-adducin on blood pressure, a beta-adducin-deficient mouse strain (-/-) was studied and compared with wild-type controls (+/+). Aortic blood pressure was measured in nonanesthetized, freely moving animals with the use of telemetry implants. It is important to note that these mice have at least 98% of C57Bl/6 genetic background, with the only difference from wild-type animals being the beta-adducin mutation. We found statistically significant higher levels of systolic blood pressure (mm Hg) (mean+/-SE values: -/-: 126.94+/-1.14, n=5; +/+: 108.06+/-2. 34, n=6; P:</=0.0001), diastolic blood pressure (-/-: 83.54+/-1.07; +/+: 74.87+/-2.23; P:</=0.005), and pulse blood pressure (-/-: 43. 32+/-1.10; +/+: 33.19+/-1.96; P:</=0.001) in beta-adducin-deficient mice. Western blot analysis showed that as a result of the introduced genetic modification, beta-adducin was not present in heart protein extracts from -/- mice. Consequently, this deficiency produced a sharp decrease of alpha-adducin and a lesser reduction in gamma-adducin levels. However, we found neither cardiac remodeling nor modification of the heart function in these animals. This is the first report showing direct evidence that hypertension is triggered by a mutation in the adducin gene family.

Mild spherocytic hereditary elliptocytosis and altered levels of α- and γ-adducins in β-adducin-deficient mice

The membrane skeleton, a dynamic network of proteins associated with the plasma membrane, determines the shape and mechanical properties of erythrocytes. Deficiencies or defects in membrane skeletal proteins are associated with inherited disorders of erythrocyte morphology and function. Adducin is one of the proteins localized at the spectrin-actin junction of the membrane skeleton. In this work we show that deficiency of β-adducin produces an 80% decrease of -adducin and a fourfold up-regulation of γ-adducin in erythrocytes. β-Adducin or any other isoform generated by translation of abnormally spliced messenger RNAs could not be detected by our antibodies either in ghosts or in cytoplasm of −/− erythrocytes. Actin levels were diminished in mutant mice, suggesting alterations in the actin-spectrin junctional complexes due to the absence of adducin. Elliptocytes, ovalocytes, and occasionally spherocytes were found in the blood film of −/− mice. Hematological values showed an increase in reticulocyte counts and mean corpuscular hemoglobin concentration, decreased mean corpuscular volume and hematocrit, and normal erythrocyte counts that, associated to splenomegaly, indicate that the mice suffer from mild anemia with compensated hemolysis. These modifications are due to a loss of membrane surface and dehydration that result in an increase in the osmotic fragility of red blood cells. The marked alteration in osmotic fragility together with the predominant presence of elliptocytes is reminiscent of the human disorder called spherocytic hereditary elliptocytosis. Our results suggest that the amount of adducin remaining in the mutant animals (presumably γ adducin) could be functional and might account for the mild phenotype.

Mild spherocytic hereditary elliptocytosis and altered levels of α- and γ-adducins in β-adducin-deficient mice

The membrane skeleton, a dynamic network of proteins associated with the plasma membrane, determines the shape and mechanical properties of erythrocytes. Deficiencies or defects in membrane skeletal proteins are associated with inherited disorders of erythrocyte morphology and function. Adducin is one of the proteins localized at the spectrin-actin junction of the membrane skeleton. In this work we show that deficiency of β-adducin produces an 80% decrease of -adducin and a fourfold up-regulation of γ-adducin in erythrocytes. β-Adducin or any other isoform generated by translation of abnormally spliced messenger RNAs could not be detected by our antibodies either in ghosts or in cytoplasm of −/− erythrocytes. Actin levels were diminished in mutant mice, suggesting alterations in the actin-spectrin junctional complexes due to the absence of adducin. Elliptocytes, ovalocytes, and occasionally spherocytes were found in the blood film of −/− mice. Hematological values showed an increase in reticulocyte counts and mean corpuscular hemoglobin concentration, decreased mean corpuscular volume and hematocrit, and normal erythrocyte counts that, associated to splenomegaly, indicate that the mice suffer from mild anemia with compensated hemolysis. These modifications are due to a loss of membrane surface and dehydration that result in an increase in the osmotic fragility of red blood cells. The marked alteration in osmotic fragility together with the predominant presence of elliptocytes is reminiscent of the human disorder called spherocytic hereditary elliptocytosis. Our results suggest that the amount of adducin remaining in the mutant animals (presumably γ adducin) could be functional and might account for the mild phenotype.

Utilization of an 86 bp exon generates a novel adducin isoform (beta 4) lacking the MARCKS homology domain

A novel isoform of beta-adducin has been amplified and characterized from a human bone marrow cDNA library (GenBank #U43959). This isoform arises from the insertion of an 86 bp alternatively spliced and previously unrecognized exon (now termed exon 15) within codon 581 of the human red blood cell beta-adducin sequence. This results in an insertion of 28 novel amino acids. The remainder of the red cell beta-adducin mRNA is then translated in a different reading frame, adding an additional 35 novel amino acids prior to the stop codon. This new isoform, thus, replaces beta 1-adducin sequence after residue 580 with a total of 63 new amino acids. Sequences from genomic clones of the human beta-adducin gene show that this alternate exon is flanked by splice consensus sequences and is appropriately located in the genomic map between exons encoding up-stream and down-stream sequences, thus defining a new exon. The COOH-terminus of this new isoform, which we designate beta 4, lacks a 22 amino acid lysine-rich sequence common to both the human red cell alpha- and beta-adducin subunits and homologous to a highly conserved region in MARCKS, a filamentous actin-cross linking protein regulated by protein kinase C and calcium/calmodulin. beta 4-adducin preserves a previously identified calmodulin binding domain. PCR analysis indicates that this new beta-adducin isoform is expressed in fetal brain and liver, bone marrow, and NT-2 (neuroepithelial) cells, but is not detected in several other tissues. We anticipate that this new beta 4 isoform of beta-adducin will display unique and tissue-specific functional properties.

Polymorphism of gamma-adducin gene in genetic hypertension and mapping of the gene to rat chromosome 1q55

Adducin (ADD) is a heterodimeric protein involved in cellular signal transduction. A mutation in the alpha subunit affects ion transport and blood pressure in primary hypertension of Milan rats (MHS) and humans. In rats this effect is modulated by another mutation in the beta subunit. The recently described gamma subunit is a new member of the ADD family that should take the place of beta subunit in cells and tissues expressing alpha but not beta-Add. A missense mutation (Q572K) has been found in the gamma subunit of the Milan rats. Nineteen normotensive and five hypertensive inbred rat strains were genotyped for the polymorphisms in alpha, beta and gamma-Add genes. A disequilibrium was evident in the distribution of MHS-like Add genotype, being more frequent between the hypertensive than the normotensive strains (Chi-Square = 13.03, p = 0.0003). In kidney, brain, spleen, liver and heart a cDNA differing from gamma subunit by an in-frame insertion of 96 nucleotides, was found by PCR amplification and confirmed by RNase protection analysis. The rat gamma-Add gene was localized to chromosome 1q55 by fluorescence in situ hybridization.

Organization of the human beta-adducin gene (ADD2)

The intron-exon organization of the human beta-adducin gene (ADD2) has been determined from overlapping genomic clones. The gene spans over 100 kb on chromosome 2p13 and comprises 17 exons. Seven of the exons are identical in size to the corresponding exons of the alpha-adducin gene (4p16.3), suggesting gene duplication. A 275-bp fragment 5' to exon 1 demonstrates strong promoter activity in a transient transfection assay. Within 333 bp 5' of the first exon can be found several putative transcription factor-binding sites: three SP1 sites, one GATA site, three MZF1 sites, one p300 site, and one c-Ets site. Alternatively spliced exons in the 3' region are described and contain distinct coding regions, stop codons, and 3'UTR, corresponding to previously published beta-adducin cDNA sequences beta-1 and beta-2. The alternative splice sites for the smallest adducin isoform, beta-3, are alternative donor and acceptor sites within exons 7 and 12. The most recently described isoform, beta-4, includes an alternative exon (exon 15) that results in a frame shift and early termination. Intron-exon splice sites are presented for all 17 exons and conform to the consensus sequences for mammalian splice sites. These results will be useful in further analysis of tissue-specific expression of adducin isoforms and in analysis of DNA from patients with diseases mapping to this region of chromosome 2.

Adducin regulation. Definition of the calmodulin-binding domain and sites of phosphorylation by protein kinases A and C

Adducin promotes association of spectrin with actin and caps the fast growing end of actin filaments. Adducin contains N-terminal core, neck, and C-terminal tail domains, is a substrate for protein kinases A (PKA) and C (PKC), and binds to Ca2+/calmodulin. Ser-726 and Ser-713 in the C-terminal MARCKS-related domains of alpha- and beta-adducin, respectively, were identified as the major phosphorylation sites common for PKA and PKC. PKA, in addition, phosphorylated alpha-adducin at Ser-408, -436, and -481 in the neck domain. Phosphorylation by PKA, but not PKC, reduced the affinity of adducin for spectrin-F-actin complexes as well as the activity of adducin in promoting binding of spectrin to F-actin. The myristoylated alanine-rich protein kinase C substrate-related domain of beta-adducin was identified as the dominant Ca2+-dependent calmodulin-binding site. Calmodulin-binding was inhibited by phosphorylation of beta-adducin and of a MARCKS-related domain peptide by PKA and PKC. Calmodulin in turn inhibited the rate, but not the extent, of phosphorylation of beta-adducin, but not alpha-adducin, by PKA and that of each subunit by PKC. These findings suggest a complex reciprocal relationship between regulation of adducin function by calmodulin binding and phosphorylation by PKA and PKC.

Genomic organisation and chromosomal localisation of the gene encoding human beta adducin

Adducin (ADD) is a heterodimeric protein of the membrane skeleton with subunits of 103 (alpha) and 97 kDa (beta). It promotes the assembly of the spectrin-actin network. We have previously shown that one point mutation in each of the alpha and beta rat ADD-encoding genes is associated with blood pressure variation in an animal model for hypertension, the Milan hypertensive strain of rats, probably due to a change in the phosphorylation pattern. In fact, the rat mutations, Y to F for alpha and R to Q for beta, are located, respectively, in a tyrosine kinase and a protein kinase A phosphorylation site. We have now determined, for the human beta-ADD-encoding gene, its chromosomal localisation, exon-intron organisation and alternative splicing patterns. We report here that human beta-ADD is localised on chromosome 2 and we also show a characteristic 3' end alternative splicing of the beta-ADD RNA that generates two distinct beta-ADD families, namely ADD 63 and 97; both of them in turn present a very complex differential splicing pattern in the internal exons.

Characterisation and chromosomal localisation of the rat alpha- and beta-adducin-encoding genes

A polymorphism in the genes encoding alpha- and beta-adducin (ADD) was described as being associated with blood-pressure variation in a genetically hypertensive strain of rats (MHS). ADD is a cytoskeletal heterodimeric protein which may be involved in cellular signal transduction and interacts with other membrane skeleton proteins which affect ion transport across the cell membrane. The cDNA encoding the alpha subunit of rat ADD was isolated using PCR methods. The cDNA consists of about 3900 bp and encodes a protein of 735 amino acids (aa) which shows 91% aa identity with the human counterpart. In spleen and kidney, three alternative spliced exons were found by PCR amplification and confirmed by RNase protection analysis. 17 inbred rat strains were genotyped for the polymorphism in the alpha- and beta-ADD genes. Chromosomal localisation mapped rat alpha-ADD on chromosome 14 and rat beta-ADD on chromosome 4.

Spatial and sub-cellular localization of the membrane cytoskeleton-associated protein alpha-adducin in the rat brain

Studies on the identification and characterization of constituents of rat brain synaptic junctions have lead to the isolation of cDNA clones encoding segments of alpha-adducin. These and other studies suggest that adducin, a protein involved in promoting the assembly of actin and spectrin filaments at the plasma membrane, may play a role in dynamic assembly-disassembly processes underlying synaptic plasticity. In order to verify that brain alpha-adducin is indeed a constituent of synaptic structures, we have generated monoclonal antibodies against epitopes in the C-terminal region of alpha-adducin and have determined its spatial and sub-cellular distribution in postnatal day-30 rat brain. Alpha-adducin is found to be highly enriched in regions with high synapse densities of the hippocampus, corpus striatum, cerebral cortex and cerebellum. Immuno-electron microscopic analysis of peroxidase stained sections of the hippocampus and the cerebellum revealed that alpha-adducin is localized at distinct sub-cellular structures. In the CA1 and CA3 regions of the hippocampus alpha-adducin immunoreactivity is found in a distinct subset of dendrites and dendritic spines. In the molecular layer of the cerebellum, a distinct fraction of pre-synaptic terminals of parallel fiber terminals is labeled. In both cases the majority of synaptic structures does not contain adducin. Significant immunoreactivity is also detected in processes of glial cells both in the hippocampus and the cerebellum.

35H, a sequence isolated as a protein kinase C binding protein, is a novel member of the adducin family

We recently cloned a partial cDNA (35H) for a protein kinase C (PKC) binding protein from a rat kidney cDNA library and demonstrated that it is a PKC substrate in vitro (Chapline, C., Ramsay, K., Klauck, T., and Jaken, S. (1993) J. Biol. Chem. 268, 6858-6861). Additional library screening and 5' rapid amplification of cDNA ends were used to obtain the complete open reading frame. Amino acid sequence analysis, DNA sequence analysis, and Northern analysis indicate that 35H is a unique cDNA related to alpha-and beta-adducins. Antisera prepared to the 35H bacterial fusion protein recognized two polypeptides of 80 and 90 kDa on immunoblots of kidney homogenates and cultured renal proximal tubule epithelial cell extracts. The 35H-related proteins were similar to alpha- and beta-adducins in that they were preferentially recovered in the Triton X-100-insoluble (cytoskeletal, CSK) fraction of cell extracts and were predominantly localized to cell borders. Phorbol esters stimulated phosphorylation of CSK 35H proteins, thus emphasizing that sequences isolated according to PKC binding activity in vitro are also PKC substrates in vivo. The phosphorylated forms of the 35H proteins were preferentially recovered in the soluble fraction, thus demonstrating that phosphorylation regulates their CSK association and, thereby, their function in regulating cytoskeletal assemblies. We have isolated another PKC binding protein partial cDNA (clone 45) from a rat fibroblast library with substantial homology to alpha-adducin. Antisera raised against this expressed sequence recognized a protein of 120 kDa, the reported size of alpha-adducin, on immunoblots of renal proximal tubule epithelial cell extracts. A 120-kDa protein that cross-reacts with the clone 45 (alpha-adducin) antisera coprecipitated with 35H immunecomplexes, indicating that alpha-adducin associates with 35H proteins in vivo. Taken together, these results indicate that 35H is a new, widely expressed form of adducin capable of forming heterodimers with alpha-adducin. We propose naming this adducin homologue gamma-adducin.

Adducin: a physical model with implications for function in assembly of spectrin-actin complexes

Adducin binds to spectrin-actin complexes, promotes association of spectrin with actin, and is subject to regulation by calmodulin as well as protein kinases A and C. Adducin is a heteromer comprised of homologous alpha and beta-subunits with an NH2-terminal protease-resistant head domain, connected by a neck region to a COOH-terminal hydrophilic, protease-sensitive region. This study provides evidence that adducin in solution is a mixture of heterodimers and tetramers. CD spectroscopy of COOH-terminal domains of alpha- and beta-adducin bacterial recombinants provides direct evidence for an unstructured random coil configuration. Cross-linking, proteolysis, and blot-binding experiments suggest a model for the adducin tetramer in which four head domains contact one another to form a globular core with extended interacting alpha- and beta-adducin tails. The site for binding to spectrin-actin complexes on adducin was identified as the COOH-terminal tail of both the alpha- and beta-adducin subunits. The capacity of native adducin to recruit spectrin to actin filaments is similar to that of adducin tail domains. Thus, adducin tail domains alone are sufficient to interact with F-actin and a single spectrin and to recruit additional spectrin molecules to the ternary complex.

Association of the alpha-adducin locus with essential hypertension

Previous studies on genetic rat hypertension have shown that polymorphism within the alpha-adducin gene may regulate blood pressure. Adducin is a cytoskeletal protein that may be involved in cellular signal transduction and interacts with other membrane-skeleton proteins that affect ion transport across the cell membrane. There is a high homology between rat and human adducin and pathophysiological similarities between the Milan hypertensive rat strain and a subgroup of patients with essential hypertension. Thus, we designed a case-control study to test the possible association between the alpha-adducin locus and hypertension. One hundred ninety primary hypertensive patients were compared with 126 control subjects. All subjects were white and unrelated. Four multiallelic markers surrounding the alpha-adducin locus located in 4p16.3 were selected: D4S125 and D4S95 mapping at 680 and 20 kb centromeric, and D4S43 and D4S228/E24 mapping at 660 and 2500 kb telomeric. Alleles for each marker were pooled into groups. Comparisons between control subjects and hypertensive patients were carried out by testing the allele-disease association relative to the marker genotype. The maximal association occurred for D4S95 (chi 2(1) 13.33), which maps closest to alpha-adducin. These data suggest that a polymorphism within the alpha-adducin gene may affect blood pressure in humans.

Two point mutations within the adducin genes are involved in blood pressure variation

The Milan hypertensive strain of rats (MHS) develops a genetic form of renal hypertension that, when compared to its normotensive control (MNS), shows renal dysfunction similar to that of a subset of human patients with primary hypertension. MHS and MNS were shown to be homozygous by multilocus minisatellite analysis and monolocus microsatellite markers. We show here that one point mutation in each of two genes coding for the membrane skeleton protein adducin is associated with blood pressure in the Milan strain of rats. Adducin is a heterodimer formed by alpha and beta subunits that promotes the assembly of actin with spectrin. MHS and MNS differ, respectively, by the amino acids Y and F at position 316 of the alpha subunit. In the beta-adducin locus, MHS is always homozygous for R at position 529 while in MNS either R or Q occurs in that position. The R/Q heterozygotes showed lower blood pressure than any of the homozygotes. In vitro phosphorylation studies suggest that both of these amino acid substitutions occur within protein kinase recognition sites. Analysis of an F2 generation demonstrated that Y alleles segregated with a significant increment in blood pressure. This effect is modulated by the presence of the R allele of the beta subunit. Taken together, these findings strongly support a role for adducin polymorphisms in causing variation of blood pressure in the Milan strain of rats.

Relationships among alterations in renal membrane sodium transport, renin and aminopeptidase M activities in genetic hypertension

Rats of the Milan Hypertensive Strain (MHS) may be considered a useful model for understanding the genetic molecular mechanism underlying a primary form of hypertension in at least a subgroup of patients. Many differences between MHS and its normotensive control strain (MNS) were found at the organ, cellular and biochemical level. In the present investigation renal cell membrane proteins (BBMV) were analysed by two-dimensional electrophoresis and a difference between MHS and MNS was shown in a polypeptide of 32 kDa, subsequently identified as the C-terminal fragment of aminopeptidase M (APM). The activity of the enzyme was higher in MHS. Genetic relationships between this enzyme and the other biochemical cellular abnormalities of MHS, namely sodium transport in BBMV and renin activity in kidney cortex were investigated in MHS, MNS and in two inbred recombinant strains. This analysis showed that faster sodium transport, low kidney levels of renin and hypertension, but not differences in two-dimensional electrophoretic pattern and in aminopeptidase M activity, cosegregated in recombinant strains. These results are consistent with the hypothesis that the faster sodium transport can be considered a primary cellular abnormality responsible for hypertension in MHS and that the aminopeptidase difference is not involved in the cellular abnormalities.

Calmodulin-binding domain of recombinant erythrocyte beta-adducin

Adducin is a 200-kDa heterodimeric protein of the cortical cytoskeleton of mammalian erythrocytes. Analogs are also abundant in brain and several other tissues. In vitro, adducin bundles F-actin and enhances the binding of spectrin to actin. Previous studies have established that the beta subunit of adducin binds calmodulin (CaM) in a Ca(2+)-dependent fashion with intermediate affinity (approximately 200 nM) and that this activity is destroyed by proteolysis. We have confirmed the trypsin sensitivity of CaM binding by beta-adducin and the existence of a 38- to 39-kDa protease-resistant core. Calpain I digestion generates a larger core fragment (49 kDa) that is also devoid of CaM-binding activity. Use of recombinant beta-adducin peptides generated from partial cDNA clones identified strong CaM-binding activity within the protease-sensitive domain in residues 425-461: KQQKEKTRWLNTPNTYLRVNVADEVQRNMGSPRPKTT in single-letter amino acid codes. This region of the molecule is highly conserved between mouse, rat, and human and shares structural features with CaM-binding sequences in other proteins. Multiple flanking PEST sequences (sequences rich in proline, glutamic acid, serine, and threonine residues that enhance proteolytic sensitivity) may contribute to the protease sensitivity of this region. Consensus sequences for phosphorylation by cAMP-dependent kinases and by protein kinase C (or CaM-dependent kinase) are also found within or near this CaM-binding domain. Collectively, these data suggest a structural basis for the regulation of adducin by Ca(2+)-dependent CaM binding and possibly by covalent phosphorylation and calpain I-mediated proteolysis as well.

Calmodulin-binding domain of recombinant erythrocyte beta-adducin

Adducin is a 200-kDa heterodimeric protein of the cortical cytoskeleton of mammalian erythrocytes. Analogs are also abundant in brain and several other tissues. In vitro, adducin bundles F-actin and enhances the binding of spectrin to actin. Previous studies have established that the beta subunit of adducin binds calmodulin (CaM) in a Ca(2+)-dependent fashion with intermediate affinity (approximately 200 nM) and that this activity is destroyed by proteolysis. We have confirmed the trypsin sensitivity of CaM binding by beta-adducin and the existence of a 38- to 39-kDa protease-resistant core. Calpain I digestion generates a larger core fragment (49 kDa) that is also devoid of CaM-binding activity. Use of recombinant beta-adducin peptides generated from partial cDNA clones identified strong CaM-binding activity within the protease-sensitive domain in residues 425-461: KQQKEKTRWLNTPNTYLRVNVADEVQRNMGSPRPKTT in single-letter amino acid codes. This region of the molecule is highly conserved between mouse, rat, and human and shares structural features with CaM-binding sequences in other proteins. Multiple flanking PEST sequences (sequences rich in proline, glutamic acid, serine, and threonine residues that enhance proteolytic sensitivity) may contribute to the protease sensitivity of this region. Consensus sequences for phosphorylation by cAMP-dependent kinases and by protein kinase C (or CaM-dependent kinase) are also found within or near this CaM-binding domain. Collectively, these data suggest a structural basis for the regulation of adducin by Ca(2+)-dependent CaM binding and possibly by covalent phosphorylation and calpain I-mediated proteolysis as well.

Na+/K+/Cl- cotransport in resealed ghosts from erythrocytes of the Milan hypertensive rats

The erythrocytes (RBC) of the Milan hypertensive rats (MHS) have a smaller volume and faster Na+/K+/Cl- cotransport than RBC from normotensive controls (MNS). The difference in Na+/K+/Cl- cotransport is no longer present in inside-out Vesicles (IOV) of RBC membrane. To differentiate between cytoplasmic or membrane skeleton abnormalities as possible causes of these differences. Resealed ghosts (RG) were used to measure ion transport systems. The following results have been obtained: (1) RG from MHS have a smaller volume than MNS (mean +/- S.E. 20.7 +/- 0.45 vs. 22.09 +/- 0.42 fl, P < 0.05). (2) RG showed a bumetanide-sensitive Na efflux that retains the characteristics of the Na+/K+/Cl- cotransport of the original RBC: it is K(+)- and Cl(-)-sensitive and dependent on the intracellular Na+ concentration. (3) The Na+/K+/Cl- cotransport was faster in RG from MHS than in those from MNS (mean +/- S.E. 0.095 +/- 0.01 vs. 0.066 +/- 0.01 rate constant h-1, P < 0.01). These results, together with those of IOV, support the hypothesis that an abnormality in the membrane skeletal proteins may play a role in the different Na+/K+/Cl- cotransport modulation between MHS and MNS erythrocytes.