Chromosome Y genetic variants: impact in animal models and on human disease

Chromosome Y (chrY) variation has been associated with many complex diseases ranging from cancer to cardiovascular disorders. Functional roles of chrY genes outside of testes are suggested by the fact that they are broadly expressed in many other tissues and correspond to regulators of basic cellular functions (such as transcription, translation, and protein stability). However, the unique genetic properties of chrY (including the lack of meiotic crossover and the presence of numerous highly repetitive sequences) have made the identification of causal variants very difficult. Despite the prior lack of reliable sequences and/or data on genetic polymorphisms, earlier studies with animal chrY consomic strains have made it possible to narrow down the phenotypic contributions of chrY. Some of the evidence so far indicates that chrY gene variants associate with regulatory changes in the expression of other autosomal genes, in part via epigenetic effects. In humans, a limited number of studies have shown associations between chrY haplotypes and disease traits. However, recent sequencing efforts have made it possible to greatly increase the identification of genetic variants on chrY, which promises that future association of chrY with disease traits will be further refined. Continuing studies (both in humans and in animal models) will be critical to help explain the many sex-biased disease states in human that are contributed to not only by the classical sex steroid hormones, but also by chrY genetics.

Advancing the spontaneous hypertensive rat model of attention deficit/hyperactivity disorder

To advance the spontaneous hypertensive rat (SHR) model of attention deficit/hyperactivity disorder (ADHD), experiments examined the SHR in tasks recognized to assess functioning of the prefrontal cortex or dorsal striatal. Tasks included odor-delayed win-shift (nonspatial working and reference memory), win-stay (habit learning), and attentional set-shifting (attention and behavioral flexibility). In Experiment 1, the SHR strain was compared with Wistar-Kyoto (WKY) and Wistar-Kyoto Hypertensive (WKHT) strains on the first 2 tasks. In Experiment 2, oral methylphenidate (1.5 mg/kg) and vehicle (water) were evaluated on all 3 tasks in SHR and WKY strains. Results demonstrated that the SHR made significantly more errors in the odor-delayed win-shift, win-stay, and attentional set-shifting tasks compared with the WKY. Similar performances in the WKY and WKHT indicated that deficits observed in the SHR were not related solely to hypertension. Treating the SHR with methylphenidate eliminated strain differences in all 3 tasks. These findings provide evidence that the SHR is a valid model for studying ADHD-associated neurocognitive deficits. Moreover, the current behavioral approach is appropriate to assess novel medications developed to target ADHD-associated neurocognitive deficits.

Distinct gene-sex interactions regulate adult rat cardiomyocyte width and length independently

Wistar-Kyoto (WKY) and WKY-derived hyperactive (WKHA) rats are two genetically-related inbred strains of rats that are both normotensive yet exhibit differences in left ventricular mass (LVM). We had shown previously that cardiomyocytes from male WKHA are wider than that of male WKY, and that there was genetic linkage between LVM and a locus on chromosome 5 (RNO5) in the male progeny of a F2 WKHA/WKY cross. We show here that cardiomyocyte width is linked to the same RNO5 locus in male reciprocal congenic rats derived from WKHA and WKY. Contrary to males, we found no genetic linkage between LVM and the RNO5 locus in female rats. However, ventricular hypertrophy in females might be of a different nature, because cardiomyocytes from female WKHA were shorter than their WKY counterparts (with no difference in width). The RNO5 locus contains that of the natriuretic peptide precursor A (Nppa) gene. In male congenic rats, changes in cardiomyocyte width always correlated with reciprocal changes in the LV concentration of atrial natriuretic factor (ANF, i.e., the peptide product of Nppa). Taken together with other functional data, the small size of the RNO5 locus (approximately 63 cR) increased the likelihood that both cardiomyocyte width and LV ANF concentration could be linked to only one gene (possibly Nppa) in male rats. Moreover, our results support the notion that genes and sex interact to regulate cardiomyocyte width and length independently from one another.

Long-chain fatty acids modify hypertrophic responses of cultured primary neonatal cardiomyocytes

In vivo, the normal heart obtains at least 60% of its energy from lipids and the remainder from glucose. Several lines of evidence indicate that an increase in the utilization of glucose [at the expense of fatty acids (FA)] may play a role in the genesis of hypertrophy. Primary cultures of neonatal cardiomyocytes have been used extensively to study the phenotype of these cells as well as their responses to hormonal hypertrophic agents. Unfortunately, such cultures are most typically cultured in glucose-rich FA-free media, and thus might be hypertrophied to start with. We therefore tested the effects of FA-albumin complexes on three different surrogate end points of hypertrophy of cardiomyocytes. Oleate-albumin complexes decreased the baseline values of all three variables, and increased the relative response of these variables to administration of norepinephrine. Oleate:palmitate-albumin complexes also affected all three variables and their responses to norepinephrine, but the effects differed somewhat from that of oleate-albumin complexes. Our results suggest that addition of long-chain FA, by providing conditions that more closely resemble physiological situations, may optimize the expression of hypertrophic responses in such cells. However, the differences between the effects of oleate and oleate:palmitate also suggest that the precise composition of FA may affect the phenotype of cardiomyocytes and how these cells respond to hypertrophic agents.

Functional alterations of the Nppa promoter are linked to cardiac ventricular hypertrophy in WKY/WKHA rat crosses

Cardiac left ventricular hypertrophy (LVH) is commonly associated with hypertension, but its variance is determined for more than 50% by blood pressure-independent genetic factors. Because it constitutes one of the most important risk factors for cardiovascular mortality, we have performed a genome-wide scan of the F2 progeny of crosses between inbred WKY and WKHA rats to detect quantitative trait loci (QTL) linked to cardiac mass. In addition to left ventricular mass (LVM), we also measured left ventricle (LV) concentration of atrial natriuretic factor (ANF), because we have previously established that there was a genetic link between these 2 traits in the same animal cross. We found 2 contiguous QTL on chromosome 5 that were linked to either LVM (logarithm of odds [LOD]=3.5) or log(n) (LV ANF) (LOD=12). The 1-LOD support intervals of both QTL shared a region overlapping the locus of natriuretic peptide precursor A (NPPA:) (ie, the ANF-coding gene). We found by sequencing 2 single nucleotide polymorphisms (SNPs) within the first 650 bp of the NPPA: minimal promoters of the genes from both strains. One of these SNPs increased the transcriptional activity of the NPPA: minimal promoter in transfected neonatal cardiomyocytes in keeping with the higher LV concentration of ANF observed in WKY versus WKHA rats. Taken together with the previous reports showing that ANF may protect cardiomyocytes against hypertrophy, our genetic data single out NPPA: as a strong candidate gene for the determination of LVM.

Cosegregation analysis in genetic crosses suggests a protective role for atrial natriuretic factor against ventricular hypertrophy

In most rat models studied to date, increased ventricular mass is associated with high ventricular expression of the atrial natriuretic factor (ANF) gene. However, it is unknown whether ANF plays a beneficial or detrimental role in the course of left ventricular hypertrophy or whether ANF gene expression could be genetically linked to cardiac mass. To address such questions, we performed a cosegregation analysis in genetic crosses of inbred strains of rats. To select strains with the appropriate phenotypic characteristics, we first compared the ventricular abundance of ANF mRNA to ventricular mass (corrected for body weight) in 2 recombinant inbred strains derived from Wistar-Kyoto (WKY)/spontaneously hypertensive rat (SHR) hybrid crosses, ie, WKY-derived hyperactive (WKHA) and WKY-derived hypertensive (WKHT) rats, as well as in their parental inbred strains. In the 2 such strains that were normotensive, we observed that ventricular mass was higher in WKHA than in WKY rats, yet ventricular ANF mRNA was less abundant in WKHA than in WKY rats. Within a segregating population of F2 animals generated from a cross between WKY and WKHA genitors, the abundance of ventricular ANF mRNA and peptide correlated inversely with left ventricular mass, in contrast to the positive correlation observed with beta-myosin heavy chain mRNA. Finally, in the equally hypertensive SHR and WKHT strains, we found that ventricular mass was higher in SHR than in WKHT, yet ventricular ANF mRNA was less abundant in SHR than in WKHT. These results demonstrate for the first time that low ventricular ANF gene expression can be linked genetically to high cardiac mass independently of blood pressure and are consistent with a protective role for ANF against left ventricular hypertrophy.

High salt intake differentially regulates kidney angiotensin IV AT4 receptors in Wistar-Kyoto and spontaneously hypertensive rats

Functional angiotensin IV (Ang IV) receptors (denoted AT4) are localized to the outer stripe of the medulla in the rat kidney, and may play a critical role in salt homeostasis. The purpose of this study was to determine if AT4 receptor binding in the kidney is differently regulated in the salt-sensitive spontaneously hypertensive (SH) rat compared to Wistar Kyoto (WKY) controls. AT4 receptor binding was determined using in vitro receptor autoradiography. AT4 receptor binding in the outer stripe of the medulla was similar in WKY and SH rats maintained on a 1% salt diet. A high salt diet (8%) resulted in a statistically significant increase (28%) in AT4 receptor binding in kidneys from WKY rats. However, there was no change in AT4 receptor binding in the kidneys of SH rats fed the same diet. The present data indicate that AT4 binding sites are regulated by salt intake. In addition, regulation of this receptor may be impaired in the kidneys of SH rats, explaining in part the salt-sensitivity of this strain.

The renin-angiotensin system in hybrid NG108-15 cells. Renin gene is from mouse neuroblastoma, angiotensinogen and angiotensin-converting enzyme genes are of rat glioma origin

Angiotensin II (Ang II) increases the level of tyrosine phosphorylation of several proteins in nondifferentiated NG108-15 cells, a hybrid derived from the fusion of mouse neuroblastoma and rat glioma cells. Conversely, incubation of NG108-15 cells with an angiotensin-converting enzyme (ACE) inhibitor decreased the basal level of tyrosine phosphorylation of proteins, suggesting that locally secreted Ang II may act as an autocrine regulator. By RT-PCR, we found that nondifferentiated NG108-15 cells contained the mRNA transcript of the rat angiotensinogen, mouse renin and rat ACE genes, thus confirming that NG108-15 cells contain all the elements of a local renin-angiotensin system.

Immunolocalization of natriuretic peptide receptor B in the rat kidney

The natriuretic peptide receptor (NPR) family consists of three receptor subtypes: two transmembrane forms that contain a guanylyl cyclase intracellular domain (NPR-A and NPR-B), and one truncated form (NPR-C). Because of the lack of specific agonists and antagonists for each receptor subtype and to the difficulty to detect the presence of small quantities of NPR-B by ligand binding studies, polyclonal antibodies against a peptide whose sequence was chosen from a region of the extracellular domain of rat NPR-B that is not homologous to sequences in NPR-A and NPR-C were developed. Western blotting with affinity-purified anti-NPR-B (413-426)-Tyr revealed a polypeptide of approximately 120 kD on COS-1 cell membranes transfected with rat NPR-B cDNA. The antibody recognized a second polypeptide, approximately 5 to 10 kD smaller, which probably represents the unglycosylated receptor. Anti-NPR-B (413-426)-Tyr did not show crossreactivity to any other NPR. Western blotting analysis with anti-NPR-B (413-426)-Tyr also identified a protein of appropriate size in renal vascular membranes. These results were supported by immunohistochemistry findings that demonstrated staining for NPR-B on papillary and medullary capillaries, glomeruli, and renal arteries. This study concludes that NPR-B is present in the rat kidney, although it was only detected in vascular structures.

Evidence for intracellular generation of angiotensin II in rat juxtaglomerular cells

The formation of the vasoactive peptide angiotensin II (AII) is dependent on the sequential action of two enzymes, renin and angiotensin converting enzyme (ACE), on the substrate angiotensinogen. Although the renin-producing cells of the kidney do not express angiotensinogen, they contain large amounts of AII in the same storage granules that contain renin. When renin expression is suppressed in these cells, AII also disappears. In the current study, we have tested whether the renin-associated disappearance of AII in renal juxtaglomerular (JG) cells is due to a renin-dependent down-regulation of granule biosynthesis and whether receptor-mediated internalization of AII could account for its concentration in these cells. Our results support a model whereby AII peptides are generated within JG cells, presumably by a mechanism which involves the action of endogenous renin on internalized, exogenous angiotensinogen.

Peptide receptors on astrocytes

In recent years, it has become apparent that astrocytes (at least in vitro) harbor functional receptors to almost all possible neurotransmitters (with the potential noticeable exception of acetylcholine nicotinic receptors). Peptides are no exception, since receptors to all neuropeptides known to be produced in the CNS have been found on cultured astrocytes, and the presence of many of these has been confirmed on astrocytes in vivo. A variety of methodologies have been used to detect peptide receptors on astrocytes, as summarized in the current review. Special emphasis is also put on the possible roles that peptides may play in the regulation of astrocyte functions. These include proliferation, morphology, release of eicosanoids and arachidonic acid, induction of calcium transients and calcium waves, and control of internal pH, glucose uptake, glycogen metabolism, and gap junctional conductance. Recent data concerning the effects of natriuretic peptides on astrocytes are reviewed, and why these peptides may constitute priviledged tools to test the effects of peptides on astrocyte-neuron interactions is also discussed.

Genetic characterization of novel strains of rats derived from crosses between Wistar-Kyoto and spontaneously hypertensive rats, and comparisons with their parental strains

Two novel strains of rats have recently been generated from hybrid crosses of spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. The breeding pairs chosen to create these strains were initially selected on the basis of two mutually exclusive phenotypic traits, so that the rats called WKHT are hypertensive but not hyperactive, whereas the rats called WKHA are hyperactive but not hypertensive. These strains have been refined by inbreeding on a strict brother-sister basis for more than 25 generations. To increase usefulness of these strains, we characterized them by use of genetic methods, including DNA finger-printing and simple sequence length polymorphism (SSLP) analyses. We found that these two novel strains are more closely related genetically to either SHR or WKY rats than the degree to which the latter two strains are related to one another; heterozygosities of SSLP marker alleles were extremely rare in WKHA and WKHT, indicating that these strains can be considered as truly inbred (in contrast to WKY rats from two sources); and it was almost always possible to match the SSLP marker alleles found in WKHA and WKHT rats with similarly sized alleles in the parental SHR and WKY alleles, indicating that the WKHA and WKHT strains constitute true mixes of the genomes of SHR and WKY rats. Furthermore, immunogenetic analyses indicated that WKY and WKHT rats belonged to the RT1 l haplotype, whereas SHR and WKHA rats belonged to the k haplotype. These results extend the usefulness of WKHA and WKHT rats for further genetic and physiologic studies.

Regulation of energy metabolism by neurotransmitters in astrocytes in primary culture and in an immortalized cell line

Evidence suggests that astrocytes might play an important role in cerebral energy metabolism. A recently developed cell line, called DI TNC1, displays several characteristic features of astrocytes. Thus, we have investigated in these cells a number of parameters related to energy metabolism. First, glycogen, the major energy reserve in the brain, is present in these cells and its levels are influenced by the glucose content of the growth medium and the presence of serum. Second, several neurotransmitters including noradrenaline and vasoactive intestinal peptide (VIP) induce a glycogenolytic response. Their effect on glycogen is paralleled by a similar effect on the formation of cyclic AMP, which is presumably the second messenger involved. Third, noradrenaline stimulates glucose utilization (as reflected by 2-deoxyglucose uptake) in DI TNC1 cells, an effect which is mimicked by the second messenger arachidonate. Interestingly, two actions of neurotransmitters, which are well characterized in primary astrocytes, are absent in DI TNC1 cells. These are the noradrenaline- and VIP-induced resynthesis of glycogen and the glutamate-stimulated glycolysis. In summary, the observations reported here lend further support to the concept that astrocytes are important for the control of brain energy metabolism. In addition, DI TNC1 cells might represent an interesting preparation to help decipher some of the astrocytic functions related to energy metabolism.

Adrenal vasoactive intestinal peptide participates in neonatal corticosteroid production in the rat

Neonatal rats (3-14 days old) exhibit a period of adrenal hyporesponsiveness characterized by blunted corticosterone (B) responses to stress and reduced adrenal sensitivity to adrenocorticotropic hormone (ACTH). Several adrenomedullary peptidergic systems like vasoactive intestinal peptide (VIP) are postulated to influence cortical function. VIP is known to stimulate corticosterone secretion in vitro and to be released from the adrenal medulla following splanchnic nerve stimulation. Here, we tested whether 1) accelerated sympathetic innervation of the adrenal gland by daily L-thyroxine (T4) treatment modified the ontogeny of adrenal VIP and 2) an increase in VIP synthesis could prematurely increase adrenal sensitivity and corticosteroid output during neonatal life. Immunohistochemical VIP staining revealed a different ontogenetic pattern between adrenal regions from days 2-18 and different sensitivities to T4 treatment. Capsular staining was most abundant at all ages and increased with T4 treatment, whereas medullary staining was seen by day 18 and was not affected by T4. Throughout development, VIP receptors were detected mostly in the capsular region, but not in the adrenal cortex. Although receptor levels were not modified by T4 injections, T4 significantly enhanced VIP mRNA levels in the whole adrenal at all ages. In vivo administration of VIP (0.1-2.0 mg/kg body wt ip) to 9- to 12-day-old neonates increased pituitary ACTH, adrenal B, and aldosterone secretion significantly. Corticotropin-releasing factor immunoneutralization before VIP injection diminished VIP-induced ACTH release but still produced small but significant B and aldosterone secretion. Our results show that 1) VIP innervation of the adrenal capsule is present soon after birth and is increased by sympathetic activity whereas VIP appears only much later in the medulla and does not coincide with the onset of splanchnic innervation and 2) exogenous VIP stimulates ACTH, B, and aldosterone release during development and the effect of VIP on steroidogenic secretion is occurring through ACTH secretion, but also, at least in part, directly at the level of the adrenal gland.

Tissue targeting of angiotensin peptides

Angiotensin II (Ang II) is an octapeptide generated by the sequential proteolytic action of renin and angiotensin converting enzyme on the glycoprotein angiotensinogen. While numerous mammalian tissues have been shown to express some or all of the components of the renin-angiotensin system (RAS), the function of most of these tissue RAS remains a matter of conjecture. To test for tissue-specific functions of Ang II and as an alternative to co-expressing all the components of RAS, we have engineered a fusion protein that leads to direct Ang II release within specific tissues. The angiotensin peptide is cleaved from the fusion protein within the secretory pathway by the ubiquitous endoprotease furin and is released from the cell by constitutive secretion. Direct injection of an expression vector encoding such a fusion protein into rat cardiac ventricles results in a highly localized expression of atrial natriuretic peptide mRNA (an angiotensin responsive marker of cardiac hypertrophy), demonstrating the utility of this approach for local targeting of mature peptides to tissues in animal models.

Cyclic GMP inhibits a pharmacologically distinct Na+/H+ exchanger variant in cultured rat astrocytes via an extracellular site of action

There is growing evidence that cyclic GMP (cGMP) plays important roles in the brain. In cultured rat astrocytes, we observed that the cGMP-inducing C-type natriuretic peptide (CNP) and cGMP analogues caused a decrease in intracellular pH (pHi). To examine whether this effect was due to inhibition of an Na+/H+ exchanger (NHE), we acidified cells by replacing extracellular Na+ by choline and examined the kinetics of the pHi recovery that occurred on reintroduction of Na+ in the extracellular medium. Both CNP and amiloride analogues inhibited the Na(+)-dependent pHi recovery, even in the nominal absence of CO2/HCO3-. This indicated that CNP inhibited the activity of an exchanger that was Na(+)-dependent, HCO3(-)-independent, and sensitive to known inhibitors of NHE. However, comparison of the potencies of four distinct amiloride analogues revealed a pharmacological profile that was different from that of any other NHE characterized to date. cGMP mimicked the effect of CNP on sodium-dependent pHi recovery, but the native nucleotide was as potent as membrane-permeant analogues. Intracellularly produced cGMP was very rapidly exported out of astrocytes. Probenecid and niflumic acid slowed down the rate of cGMP egression and inhibited the effect of CNP on Na(+)-dependent recovery, but not that of extracellular cGMP. Altogether, our data indicate that cGMP inhibits a novel type of NHE in astrocytes via an extracellular site of action. If these results with primary cultures transfer to brain, this phenomenon may constitute a mechanism by which natriuretic peptides exert some of their actions in the brain, as pHi transients have been shown to modulate several important astrocytic functions.

Comparison of ANP binding and sensitivity in brains from hypertensive and normotensive rats

We compared the abundance and sensitivity of atrial natriuretic peptides (ANP) receptors in the brains of spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats and examined the effect of blood pressure on the abundance of brain ANP receptors in several other experimental rat models. Brain slices from SHR generated more guanosine 3',5'-cyclic monophosphate in response to ANP than brain slices from WKY rats. No differences were found in brain particulate guanylate cyclase activity in both strains of rats. In rat brain homogenates, we observed that ANP bound in a specific and saturable fashion to samples from WKY rats, but not in samples from SHR. In vitro receptor autoradiography revealed that ANP binding was reduced in the subfornical organ, the choroid plexus, and the paraventricular nucleus of SHR compared with WKY rat brains. Correction of hypertension in SHR or induction of hypertension in other strains did not affect ANP binding in any of these brain regions. Altogether, our data suggest that the increased sensitivity of SHR brains to the action of ANP may be a consequence of factors other than the abundance of receptors and that it is not secondary to the elevation of blood pressure.

Prorenin activation and prohormone convertases in the mouse As4.1 cell line

The precise identification of prorenin-processing enzymes has been hampered by the very low abundance of juxtaglomerular cells in the kidney. Recently, an immortalized renin-producing renal tumor cell line (As4.1) has been proposed as a model to carry out such studies. Despite the fact that they contain secretory granules, we found no evidence (on the basis of enzymatic assays of renin activity in the supernatant of the cells and of immunoprecipitations experiments) that the As4.1 cells can secrete active renin through the regulated pathway. As4.1 cells produce only renin-1, as they derive from a strain of mice expressing only one renin gene. However, stable transfection of these cells with a renin-2 expression plasmid increased the capacity of this cell line to secrete active renin in the regulated pathway. Northern blot and reverse transcriptase-polymerase chain reaction amplification (RT-PCR) assays revealed that furin, PACE4 and PC5 were the only members of the proprotein convertase (PC) family to be present in these cells. As PC5 is the only such enzyme with the demonstrated ability to process mouse prorenin 2, it may constitute a candidate enzyme for the processing of prorenin-2 in mouse juxtaglomerular cells. However, it is not likely to be involved in the processing of mouse prorenin 1.

Comparison of effect of endothelin antagonism and angiotensin-converting enzyme inhibition on blood pressure and vascular structure in spontaneously hypertensive rats treated with N omega-nitro-L-arginine methyl ester. Correlation with topography of vascular endothelin-1 gene expression

Inhibition of nitric oxide synthase by L-arginine analogues such as N omega-nitro-L-arginine methyl ester (L-NAME) in spontaneously hypertensive rats (SHR) is associated with malignant hypertension and enhanced expression of the endothelin-1 gene in some blood vessels. In this study, SHR treated chronically with L-NAME (SHR-L-NAME) were given the angiotensin I-converting enzyme inhibitor cilazapril or the endothelin-A/endothelin-B receptor antagonist bosentan for 3 weeks. Systolic pressure was lowered slightly by cilazapril (213 +/- 2 versus 229 +/- 2 mm Hg in untreated SHR-L-NAME, P < .01) but was not significantly lowered by bosentan (223 +/- 2 mm Hg). Hypertrophy of aorta and small arteries (coronary, renal, mesenteric, and femoral) was decreased by cilazapril treatment and unaffected by bosentan. Expression of the endothelin-1 gene was evaluated in SHR-L-NAME by in situ hybridization histochemistry, which showed that endothelin-1 expression was enhanced in the endothelium of aorta but not in small mesenteric arteries in these rats. The absence of enhancement of endothelin-1 gene expression in small arteries may account for the absence of increased severity of hypertrophy of small vessels in SHR-L-NAME and may be a mechanism whereby L-NAME inhibits cardiovascular growth. These results suggest that in the absence of enhanced small-artery endothelin-1 expression, endothelin antagonism does not lower blood pressure. The blood pressure-lowering effect of angiotensin I-converting enzyme inhibition suggests a role for the renin-angiotensin system in the malignant form of hypertension that develops in SHR treated with L-NAME.

Vascular structure and expression of endothelin-1 gene in L-NAME-treated spontaneously hypertensive rats

Inhibition of nitric oxide synthase by L-arginine analogues is associated with elevation of blood pressure in rats. Deoxycorticosterone acetate (DOCA)-salt hypertensive rats and DOCA-salt-treated spontaneously hypertensive rats (SHR) overexpress the endothelin-1 gene in blood vessels, and this is associated with severe vascular hypertrophy, whereas SHR do not overexpress endothelin-1 and exhibit limited vascular hypertrophy. In this study malignant hypertension was induced in SHR by chronic administration of the L-arginine analogue NG-nitro-L-arginine methyl ester (L-NAME), a potent inhibitor of nitric oxide synthase, to determine whether malignant hypertension would result in endothelin-1 gene overexpression in blood vessels and in greater severity of vascular hypertrophy, as found in malignant DOCA-salt-treated SHR. L-NAME treatment induced malignant hypertension in SHR, with a systolic blood pressure of 246 +/- 2 mm Hg, compared with 211 +/- 2 mm Hg (P < .01) in untreated SHR. Plasma renin activity was very high in L-NAME-treated SHR, and their plasma immunoreactive endothelin concentration was slightly but significantly elevated (P < .01). After 3 weeks of treatment, aortic and to a lesser degree mesenteric artery weights were significantly increased in L-NAME-treated SHR compared with untreated SHR. However, cardiac weight and the media cross-sectional area or media width-to-lumen diameter ratio of small arteries from the coronary, renal, mesenteric, or femoral vasculature were not increased in L-NAME-treated SHR in comparison with untreated SHR. The abundance of endothelin-1 mRNA measured by Northern blot analysis was significantly increased in L-NAME-treated SHR in aorta and with less magnitude in the mesenteric arterial tree. The absence of accentuation of cardiac and small artery hypertrophy in malignant hypertension in L-NAME-treated SHR, despite enhanced expression of the endothelin-1 gene in blood vessels, may suggest a direct or indirect inhibitory effect of L-NAME on cardiovascular growth, probably independent of its effects on nitric oxide synthase, counterbalanced in aorta and large mesenteric arteries by the hypertrophic effect of enhanced vascular endothelin-1 gene expression. These results also suggest a role for blood pressure and potentially for nitric oxide in the regulation of endothelin-1 gene expression in blood vessels.

Reduced affinity of iodinated forms of Tyr0 C-type natriuretic peptide for rat natriuretic peptide receptor B

Tyr(O)CNP is an analogue of C-type natriuretic peptide (CNP) with a tyrosine residue added to the NH2 terminus to allow its iodination. In the present study, the suitability of iodinated Tyr(O)CNP as a ligand was tested, and its potency was compared with that of other natural rat natriuretic peptides or structural analogues by radioligand binding experiments. Binding studies were performed on membranes of COS-1 cells transfected with expression plasmids for either rat natriuretic peptide receptor (NPR)-A, rat NPR-B, or bovine NPR-C. 125I-ANP(99-126) was used as a ligand to assess the binding characteristics of NPR-A and -C, and 125I-Tyr(O)CNP was used to study NPR-B. Binding associated to membranes of nontransfected COS cells was always < 3% of the total binding observed in membranes from cells transfected with receptor expression plasmids. Receptor densities in transfected cells ranged from 500 to 2500 fmol/mg of protein. High performance liquid chromatography and ionspray mass spectrometry analyses revealed that the reagents used in the course of iodination (lactoperoxidase, chloramine T, or N-chloromorpholine altered the structure of Tyr(O)CNP, most likely by changing the thiol of the Met17 residue into a sulfoxide. To further evaluate the usefulness of forms of iodinated Tyr(O)CNP on the cGMP responses in cells transfected with NPR-B. In conclusion, the suitability iodinated forms of Tyr(O)CNP as radioligands, we performed iodination of the peptide with cold iodine (Na-127I-). After purification by high performance liquid chromatography, three different modified peptides (i.e. Tyr(O)Met(O)17CNP, 127I-Tyr(O)Met(O)17CNP, and 127I2-Tyr(O)Met(O)17CNP) were recovered, and they were compared with CNP-22, Tyr(O)CNP, ANP(99-126), BNP-32, and des[Gin18, Ser19, Gly20, Leu21, Gly22]ANP(4-23) NH2 (c-ANP) for their ability to bind to transfected receptors. The binding affinity of Tyr(O)CNP for NPR-A and -B receptors is similar to that of CNP. However, oxidation of the Met17 residue into methionine sulfoxide reduces the affinity of the compound for NPR-B by > 10-fold, whereas the addition of one or two iodines did not further reduce its affinity. Similar results were obtained on evaluation of the ability of the oxidized form of monoiodinated Tyr(O)CNP on the cGMP responses in cells transfected with NPR-B. In conclusion, the suitability of iodinated forms of Tyr(O)CNP as radioligands for binding studies on rat NPR-B is not optimal, and the results of studies using such compounds for the detection, identification, and quantification of this receptor should be interpreted with caution.

Identification and cellular localization of protein kinase C isoforms in cultures of rat type-1 astrocytes

We have examined which isoforms of protein kinase C were present in rat brain astrocytes and found that: (1) the total of calcium-independent isoforms was greater than the total of calcium-dependent isoforms; (2) there were differences in the intracellular distribution of different isoforms; and (3) the abundance of total protein kinase C was greater in astrocytes from cortex than astrocytes from diencephalon.

Generation of cyclic guanosine monophosphate in brain slices incubated with atrial or C-type natriuretic peptides: comparison of the amplitudes and cellular distribution of the responses

Natriuretic peptides have been demonstrated to induce a variety of effects when administered into the brain. Most studies to date have tested the effects of 'atrial' natriuretic peptide (ANP), but C-type natriuretic peptide (CNP) has recently been suggested to be the predominant form of natriuretic peptides within the brain. We therefore have compared the amplitudes of the cyclic guanosine monophosphate (cGMP) responses induced by either ANP or CNP in slices form different rat brain regions. Whereas both peptides induced the generation of cGMP, CNP-evoked responses were never greater than those obtained with ANP, regardless of the brain region used or the age of the animal. In diencephalon, ANP even induced a significantly higher cGMP response than CNP. To test which cells were targets to the actions of the peptides, brain slices were incubated with fluorocitrate (a drug that selectively blocks the metabolism of glial cells). Fluorocitrate totally blocked the ANP-evoked cGMP responses in brain slices. In contrast, fluorocitrate reduced only partially the responses evoked by sodium nitroprusside (a drug that stimulates soluble guanylate cyclase, which is contained predominantly in neurons). Likewise, the cGMP response induced by CNP was only partially affected by fluorocitrate. These results indicate that: (1) CNP is not more potent than ANP in terms of its ability to generate cGMP in rat brains; (2) brain cells generating cGMP upon exposure to ANP are predominantly glial; and (3) CNP-responsive cells are partly glial, but belong at least in part to a different compartment than ANP-responsive cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The membranes of cultured rat brain astrocytes contain endothelin-converting enzyme activity

Both endothelins and their big-endothelin precursors were found capable of inducing the release of arachidonic acid from purified cultures of rat astrocytes. Their order of potency was as follows: big-endothelin-3 < big-endothelin-1 < endothelin-1 = endothelin-3. Mature endothelins induced the release of arachidonic acid in a rapid fashion. In contrast, much longer incubation times were required for big-endothelins to exert an effect, suggesting that their activity was dependent on their conversion. When big-endothelin-1 was added to the incubation medium of intact live astrocytes, it was converted into mature endothelin-1 in a time-dependent manner and the conversion was inhibited by phosphoramidon. This suggests that astrocytic endothelin-converting enzyme is (at least in part) an external membrane-bound metalloprotease. Some conversion of big-endothelin-3 into endothelin-3 also occurred. However, it was less efficient than the conversion of big-endothelin-1, which is compatible with the lower bioactivity of big-endothelin-3 vs. that of big-endothelin-1 in astrocytes.

Angiotensinogen: hormonal regulation and relative importance in the generation of angiotensin II

The production of angiotensinogen is controlled mainly by hormones that affect the concentration of its mRNA in tissues. Accordingly, hormones that act upon gene transcription play a prominent role. However, other factors may modulate the transcriptional effects of hormones, and these should be considered to appreciate the final effects of hormones on the secretion of angiotensinogen. The most important role played by hormones in the regulation of angiotensinogen may be to maintain its production in the face of rapid consumption by high levels of renin. However, elevated levels of angiotensinogen may become a risk factor in situations where the normal feedback regulation of renin does not operate normally. Finally, the synthesis of angiotensinogen in tissues may be regulated differentially than that in liver, although the exact importance of these observations is still poorly understood.

Hypertension induced by brain grafts from fetal spontaneously hypertensive rats

Hypothalami from fetal rats were grafted into the third ventricle of four strains of adult rats. Grafts from spontaneously hypertensive rats (SHR), in contrast to grafts from Wistar-Kyoto (WKY) rats, induced an elevation of systolic blood pressure and a thickening of the media of resistance arteries, along with corresponding alterations in the contractile properties of these vessels. However, no cardiac hypertrophy was observed. The resistance arteries of rats grafted with hypothalamic from SHR also displayed functional alterations that were similar to what is typically found in the resistance arteries of young prehypertensive SHR, ie, an increase in the sensitivity to cocaine and an impairment in the ability to relax in the presence of acetylcholine. This suggests that the brain may play a causal role in these alterations. Histological examination of sections of brains grafted with previously labeled tissue revealed that (1) there was no brain area that was systematically infiltrated by grafts from SHR and not by grafts from WKY rats; (2) the volume of the transplants appeared larger 2 weeks after the graft than the volume of the tissue originally implanted; and (3) grafts from SHR were slightly larger, displayed more individual foci, and extended farther along the anteroposterior axis than grafts from WKY rats. In addition, glial cultures derived from the hypothalami of SHR had a higher in vitro growth rate than equivalent cultures from WKY rats. It is therefore possible that the ability of brain grafts from SHR to induce hypertension is related to a higher proliferative and/or migratory potential of nonneuronal cells within the hypothalamus.

Effects of C-type natriuretic peptide on rat astrocytes: regional differences and characterization of receptors

We have compared the effects of atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) on the accumulation of cyclic GMP (cGMP) in secondary cultures of rat astrocytes. The order of potency of these peptides was CNP > ANP > BNP, which would be compatible with a predominance of guanylate cyclase B (GC-B)- versus guanylate cyclase A (GC-A)-type receptors in these cells. Accordingly, we found by northern blot analysis that the mRNA transcripts of GC-B were much more abundant in astrocytes than the transcripts of GC-A. In addition, astrocytes from diencephalon accumulated two times more cGMP in response to CNP than astrocytes from cortex. Binding experiments with 125I-labeled ANP or [Tyro]-CNP established that these ligands recognized only clearance-type receptors on astrocytes. However, the number of binding sites was approximately 100 times higher in astrocytes from cortex than in astrocytes from diencephalon and thus was inversely correlated to the amplitude of the cGMP response in the same cells. We found no further evidence for differences in the levels of GC-B receptors in astrocytes from the two regions because (a) the abundance of GC-B mRNA was similar and (b) there was no difference in particulate guanylate cyclase activity in astrocytes from each region. In addition, occupancy of clearance receptors with C-ANP4-23 did not affect the accumulation of cGMP in response to CNP; this makes it unlikely that the differences in cGMP responsiveness can be accounted for by binding and sequestration of CNP to the clearance receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunocytochemical and functional characterization of an immortalized type 1 astrocytic cell line

The DI TNC1 cell line has been derived from cultures of rat brain astrocytes by targeted oncogenesis. Cultured astrocytes are known to promote neurite outgrowth via the production of adhesion molecules found either on the cell surface or in the extracellular matrix. We sought to investigate whether DI TNC1 cells retained the ability to produce such neurite-inducing molecules, and promote neurite growth. We found by immunofluorescence that DI TNC1 cells expressed laminin, N-CAM and 1A1. The latter is a cell adhesion molecule that is expressed exclusively on astrocytes of the type 1 lineage. In vitro neurite outgrowth assays were also used to assess the functional properties of these cells. Monolayers of DI TNC1 cells were almost as effective a substrate as monolayers of astrocytes purified from the neonatal rat brain in their ability to support neurite outgrowth. In addition, PC12 cells grown on extracellular matrix derived from either DI TNC1 cells or neonatal astrocytes displayed significantly more neurite growth than cells plated on plastic. This effect was partially inhibited by preincubation of the extracellular matrix with anti-laminin antibodies. Taken together, these results suggest that the immortalized DI TNC1 cells show many similarities to neonatal astrocytes. Given the heterogeneity of cultured astrocytes, this homogeneous cell line may prove to be particularly useful for future investigations on interactions between glia and neurons.

Neurosteroid biosynthesis: genes for adrenal steroidogenic enzymes are expressed in the brain

To determine if neurosteroids (steroids synthesized in the brain) are produced by enzymes found in steroidogenic tissues, we determined if mRNA for five steroidogenic enzymes could be detected in brain tissues or cultured cells. We detected mRNAs for adrenodoxin, P450scc (cholesterol side-chain cleavage enzyme) and P450c11 beta (11 beta-hydroxylase) but not for P450c17 (17 alpha-hydroxylase/17,20 lyase) or P450c11AS (aldosterone synthase) in rat brains and cultures of rat glial cells. P450scc mRNA abundance in brain or primary glial cultures was approximately 0.01% of that found in the adrenal, but more P450scc mRNA was detected in C6 glial cells. Both P450scc and P450c11 beta mRNAs were most abundant in the cortex, but there were region-specific differences for both mRNAs, and sex-specific differences for P450c11 beta mRNA. P450scc mRNA was equally abundant in mixed glial cultures containing both astrocytes and oligodendrocytes as in astrocyte-enriched cultures, and P450scc immunoreactivity co-localized with GFAP immunoreactivity in cultured astrocytes. P450c11 beta mRNA was not detected in the mixed primary glial cultures for the C6 glioma cell line that synthesize P450scc mRNA, suggesting that glial cells do not synthesize P450c11 beta mRNA. Thus some of the same enzymes involved in steroidogenesis in classic endocrine tissues are found in a cell-specific and region-specific fashion in the brain. Neurosteroids may be derivatives of known classic steroids, and/or may function through non-classic steroid hormone receptors, such as GABAA, N-methyl-D-aspartate, and corticosterone receptors.

Regulation of the angiotensinogen gene by estrogens in rat liver and different brain regions

Estrogens regulate the production of angiotensinogen (AOG) in liver and other tissues. However, evidence suggests that the effect is tissue specific and that it may depend on a variety of factors. We have tested the effects of ethynylestradiol (EE) on liver AOG mRNA and plasma AOG in intact male and ovariectomized female rats, as well as in hypophysectomized male rats. EE stimulated both variables to a comparable extent and in a dose-dependent manner. However, its effect on plasma AOG was significantly higher in female than in male rats. In addition, there was no response in hypophysectomized male rats; however, responsiveness was restored by pretreatment of the animals with prolactin. AOG mRNA concentration was also affected by EE in brain, but there were striking time- and region-specific differences. These results indicate that cell-specific factors also modulate the response of the AOG gene to EE in extrahepatic tissues.

Characterization of "plasma proteins" secreted by cultured rat macroglial cells

The brain is isolated behind a blood-tissue barrier that restricts the access of circulating proteins to neural cells. There is evidence that some of these proteins are synthesized within the central nervous system. The present study examines the synthesis and secretion of such proteins by cultured macroglial cells. Primary glial cultures were derived from cortical and subcortical regions of neonatal rat brains, and subsequent secondary cultures were enriched in type-1 astrocytes, type-2 astrocytes, or oligodendrocytes. Newly synthesized proteins were immunoprecipitated from the culture media using antisera directed against whole rat serum. All three types of glial cells secreted a range of plasma proteins. In general, type-1 astrocytes secreted more of these proteins than did type-2 astrocytes or oligodendrocytes, although the one-dimensional polyacrylamide gel electrophoresis (PAGE) profiles were specific for each cell type. Anti-sera directed against specific plasma proteins identified three of the most abundant proteins secreted by type-1 astrocytes as transferrin, alpha-2-macroglobulin, and ceruloplasmin. Northern blot analysis of cellular RNA confirmed that type-1 astrocytes contained transferrin mRNA, and that it was more abundant in cultures derived from subcortical regions than from cortical regions. In situ hybridization studies revealed that virtually all type-1 and type-2 astrocytes contained transferrin mRNA. Since the proteins identified in this study have been proposed to have a variety of neurotrophic roles in the central nervous system, these data further extend the range of possible functions that glial cells may serve in the CNS.

The stimulation of liver angiotensinogen by glucocorticoids depends on the type of steroid and its mode of administration

Glucocorticoids are well known inducers of transcription of the liver angiotensinogen (AOG) gene. However, the doses and the conditions under which they exert this effect in vivo are not known. To investigate this question, we have implanted rats with wax pellets containing 80% corticosterone (B). These pellets increased plasma B and induced clear signs of hypercorticism. However, they did not stimulate plasma AOG, whereas acute injections of dexamethasone (DEX) had a robust effect. In additional experiments, we have determined that: 1) chronic exposure to DEX was less effective than acute DEX in stimulating the production of liver AOG in rats and AOG secretion by rat hepatoma cells; 2) at maximally effective doses, B stimulated the production of AOG by hepatoma cells less effectively than DEX; and 3) DEX had less effect on AOG secretion than on AOG messenger RNA concentration, both in vivo and in vitro. All three mechanisms may have contributed concomitantly to the absence of response of plasma AOG to mild and chronic elevations of plasma B. These results suggest that glucocorticoids are unlikely to be primary regulators of liver AOG.

Directed establishment of rat brain cell lines with the phenotypic characteristics of type 1 astrocytes

Interest in obtaining cell lines for use in studies on the development and biochemistry of the central nervous system has motivated efforts to establish cells from primary brain cultures by the use of oncogene-transfer techniques. In previous reports, cell lines derived from astrocytes in this way have had immature or abnormal phenotypes. We have explored the possibility of specifically "targeting" expression of exogenous oncogenes to differentiated astrocytes by using the promoter of the gene encoding glial fibrillary acidic protein, which is expressed almost exclusively in such cells. We report here that cell lines displaying the phenotypic characteristics of type 1 astrocytes can be established reproducibly in this manner. Given the heterogeneity of primary cultures, the availability of clonal cell lines displaying characteristics of type 1 astrocytes should greatly facilitate our understanding of the biology of these cells.

Characterization and distribution of angiotensin II binding sites in fetal and neonatal astrocytes from different rat brain regions

Although angiotensin II (Ang II) binding sites have been extensively investigated in brain, revealing the presence of both AT1 and AT2 subtypes in various areas, the question as to which cells express AT1 and AT2 sites is still open. We report here that primary cultures of astrocytes obtained from various brain regions of fetal (F17) and one-day-old rats express Ang II binding sites belonging only to the AT1 subtype. The binding sites have the same binding profile in all regions tested; however, much less binding was observed in membranes of astrocytes derived from cortical than from subcortical regions and almost none were found in neonatal cortex. In addition, the dispersion method used at the onset of culture affects the number of binding sites present at the end of the culture period.

Atrial natriuretic peptide synthesis in atrial tumors of transgenic mice

Transgenic mice harboring a chimeric gene linking mouse protamine 1 5'-flanking sequence to the coding sequence of the simian virus 40 T-antigen develop spontaneous rhabdomyosarcomas of the right atria. The presence of the tumors is accompanied by dramatic elevations in plasma atrial natriuretic peptide (ANP) immunoreactivity (1,698 +/- 993 vs. 60 +/- 18 fmol/ml for controls) and hematocrit (56 +/- 8 vs. 51 +/- 2 for controls). The immunoreactive ANP (irANP) present in the tumors is similar in size to irANP found in normal mouse atria. ANP mRNA transcripts present in the tumors also appear to be very similar in overall size and 5'-termini to those produced in normal cardiac tissue. Microscopically, the tumors are composed of a disorganized array of densely packed abnormal-appearing cells. Immunocytochemistry and in situ hybridization analysis reveal considerable heterogeneity in ANP gene expression. ANP peptide and mRNA are detectable throughout the parenchyma of the tumors, but absolute levels of expression vary widely among different cells in the population. These tumors represent a potentially valuable model for the study of inappropriate ANP secretion and may provide a tissue source for the development of an ANP-producing atrial cell line.

Effects of thyroid hormones on angiotensinogen gene expression in rat liver, brain, and cultured cells

We examined the effect of chronic hypo- and hyper- thyroidism on angiotensinogen (AOG) gene expression in rat liver and brain. Chronic hypothyroidism resulted in approximately a 50% decrease in plasma AOG and AOG messenger RNA (mRNA) concentrations in liver, diencephalon, and brain stem. In contrast, plasma AOG and liver AOG mRNA concentrations were elevated by about 75% during hyperthyroidism, but no change was seen in diencephalon and brain stem. In vitro, the effect of T3 on AOG secretion by rat hepatoma cell lines H35 and H4IIEC-3 depended on the type of cell line used and on the growth status of the cells. At confluency, H35 cells were more responsive to T3 than H4IIEC-3 cells. In addition, subconfluent H35 cells were less responsive to T3 than confluent ones, although no difference was observed in the number of nuclear T3 binding sites or in the responsiveness to dexamethasone. T3 also increased AOG mRNA concentration in confluent H35 cells. Finally, AOG secretion by primary cultures of rat astrocytes increased approximately 1.8-fold following exposure to T3. The fact that T3 increased the production of AOG by these various types of cell culture in vitro suggests that it acted directly upon these cells, and that the effect of thyroid hormone was not dependent on the prior stimulation of another hormone. However, the difference in responsiveness between confluent and subconfluent H35 cells indicates that the action of thyroid hormones may be dependent on the induction of secondary genes within these cells.

Experimental basis for increasing the therapeutic index of cis-diamminedicarboxylatocyclobutaneplatinum(II) in brain tumor therapy by a high-zinc diet

Metallothionein (MT), a ubiquitous intracellular protein, confers resistance to the toxic effects of platinum compounds. Since a high-zinc diet has been shown to induce MT synthesis in extracerebral tissues but not in brain, we investigated whether it could provide an experimental basis for decreasing the hematotoxicity of carboplatin without impairing its activity against brain tumors. After 2 weeks on either a high-zinc diet or a control diet (zinc content, 180 vs 10 ppm), mice and rats received various doses of carboplatin or Hanks' balanced salt solution by i.p. injection. The hematotoxicity of carboplatin was evaluated with an assay of colony-forming units of granulocytes and mononuclear cells in mice. The high-zinc diet enabled a 50% increase in the carboplatin dose without increasing hematotoxicity. The antitumor activity was evaluated with an assay of the colony-forming efficiency of gliosarcoma cells from 9L brain tumors in rats. The high-zinc diet did not alter the efficacy of carboplatin against this brain tumor. Northern blot analysis confirmed that the high-zinc diet induced MT mRNA in the kidney but not in the brain of mice and rats; it also showed MT mRNA induction in bone marrow cells of mice but not in rat 9L brain tumors. These results suggest that increasing the dietary intake of zinc might increase the therapeutic index of carboplatin in the treatment of brain tumors.

Production of thrombin and antithrombin III by brain and astroglial cell cultures

There is increasing evidence that some proteases and protease inhibitors are produced within the central nervous system. It has been proposed that the balance between these two classes of proteins may be an important modulator of brain cell growth and differentiation. Here we report that antithrombin III (ATIII) is produced in brain and primary astroglial cultures. In addition, we show that human astroglial cultures contain prothrombin mRNA, and secrete a thrombin-like protein that makes complexes with antithrombin III.

Distribution of angiotensinogen immunoreactivity in rat anterior pituitary glands

Angiotensin II (AII) has been previously shown to be localized in the gonadotropes of the rat anterior pituitary gland. Renin and angiotensin-converting enzyme, two enzymes that participate in the generation of AII, also have been shown to be present in gonadotropes. To determine whether angiotensinogen, the precursor to AII, is present in the same cells, we have stained rat anterior pituitary sections with an antirat angiotensinogen antiserum. Angiotensinogen staining was observed in cells that had a distinctive distribution at the periphery of the gland; the number of these cells and the intensity of the staining were increased in the pituitaries of rats that had been nephrectomized 24 hr before sacrifice. When double staining was performed, we never observed colocalization of angiotensinogen with any of the known pituitary hormones or with S100 protein. The results show that in the rat anterior pituitary gland, angiotensinogen is present, at least for the most part, in cells that are different from those containing renin, angiotensin-converting enzyme, and AII.

The renin-angiotensin system in the pituitary gland

There is evidence that angiotensin II, in addition to being generated in the circulating blood, is synthesized in the anterior pituitary lobe and other endocrine tissues. Angiotensin II produced locally may act on pituitary cell receptors to modulate or mediate the action of other hormonal factors. However, tissue angiotensins may be synthesized by a different mechanism than most other neuroendocrine peptides. A precise understanding of the mode of formation of local angiotensin II is necessary for the comprehension of its physiologic role in the pituitary gland.

Production and differential endocrine regulation of atrial natriuretic peptide in neuron-enriched primary cultures

To identify factors that directly regulate the synthesis and secretion of atrial natriuretic peptide (ANP) in neuronal cells, we have developed a neuron-enriched primary culture system from fetal rat brains. A number of factors proved of importance in maintaining adequate levels of ANP secretion in such cultures: 1) cultures derived from diencephalon produced much more ANP than cultures derived from diencephalon produced with the distribution of ANP-containing cells in the rat brain; 2) brains from rats at gestational day 17 proved a better source of ANP-secreting cells than brains from rats at gestational day 16; 3) the presence of serum was required in the latter stages of the culture period to allow expression of the ANP gene; and 4) the cultures secreted more ANP when maintained at 39 C vs. 37 C. ANP mRNA transcripts in the neuron-enriched primary cultures were analyzed by S1 nuclease protection and shown to have a transcription start site similar to that employed by rat atrium and fetal hypothalamus in vivo. Dexamethasone and T3, in contrast to their stimulatory effect on ANP production in cardiocyte cultures, suppressed both the release of immunoreactive ANP and the levels of ANP mRNA in the neuron-enriched primary cultures. The cultures incorporated [35S]cysteine into immunoprecipitable ANP. HPLC analysis of 35S-labeled products in the medium revealed that, unlike neonatal cardiocyte cultures, the majority of secreted immunoreactive ANP migrated with the processed form(s) of ANP rather than the prohormone.

Rat angiotensinogen is secreted only constitutively when transfected into AtT-20 cells

To test whether angiotensinogen might be targeted to dense core secretory granules in cells containing a regulated secretory pathway, we expressed rat angiotensinogen in AtT-20 cells, a mouse pituitary cell line that has the demonstrated ability to correctly sort proteins to the constitutive or regulated pathway. We compared the pattern of secretion of angiotensinogen with that of endogenous adrenocorticotropin hormone, which is secreted by AtT-20 cells through the regulated pathway. When cells were incubated for 5 hours with dibutyryladenosine cyclic monophosphate or KCl, adrenocorticotropin hormone secretion was significantly higher than control, whereas monensin had no effect. In contrast, angiotensinogen secretion was markedly reduced by monensin, but no stimulation was observed with dibutyryladenosine cyclic monophosphate or KCl. These results make it unlikely that angiotensinogen could be cotargeted with active renin in the dense core granules of the regulated pathway. Alternative mechanisms must explain how angiotensin II is synthesized locally by tissue renin-angiotensin systems.

Glucocorticoid regulation of rat diencephalon angiotensinogen production

To investigate whether glucocorticoids can stimulate rat brain angiotensinogen production directly, we have studied the effect of dexamethasone on angiotensinogen secretion and angiotensinogen mRNA concentration in primary astroglial cultures from rat diencephalon. Dexamethasone stimulated angiotensinogen secretion by astroglial cells in a dose-related fashion. The half-maximally effective concentration was 11 nM, and the effect was blocked by RU 486, an antagonist of type II glucocorticoid receptors. This was similar to what was observed in rat hepatoma H4IIEC cells, where the half-maximally effective concentration of dexamethasone on angiotensinogen secretion was 10 nM. At maximal concentrations, dexamethasone increased angiotensinogen secretion and angiotensinogen mRNA concentration 2-fold in astroglial cells. In the hepatoma cells, however, the increase in angiotensinogen secretion was 5-fold. The in vivo diencephalon angiotensinogen mRNA concentration was decreased after adrenalectomy. Dexamethasone restored those levels to normal and induced a modest increase when the animals were killed 6 h after drug administration. In contrast, dexamethasone induced a robust increase in liver angiotensinogen mRNA concentration in the same animals. These results indicate that glucocorticoids increase angiotensinogen production through a direct receptor-mediated mechanism in both liver and brain. However, the angiotensinogen gene appears much more responsive to the action of glucocorticoids in liver than in brain.

Stage-specific expression of the lactate dehydrogenase-X gene in adult and developing mouse testes

Lactate dehydrogenase-X (LDH-X), a glycolytic enzyme found only in mammalian testes and spermatozoa, is encoded by a single gene (Ldh-x) in the mouse haploid genome. Several studies have demonstrated that LDH-X is associated with germ cells at specific stages of development. We have examined the expression of the Ldh-x gene during mouse spermatogenesis and testis maturation using in situ mRNA hybridization and immunocytochemistry. The results showed that transcription and translation of the Ldh-x gene are initiated at the pachytene stage of germ cell differentiation. However, although the amount of LDH-X protein increased as the germ cells progressed to maturation, its mRNA level was greatly decreased. These observations were confirmed by Northern analysis of total RNA derived from fractionated spermatogenic cells and developing testes. Furthermore, Northern studies also indicated two sizes of Ldh-x transcripts among different populations of spermatogenic cells in mature mouse testis.

Angiotensinogen production by rat astroglial cells in vitro and in vivo

To investigate the production of angiotensinogen by the brain, primary cultures were prepared from the brains of one-day-old rats. Two to four weeks after plating, they were transferred to serum-free medium. The cultures, which contained approximately 15% neurons, 80% astroglia and 5% other types of cells, produced angiotensinogen at a steady rate for three to four days in serum-free medium. Cultures prepared from subcortical tissue produced more angiotensinogen than cultures prepared from cerebral cortical tissue. Angiotensinogen mRNA was also identified in those cultures. Forskolin treatment had no effect on angiotensinogen production. Astroglia-enriched cultures that contained no identifiable neurons also produced angiotensinogen and its mRNA. Astroglial cells from hypothalamus and thalamus produced more of both than astroglial cells from the cerebral cortex. In situ hybridization histochemistry on sections of the hypothalamus of adult male rats showed a diffuse distribution of cells containing angiotensinogen mRNA that was more consistent with a glial than a neuronal distribution. The data indicate that most if not all of the angiotensinogen in rat brain is produced by astrocytes.

Extracellular calcium regulates expression of the gene for atrial natriuretic factor

Primary cultures of neonatal rat cardiocytes were exposed for 24 hours to culture media containing 0-2.0 mM extracellular calcium. Both atrial natriuretic factor (ANF) messenger RNA (mRNA) and ANF secretion were increased approximately threefold in the presence of 2.0 mM CaCl2 (vs. Ca2(+)-free medium). When cardiocytes were treated with the calcium channel blockers diltiazem, nifedipine, or verapamil, both ANF synthesis and secretion fell to 25-40% of control values. The choice of transcription start site on the ANF gene was not altered by the calcium channel blockers. When exogenous calcium was added to cardiocytes treated with verapamil, secretion of ANF was partially restored to control levels. High-performance liquid chromatographic analysis of medium from cardiocytes exposed to varying extracellular calcium concentrations or treated with calcium channel blockers for 24 hours revealed that the majority of secreted immunoreactivity (60-70%) migrated with pro-ANF (17 kDa) and that none of the various experimental manipulations significantly changed the ratio of pro-ANF to ANF in the media. These results were confirmed by immunoprecipitation analysis of the culture medium from the individual treatment groups. Treatment of cardiocytes for 24 hours with either the calcium ionophore A23187 or the phorbol ester 12-O-tetradecanoylphorbol 13-acetate increased ANF secretion. The combined use of these agents resulted in stimulation of both ANF secretion and ANF mRNA accumulation.

Expression of the atrial natriuretic peptide gene in human fetal heart

Expression of the human atrial natriuretic peptide (hANP) gene was examined in tissues of 19 to 28-week-old human fetuses. As expected the fetal atria express the hANP gene at a high level, accruing substantial quantities of ANP immunoreactivity and ANP mRNA. The neonatal ventricle also expresses the hANP gene at a significant level. ANP mRNA levels in the ventricle were, on the average, about 20% of those in the right atria. ANP immunoreactivity in ventricle was less that 5% of that in the right atria, suggesting important differences in the way these tissues synthesize and store the ANP peptide. Much lower levels of hANP transcripts were also detected in the lung and aortic arch. Analysis of the 5' termini of cardiac hANP transcripts using three independent techniques suggests the presence of two transcription start sites. A major start site is located approximately 28 basepairs downstream from the primary TATA sequence. A second minor start site is positioned about 110 basepairs further upstream. Immunocytochemistry and in situ hybridization analysis indicate that the hANP gene is expressed in a homogeneous distribution throughout the atrial myocardium. Diffuse low level expression is also present within the ventricular myocardium. In addition, there are scattered foci of increased expression in the ventricle which tend to be concentrated in the subendocardium of that organ. These findings indicate that the developing human ventricles as well as the atria possess the capacity to express the hANP gene at a substantial level and suggest that the ventricle may contribute significantly to the circulating pool of plasma ANP.

Renin-angiotensin system in the anterior pituitary of the rat

In rats, angiotensin II appears to be synthesized in the anterior pituitary gland and stored in gonadotropes in the same granules as the beta-subunit of luteinizing hormone (LH). The gonadotropes also contain renin-like and angiotensin-converting enzyme-like immunoreactivity, but angiotensinogen-like immunoreactivity is found in a separate population of cells and does not colocalize with any of the known anterior pituitary hormones. This suggests that angiotensinogen shuttles to the gonadotropes in a paracrine fashion. There are angiotensin II receptors on lactotropes and corticotropes, but no definite function has been established for pituitary angiotensin II in the regulation of prolactin and adrenocorticotropic hormone.

Gene for the rat atrial natriuretic peptide is regulated by glucocorticoids in vitro

Glucocorticoids regulate the expression of the gene for atrial natriuretic peptide (ANP) in neonatal cardiocytes. Dexamethasone (Dex) increased cytoplasmic ANP mRNA levels and media ANP immunoreactivity in a dose-dependent fashion. These effects were not shared by the other classes of steroid hormones and were reversed by the glucocorticoid antagonist RU 38486. The effect on ANP mRNA levels resulted, at least in part, from enhanced transcription of the gene. Dex effected a two-fold increase in ANP gene activity assessed using a run-on transcription assay. The turnover of the ANP transcript was approximated using a standard pulse-chase technique. The half-life of the ANP mRNA was 18 h in hormone-free media. In the presence of Dex this half-life increased modestly to 30 h, although the increase relative to the control did not reach statistical significance. The effect of Dex at the level of the individual myocardial cell was assessed by in situ hybridization analysis using a specific [3H]cRNA probe. These studies demonstrated a significant level of ANP expression within a subpopulation of cells in the cultures. Exposure of the cells to Dex for 24 h did not recruit additional cells into the expressing pool (27.3% cells/high power field vs. 31.3% for the control) but did increase the level of expression (i.e., grain density) within individual cells. These findings indicate that glucocorticoids stimulate expression of the ANP gene directly at the level of the myocardial cell. This results predominantly from transcriptional activation in cells already expressing the gene rather than through recruitment of previously quiescent cells.