Tissue distribution and functional expression of a cDNA encoding a novel mixed lineage kinase

Hypertrophy is an adaptive response of the heart to myocardial injury or hemodynamic overload that may progress and contribute to cardiac decompensation and eventually to heart failure. The signaling pathways controlling this response in the cardiac myocyte are poorly understood. A data mining effort of a human failed heart cDNA library was undertaken in an effort to identify novel signaling molecules involved in cardiac hypertrophy. This effort identified a novel kinase (MLK7) homologous to the mixed lineage kinase family of proteins. The mixed lineage kinases are mitogen-activated protein kinase kinase kinases (MAPKKKs) which activate stress activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) and p38 kinase pathways. They contain a catalytic domain with homology to both serine/threonine and tyrosine-specific kinases and a dual leucine zipper. MLK7 is identical to leucine zipper and sterile-alpha motif protein kinase (ZAK) through the leucine zipper domain but has a completely divergent COOH-terminus and shares approximately 40% homology with the other MLKs overall. Expression of MLK7 mRNA is most abundant in skeletal muscle and heart, with expression restricted to the cardiac myocyte. The recombinant histidine tagged MLK7 expressed and purified from insect cells exhibited serine/threonine kinase activity in vitro with myelin basic protein as substrate. When expressed in cardiac myocytes, MLK7 activated SAPK/JNK1, and ERK and p38 to a lesser extent. Additionally, MLK7 altered fetal gene expression and increased protein synthesis in cardiac myocytes. These data suggest that MLK7 is a new member of the mixed lineage kinase family that modulates cardiac SAPK/JNK pathway and may play a role in cardiac hypertrophy and progression to heart failure.

Decreased p38 MAPK activity in end-stage failing human myocardium: p38 MAPK alpha is the predominant isoform expressed in human heart

Short duration exposure to cellular stresses have been shown to activate p38 mitogen-activated protein kinase (MAPK) in cultured rat ventricular cardiomyocytes and isolated perfused hearts; however, effects of chronic stress on p38 MAPK are not well understood. This study determined whether alterations in the p38 MAPK pathway occurred prior to end-stage human heart failure. The p38 MAPK alpha isoform was detectable in human cardiac tissue. However, carefully controlled analysis of protein and message in this study demonstrated an absence of the p38 MAPK beta -isoform. Low levels of message for the non-SB203580 sensitive p38 MAPK gamma and delta isoforms were also detected in both normal and failing human myocardium. Ischemic and idiopathic end-stage failing human hearts were compared to non-failing hearts for both p38 alpha MAPK protein level and total p38 MAPK activity. Western blotting techniques demonstrated no significant changes in total p38 alpha MAPK content. However, approximately 75% decreases in active/phosphorylated p38 MAPK (P<0.005) were observed in both ischemic and idiopathic failing hearts compared to non-failing hearts. In-gel kinase assays confirmed that activated p38 MAPK, detected by Western blotting, phosphorylated its potential downstream targets. When compared to non-failing hearts, approximately 46% decreases in p38 MAPK phosphorylation of mitogen-activated protein kinase-activated protein kinase-2 (MAPKAPK-2) were observed in ischemic and idiopathic failing hearts (P=0.03 and P=0.04 respectively). Active p38 MAPK was localized to sarcomeric structures in the cytosol of myocytes by confocal immunofluorescence microscopy. The correlation between decreased MAPKAPK-2 phosphorylation and loss of active p38 MAPK in failing human myocytes suggests that decreases in the activation of p38 MAPK alpha, the predominant cardiac isoform, occur prior to end-stage heart failure.

Functional analysis of a mutation occurring between the two in-frame AUG codons of human angiotensinogen

Angiotensinogen (ANG) is the specific substrate of the renin-angiotensin system, a major participant in blood pressure control. We have identified a natural mutation at the -30 amino acid position of the angiotensinogen signal peptide, in which an arginine is replaced by a proline (R-30P). Heterozygous individuals with R-30P showed a tendency to lowered plasma angiotensinogen level (1563 ng of ANG I/ml (range 1129-1941)) compared with normal individuals in the family (1892 ng of ANG I/ml (range 1603-2072)). Human angiotensinogen mRNA has two in-phase translation initiation codons (AUG) starting upstream 39 and 66 nucleotides from the cap site. R-30P occurs in a cluster of basic residues adjacent to the first AUG codon that may affect intracellular sorting of the nascent protein. Pulse-chase experiments in transiently transfected cultured cells revealed that the R-30P mutation was associated with reduced amounts of both intra- and extracellular protein. In a cell-free system, we found that two forms of native angiotensinogen were generated by alternative initiation of translation at either AUG codon. Alteration of either the first or second AUG codons abolished the synthesis of the longer and the shorter form of native angiotensinogen, respectively. Furthermore, the rate of secretion of the shorter form was lower than that of the longer form. By transplanting angiotensinogen signal peptide onto green fluorescence protein, however, we found that both forms of the signal peptide could target green fluorescence protein, normally localized in the cytoplasm, to the secretory pathway. Although the R-30P mutation may not affect intracellular sorting of angiotensinogen in a qualitative manner, it leads to a quantitative reduction in the net secretion of mature angiotensinogen through decreased translocation or increased residence time in the endoplasmic reticulum.

Genetic variants in the epithelial sodium channel in relation to aldosterone and potassium excretion and risk for hypertension

Renin and aldosterone secretion is often lower in blacks than in whites, characteristics that resemble a milder form of Liddle syndrome in which a mutation in the amiloride-sensitive epithelial sodium channel (ENaC) of the kidney results in enhanced resorption of sodium. In the present study, we looked for evidence that the intrinsic level of ENaC activity is indeed higher in blacks than in whites. In overnight urine samples collected from young people (249 white and 181 black subjects, mean age 13.4 years), the urinary aldosterone/potassium ratio, which is typically very low in Liddle syndrome, was lower in blacks than in whites: 0.421+/-0.024 (mean+/-SE) versus 0.582+/-0.016 nmol/mmol (P<0.0001). In addition, all but 1 of 5 molecular variants in ENaC were much more common in blacks than in whites. G442V in the beta-subunit, present in 16% of the blacks and in only 1 white, was associated with parameters reflective of a greater Na retention and potentially a higher ENaC activity: a lower plasma aldosterone concentration (P=0.070), a lower urinary aldosterone excretion rate (P=0.052), a higher potassium excretion rate (P=0.048), and a lower urinary aldosterone/potassium ratio (P=0.027). In a second cohort consisting of 126 black and 161 white normotensive subjects and 232 black and 188 white hypertensive subjects, betaG442V did not show a significant association with hypertension (P=0.089). On the other hand, a variant that was twice as common in whites, alphaT663A, was associated with being normotensive both in blacks (P=0.018) and in whites (P=0.034). Expression of either betaG442V or alphaT663A in Xenopus oocytes did not result in a change in basal Na current, consistent with the variants being in linkage disequilibrium with alleles at active loci. In conclusion, several lines of evidence are presented to suggest that ENaC activity is higher in blacks than in whites, which could contribute to racial differences in Na retention and the risk for hypertension.

Structure-function studies of the eighth hydrophobic domain of a serotonin receptor

The most prominent structural feature of the G protein-coupled receptor superfamily is their seven hydrophobic domains, which are postulated to form membrane-spanning alpha helices. Some members of the G protein-coupled receptor family, specifically several serotonin (5-HT) receptors, possess eight hydrophobic domains. The importance of this extra hydrophobic domain, located at the N terminus of the receptor, is unknown. This question was addressed by deleting the extra hydrophobic region from the 5-HT2C receptor and comparing its function and topology with those of the wild-type receptor. Immunofluorescence microscopy was used to determine the location of the N terminus of the epitope-tagged wild-type and mutant receptors. The N terminus of both receptors was extracellular, suggesting that the extra hydrophobic domain does not change the topology of this receptor and is unlikely to be a membrane-spanning alpha helix. Radioligand-binding studies in transfected cells and expression studies in Xenopus oocytes demonstrated that seven hydrophobic domains were sufficient for normal function in these assays. Interestingly, the mutant receptor, now containing seven hydrophobic domains, is expressed at higher levels in transfected cells than the wild-type receptor containing eight hydrophobic domains, suggesting that the extra hydrophobic domain does impact the activity of this receptor by regulating its expression.

Association of the angiotensinogen gene to serum angiotensinogen in blacks and whites

A variant of the angiotensinogen gene (AGT) that encodes for threonine at codon 235 (T235) has been associated with a higher serum angiotensinogen concentration and with hypertension in white subjects. The frequency of T235 is about two times higher in blacks than whites, suggesting that AGT may contribute to the susceptibility to hypertension in blacks more than it does in whites. However, an association of T235 with angiotensinogen level or blood pressure has not been observed in blacks, possibly because the high prevalence of T235 makes it insufficiently informative as a marker. For this reason, we undertook to further differentiate the T235 carrier state by constructing haplotypes with alleles in the 5' upstream region of AGT. One such haplotype, -1074t;T235, showed a significant association with angiotensinogen level in a cohort of black and white children and adolescents (76 blacks, mean age = 12.3 +/- 2.0 [SD] years; 139 whites, mean age = 12.4 +/- 1.8 years). With a linear regression model, the level of serum angiotensinogen was significantly related to body mass index (P = .0017) and the haplotype (P = .0001). Within specific race groups, the haplotype was significantly related to serum angiotensinogen in both the blacks (P = .0277) and whites (P = .0001). The mean level of angiotensinogen was higher in the blacks carrying a single copy of the haplotype than in those without the haplotype (1472.2 +/- 68.4 versus 1274.9 +/- 46.7 nmol angiotensin I/L), a difference that was marginally significant (P = .0609). In the whites, the level of angiotensinogen was also higher in carriers of a single copy than in those with no copy (1527.9 +/- 71.2 versus 1099.2 +/- 20.1 nmol angiotensin I/L) (P = .0003). Serum angiotensinogen level did not increase with two copies of the haplotype, but in each racial group, there were only four individuals who were homozygous. The haplotype showed a marginally significant relation (P = .0757) to the mean of longitudinally determined diastolic pressures adjusted for body mass index, race, sex, and age. In summary, using a haplotype to differentiate further the T235 carrier state, we observed an association of genotype with serum angiotensinogen level and blood pressure in blacks and whites. The findings suggest that AGT may play an important role in blood pressure regulation in both racial groups.

Racial difference in the relationship of an angiotensin I-converting enzyme gene polymorphism to serum angiotensin I-converting enzyme activity

An insertion (I)/deletion (D) polymorphism of the angiotensin I-converting enzyme (ACE) gene that has been associated with certain cardiovascular disorders accounts for nearly half the variation in serum ACE level in white subjects. Whether a similar association of serum ACE with the I/D polymorphism occurs in other racial groups is not known. We studied the I/D polymorphism of ACE in relation to serum ACE activity in 141 white and 62 black healthy, unrelated children and adolescents (mean age, 14.7 years). The mean level of ACE activity in whites homozygous for the D allele was higher than in heterozygotes (P = .002) and in homozygotes for the I allele (P = .0001), consistent with an earlier study. In blacks, on the other hand, no significant difference in serum ACE activity between genotypes was observed. An additional finding was a significantly positive relationship between serum ACE activity and diastolic pressure (P = .009). In children and adolescents, serum ACE activity is related to the ACE gene I/D polymorphism in whites but not in blacks. The results indicate a potentially important ethnic variation in genetic regulation of serum ACE activity and the relationship of the I/D polymorphism to cardiovascular disease.

Identification of a splice variant of the rat and human mineralocorticoid receptor genes

The sequence of a splice variant of the rat mineralocorticoid receptor (MR) gene is presented. A cDNA clone corresponding to rat MR was isolated from a rat brain cDNA library. Sequence analysis of the region corresponding to the DNA binding domain revealed the presence of a 12 base pair (bp) insertion. Analysis of mRNA from several rat tissues suggests that the variant is less abundant than the wild type in most tissues. The insertion variant is also a product of the human MR gene, the identical splice variant was also observed in human white blood cell mRNA. Unlike other splice variants reported for the MR, this variant alters the encoded protein by the addition of four amino acid residues in the DNA binding domain. The altered protein may influence the affinity of the MR for mineralocorticoid or glucocorticoid response elements.

The serum angiotensinogen concentration and variants of the angiotensinogen gene in white and black children

The T235 allele of the angiotensinogen gene (AGT) has been associated with hypertension. Blood pressure increases faster over time in black children than in white children, and in adults hypertension is more prevalent in blacks. We sought evidence for a role for angiotensinogen to contribute to racial differences in blood pressure in a study of 148 white and 62 black normotensive children (mean age, 14.8 yr). The frequency of the T235 allele was 0.81 in blacks and 0.42 in whites (chi 2 = 77.3, P = 0.0001). The mean angiotensinogen level was 19% higher in blacks than in whites (P = 0.0001 for males, P = 0.004 for females). Genotype was positively related to serum angiotensinogen in white children (P = 0.0001 for males, P = 0.004 for females), but a similar relationship was absent in blacks where the frequency of M235 may have been too low to discern an association. Longitudinal blood pressure (measured twice yearly) adjusted for body mass index showed a marginally significant relationship to the angiotensinogen level (P = 0.07). An independent relationship of serum angiotensinogen with body mass index (P = 0.0001) and race (P = 0.0003) was also observed. In summary, T235 was more frequent, and the level of angiotensinogen was higher in blacks than in whites. Such a racial difference in the renin-angiotensin system may contribute to the disparity in blood pressure levels of white and black young people.

Racial differences in aldosterone excretion: a longitudinal study in children

Aldosterone production, estimated from urinary excretion of aldosterone and the plasma aldosterone level, was found in a previous cross-sectional study to be lower in black children than white children. The present study examined aldosterone excretion longitudinally to determine whether the aldosterone excretion rate changed with time and if the racial difference in aldosterone excretion persisted. Urine samples were collected every 6 months for up to 5.5 yr in 351 white and 170 black children for measurements of aldosterone, sodium (Na+), and potassium (K+) excretion. Results were expressed per mumol urinary creatinine. Mean values for excretion rates for the total longitudinal period were determined. Na+ excretion was not significantly different in the two groups, whereas K+ excretion was 18% lower in blacks than whites (P = 0.0001). Body weight and urinary Na+ and K+ excretion were significantly related to aldosterone excretion. After adjusting for these variables, the aldosterone excretion rate was 35% lower in blacks than whites (P = 0.0001), a racial difference that did not change with age. Aldosterone excretion rates showed no longitudinal trend to either increase or decrease. The physiological relevance of the lower aldosterone excretion rate in black children remains unknown.

Analysis of the promoter sequence and the transcription initiation site of the mouse 5-HT1C serotonin receptor gene

The serotonin 1c (5-HT1C) receptor is found in many brain regions, but is particularly enriched on the epithelial cells of the choroid plexus. A major challenge in neurobiology is to delineate the molecular processes that regulate the specific pattern of neuronal gene expression in the brain. As an initial step towards identifying cis-acting DNA sequences that control the expression of the 5-HT1C receptor, we have isolated the promoter sequence of its gene. Sequence analysis of a 1.8 kb fragment indicated that the 3' end of this fragment overlaps with the 5' untranslated region of the 5-HT1C receptor mRNA, and primer extension using mouse brain poly(A)+ RNA mapped the transcription initiation site within this fragment. There are a number of sequence elements upstream from the transcription initiation site that are homologous to regulatory elements found in other eucaryotic genes. To determine the promoter activity, a plasmid was constructed that contains this fragment as promoter region and the cDNA for the 5-HT1C receptor as the reporter. When injected into the nucleus of Xenopus oocytes, this construct resulted in functional expression of the reporter gene. Primer extension using the RNA extracted from the injected oocytes indicated a single transcription initiation site of the reporter mRNA. These results suggest that the 5-HT1C receptor was functionally expressed under the promoter activity of the 1.8 kb 5' sequence of its gene. This system will be useful for further analysis of the cis-acting elements in the promoter region of the 5-HT1C receptor gene and the trans-acting factors that regulate tissue-specific expression of the receptor.

Use of polymerase chain reaction to screen phage libraries

Isolating a clone from a cDNA or genomic library often involves screening the library by several rounds of plating and filter hybridization. This is not only laborious and time-consuming, but also is prone to artifacts such as false positives commonly encountered in filter hybridization. These problems can be alleviated by using polymerase chain reaction (PCR) in the early rounds of screening prior to conventional filter hybridization. The advantages of PCR screening are threefold: (1) Positive clones are identified by DNA bands of correct sizes in gel, thus avoiding the confusion from false positive spots in filter hybridization; (2) it saves time, especially in initial rounds of screening; and (3) screening of multiple genes can be performed in the same PCR by using appropriate primers for these genes. After the complexity of the phage pool is reduced and the existence of true positives in the pool confirmed, individual clones can be isolated by conventional methods.

The mouse 5-HT1C receptor contains eight hydrophobic domains and is X-linked

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) exerts diverse physiological effects in the central and peripheral nervous systems and in smooth muscle by interacting with pharmacologically distinct membrane receptors. We report here the cDNA cloning of the mouse 5-HT1C receptor and its functional expression in Xenopus oocytes. This receptor possesses the unusual feature of containing eight hydrophobic domains capable of forming membrane-spanning alpha-helices, contrary to the usual '7-helix' paradigm for other membrane receptors that function through coupling to GTP-binding proteins. By hybridization analysis of Chinese hamster x mouse somatic cell hybrid lines, the gene for the receptor, designated Htr1c, has been assigned to the mouse X chromosome.

Serum stimulation of quiescent human fibroblasts induces the synthesis of tissue factor mRNA followed by the appearance of tissue factor antigen and procoagulant activity

Expression of the gene for tissue factor, the cell-surface initiator of blood coagulation, was examined by stimulating growth-arrested human fibroblasts with serum and measuring changes in the cellular content of tissue factor mRNA, antigen, and activity. Maximum tissue factor mRNA levels were reached within 1 h following serum induction and slowly declined to basal levels from 24 to 48 h after stimulation. The appearance of the tissue factor mRNA was followed by an increase in tissue factor antigen and activity. The parallel rise in antigenically positive protein and procoagulant activity was first observed about 2 h after serum stimulation with a peak at 12 h followed by a slow decline during the next 36 h. The serum-induced synthesis of the tissue factor mRNA was independent of de novo protein synthesis as demonstrated by the increased tissue factor mRNA levels generated in the presence of cycloheximide. The results of this study suggest that the synthesis of tissue factor in human fibroblasts is regulated principally at the level of transcription. In one strain of fibroblasts the activity/antigen ratio, during the period of maximum synthesis, was indistinguishable from that of tissue factor which had been immunoaffinity purified from human brain and reconstituted into phospholipid vesicles. However, during serum starvation the activity/antigen ratio in these cells was significantly reduced. Western blot analysis revealed that in serum-starved cells there was an accumulation of truncated forms of the tissue factor antigen while in the serum-stimulated cells only the full-length antigen was observed.