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

Retinal degeneration protein 3 controls membrane guanylate cyclase activities in brain tissue

The retinal degeneration protein RD3 is involved in regulatory processes of photoreceptor cells. Among its main functions is the inhibition of photoreceptor specific membrane guanylate cyclases during trafficking from the inner segment to their final destination in the outer segment. However, any physiological role of RD3 in non-retinal tissue is unsolved at present and specific protein targets outside of retinal tissue have not been identified so far. The family of membrane bound guanylate cyclases share a high homology of their amino acid sequences in their cytoplasmic domains. Therefore, we reasoned that membrane guanylate cyclases that are activated by natriuretic peptides are also regulated by RD3. We analyzed transcript levels of the rd3 gene and natriuretic peptide receptor genes Npr1 and Npr2 in the mouse retina, cerebellum, hippocampus, neocortex, and the olfactory bulb during development from the embryonic to the postnatal stage at P60. The rd3 gene showed a lower expression level than Npr1 and Npr2 (encoding for GC-A and GC-B, respectively) in all tested brain tissues, but was at least one order of magnitude higher in the retina. RD3 and natriuretic peptide receptor GCs co-express in the retina and brain tissue leading to functional tests. We expressed GC-A and GC-B in HEK293T cells and measured the inhibition of GCs by RD3 after activation by natriuretic peptides yielding inhibitory constants around 25 nM. Furthermore, endogenous GCs in astrocytes were inhibited by RD3 to a similar extent. We here show for the first time that RD3 can inhibit two hormone-stimulated GCs, namely GC-A and GC-B indicating a new regulatory feature of these hormone receptors.

Regulation of the Natriuretic Peptide Receptor 2 (Npr2) by Phosphorylation of Juxtamembrane Serine and Threonine Residues Is Essential for Bifurcation of Sensory Axons

cGMP signaling elicited by activation of the transmembrane receptor guanylyl cyclase Npr2 (also known as guanylyl cyclase B) by the ligand CNP controls sensory axon bifurcation of DRG and cranial sensory ganglion (CSG) neurons entering the spinal cord or hindbrain, respectively. Previous studies have shown that Npr2 is phosphorylated on serine and threonine residues in its kinase homology domain (KHD). However, it is unknown whether phosphorylation of Npr2 is essential for axon bifurcation. Here, we generated a knock-in mouse line in which the seven regulatory serine and threonine residues in the KHD of Npr2 were substituted by alanine (Npr2-7A), resulting in a nonphosphorylatable enzyme. Real-time imaging of cGMP in DRG neurons with a genetically encoded fluorescent cGMP sensor or biochemical analysis of guanylyl cyclase activity in brain or lung tissue revealed the absence of CNP-induced cGMP generation in the Npr2^7A/7A mutant. Consequently, bifurcation of axons, but not collateral formation, from DRG or CSG in this mouse mutant was perturbed at embryonic and mature stages. In contrast, axon branching was normal in a mouse mutant in which constitutive phosphorylation of Npr2 is mimicked by a replacement of all of the seven serine and threonine sites by glutamic acid (Npr2-7E). Furthermore, we demonstrate that the Npr2^7A/7A mutation causes dwarfism as described for global Npr2 mutants. In conclusion, our in vivo studies provide strong evidence that phosphorylation of the seven serine and threonine residues in the KHD of Npr2 is an important regulatory element of Npr2-mediated cGMP signaling which affects physiological processes, such as axon bifurcation and bone growth.SIGNIFICANCE STATEMENT The branching of axons is a morphological hallmark of virtually all neurons. It allows an individual neuron to innervate different targets and to communicate with neurons located in different regions of the nervous system. The natriuretic peptide receptor 2 (Npr2), a transmembrane guanylyl cyclase, is essential for the initiation of bifurcation of sensory axons when entering the spinal cord or the hindbrain. By using two genetically engineered mouse lines, we show that phosphorylation of specific serine and threonine residues in juxtamembrane regions of Npr2 are required for its enzymatic activity and for axon bifurcation. These investigations might help to understand the regulation of Npr2 and its integration in intracellular signaling systems.

Brain-specific natriuretic peptide receptor-B deletion attenuates high-fat diet-induced visceral and hepatic lipid deposition in mice

C-type natriuretic peptide (CNP) and its receptor, natriuretic peptide receptor-B (NPR-B), are abundantly distributed in the hypothalamus. To explore the role of central CNP/NPR-B signaling in energy regulation, we generated mice with brain-specific NPR-B deletion (BND mice) by crossing Nestin-Cre transgenic mice and mice with a loxP-flanked NPR-B locus. Brain-specific NPR-B deletion prevented body weight gain induced by a high-fat diet (HFD), and the mesenteric fat and liver weights were significantly decreased in BND mice fed an HFD. The decreased liver weight in BND mice was attributed to decreased lipid accumulation in the liver, which was confirmed by histologic findings and lipid content. Gene expression analysis revealed a significant decrease in the mRNA expression levels of CD36, Fsp27, and Mogat1 in the liver of BND mice, and uncoupling protein 2 mRNA expression was significantly lower in the mesenteric fat of BND mice fed an HFD than in that of control mice. This difference was not observed in the epididymal or subcutaneous fat. Although previous studies reported that CNP/NPR-B signaling inhibits SNS activity in rodents, SNS is unlikely to be the underlying mechanism of the metabolic phenotype observed in BND mice. Taken together, CNP/NPR-B signaling in the brain could be a central factor that regulates visceral lipid accumulation and hepatic steatosis under HFD conditions. Further analyses of the precise mechanisms will enhance our understanding of the contribution of the CNP/NPR-B system to energy regulation.

Natriuretic peptides in the central nervous system: Novel targets for cognitive impairment

Natriuretic peptides (NPs) are traditionally known as cardiac hormones with diuretic, natriuretic and blood pressure lowering properties. Evidence indicates that NPs and their receptors are abundant in the central nervous system, suggesting their involvement in regulation of various brain functions. It has been shown that NPs are involved in the regulation of neurovascular and blood-brain barrier integrity, neuro-inflammation, neuroprotection, synaptic transmission and brain fluid homeostasis. In addition, NPs might contribute to the brain's inhibitory control over the hypothalamic-pituitary-adrenal axis. Studies have also shown that high systemic levels of NPs are associated with cognitive impairment independent of cardiovascular risk factors. In this review we discuss the potential roles of NPs in regulating structural and functional integrity of the brain. Based on the available neurobiological and clinical evidence, we propose that NPs might represent as potential novel diagnostic and therapeutic targets for cognitive impairment.

Glial cells as sources and targets of natriuretic peptides

Natriuretic peptides and their receptors are widely expressed in mammalian CNS and increasing evidence implicates them in the regulation of neural development, synaptic transmission and processing of information, and neuroprotection. Although the peptides have been mainly localized in neuronal populations they are also produced in glial cells. Astroglia and microglia also express functional natriuretic peptide receptors that can regulate important physiological responses. In this article we review evidence on the localization of natriuretic peptides and their receptors in astroglial and microglial cells and summarize data supporting the participation of this signalling system in neuron-glia and glia-brain blood vessel communication relevant to CNS function.

Cyclic GMP phosphodiesterase inhibition alters the glial inflammatory response, reduces oxidative stress and cell death and increases angiogenesis following focal brain injury

Recent evidence obtained in cultured glial cells indicates that cGMP-mediated pathways regulate cytoskeleton dynamics, glial fibrillary acidic protein expression and motility in astrocytes, as well as inflammatory gene expression in microglia, suggesting a role in the regulation of the glial reactive phenotype. The aim of this work was to examine if cGMP regulates the glial inflammatory response in vivo following CNS damage caused by a focal cryolesion onto the cortex in rats. Results show that treatment with the cGMP phosphodiesterase inhibitor zaprinast (10 mg/kg i.p.) 2 h before and 24 and 48 h after the lesion results 3 days post-lesion in notably enhanced astrogliosis manifested by increased glial fibrillary acidic protein immunoreactivity and protein levels around the lesion. In contrast, zaprinast decreased the number of round/ameboid lectin-positive cells and the expression of the activated microglia/macrophage markers Iba-1 and CD11b indicating decreased recruitment and activation of these cells. This altered inflammatory response is accompanied by a decrease in protein oxidative stress, apoptotic cell death and neuronal degeneration. In addition, zaprinast enhanced angiogenesis in the lesioned cortex probably as a result of vascular endothelial growth factor expression in reactive astrocytes. These results suggest that regulation of the glial inflammatory response may contribute to the reported neuroprotective effects of cGMP-phosphodiesterase inhibitors in brain injury.

Role of natriuretic peptide signaling in modulating asthma and inflammation

Atrial natriuretic peptide (ANP), the C-terminal peptide comprising residues 99-126 of the pro-ANP hormone, has been studied for 3 decades for its cardiovascular effects. Recent reports suggest that it plays a significant role in modulation of the immune system. Immune cells, including macrophages, dendritic cells, and T lymphocytes, express receptors for ANP. ANP plays a significant role in shaping the early immune response to environmental antigens and may play a critical role in the interaction between cells of the innate and adaptive immune systems; it also appears to be involved in polarizing the immune response to allergens. Thus, ability to alter the magnitude of natriuretic peptide receptor A (NPRA) signaling could be exploited to develop therapeutics for several allergic diseases, including asthma. This report will review and critically evaluate the role of the ANP pathway in asthma and inflammation.

ANP-mediated cGMP signaling and phosphodiesterase inhibition in the rat cervical spinal cord

Natriuretic peptides (NP) and the corresponding receptors are present in the rodent spinal cord. We have studied the structures which respond to atrial natriuretic peptide, brain natriuretic peptide, or C-type natriuretic peptide with an increased synthesis of cGMP. NP-responsive cGMP-producing structures were observed in laminae I-III, and X, and in addition in ependymal cells, astrocytes and a subpopulation of dorsal root ganglion cells. As the cGMP concentration is controlled by the rate of synthesis and the rate of breakdown by phosphodiesterases, we studied NP-responsive structures in spinal cord slices incubated in the presence of different phosphodiesterase inhibitors. We studied EHNA and BAY 60-7550 as selective PDE2 inhibitors, sildenafil as a selective PDE5 inhibitors, dipyridamole as a mixed type PDE5 and PDE10 inhibitor, rolipram as a PDE4 inhibitor, and SCH 81566 as a selective PDE9 inhibitor. Double immunostainings showed that cGMP-IR colocalized partial with the vesicular acetylcholine transporter molecule in lamina X, with Substance P in a subpopulation of neuronal fibers situated dorsolateral, and with a subpopulation of CGRP-IR dorsal root ganglion neurons. Colocalization of cGMP-IR was absent with parvalbumin, synaptophysin, and the vesicular transporter molecules for GABA and glutamate. It is concluded that NPs in the spinal cord are probably involved in integrating intersegmental sensory processing in the spinal cord although the greater part of the NP-responsive cGMP-producing fibers could not be characterized. PDE2, 5, and 9 are involved in regulating NP-stimulated cGMP levels in the spinal cord. NPs may have a role in regulating cerebrospinal fluid homeostasis.

Effect of C-type natriuretic peptide (CNP) on water channel aquaporin-4 (AQP4) expression in cultured astrocytes

Astrocytes play a vital role in volume and ion control in the central nervous system. C-type natriuretic peptide (CNP) may be involved in neuronal-glial signaling, but its physiological role has not yet been characterized. In our study, we found that CNP can regulate the water channel aquaporin-4 (AQP4) expression in cultured astrocytes. Using immunocytochemistry and enzyme immunoassay, we found that primary neuronal cultures exhibited a high level of reactivity to CNP, and that cultured astrocytes exhibited reactivity to cyclic GMP after exposure of CNP. Using RT-PCR, immunoblot and immunocytochemistry, we detected increased levels of AQP4 mRNA and AQP4 immunoreactivity in the cultured astrocytes after they had been exposed to CNP or cyclic GMP. These results suggest that CNP, which is mainly produced by the neurons, effects the level of AQP4 in the astrocytes. Therefore, CNP may be a regulator of water homeostasis in the central nervous system.

Expression of constitutively active guanylate cyclase in cardiomyocytes inhibits the hypertrophic effects of isoproterenol and aortic constriction on mouse hearts

Evidence from several rodent models has suggested that a reduction of either atrial natriuretic peptide or its receptor in the heart affects cardiac remodeling by promoting the onset of cardiac hypertrophy. The atrial natriuretic peptide receptor mediates signaling at least in part via the generation of intracellular cyclic GMP. To directly test whether accumulation of intracellular cyclic GMP conveys protection against cardiac hypertrophy, we engineered transgenic mice that overexpress a catalytic fragment of constitutively active guanylate cyclase domain of the atrial natriuretic peptide receptor in a cardiomyocyte-specific manner. Expression of the transgene increased the intracellular concentration of cyclic GMP specifically within cardiomyocytes and had no detectable effect on cardiac performance under basal conditions. However, expression of the transgene attenuated the effects of the pharmacologic hypertrophic agent isoproterenol on cardiac wall thickness and prevented the onset of the fetal gene expression program normally associated with cardiac hypertrophy. Likewise, expression of the transgene inhibited the hypertrophic effects of abdominal aortic constriction, since it abolished its effects on ventricular wall thickness and greatly attenuated its effects on cardiomyocyte size. Altogether, our results suggest that cyclic GMP is a cardioprotective agent against hypertrophy that acts via a direct local effect on cardiomyocytes.

Atrial natriuretic peptides elevate cyclic GMP levels in primary cultures of rat ependymal cells

The aim of this study was to examine the effect of atrial natriuretic peptides on primary cultures of ependymal cells, as measured by changes in intracellular levels of cyclic GMP. Incubation of ependymal cells with rat atrial natriuretic peptide-(1-28) (rANP) elicited a 30-fold increase in ependymal cGMP content within 1 min and more than a 100-fold increase within 10 min to a plateau value of approximately 30 pmol/mg protein. The C-type natriuretic peptide (CNP) elicited a similar increase in cGMP levels; however the maximal effect was observed within 1 min and the levels subsequently dropped by 90% to a low plateau within 10 min. A comparison of the concentration-response curves for rANP, human ANP-(1-28) (hANP) and CNP showed that rANP, hANP and CNP had similar effects, with regards to elevation of cGMP levels at high concentrations, but with differing EC50 values. These results demonstrate the presence of a heterogenous population of functional ANP receptors i n cultured ependymalcells suggesting that ANP may regulate specific ependymal cell activity.

Natriuretic peptide signalling: molecular and cellular pathways to growth regulation

The natriuretic peptides (NPs) constitute a family of polypeptide hormones that regulate mammalian blood volume and blood pressure. The ability of the NPs to modulate cardiac hypertrophy and cell proliferation as well is now beginning to be recognized. The NPs interact with three membrane-bound receptors, all of which contain a well-characterized extracellular ligand-binding domain. The R1 subclass of NP receptors (NPR-A and NPR-B) contains a C-terminal guanylyl cyclase domain and is responsible for most of the NPs downstream actions through their ability to generate cGMP. The R2 subclass lacks an obvious catalytic domain and functions primarily as a clearance receptor. This review focuses on the signal transduction pathways initiated by ligand binding and other factors that help to determine signalling specificities, including allosteric factors modulating cGMP generation, receptor desensitization, the activation and function of cGMP-dependent protein kinase (PKG), and identification of potential nuclear or cytoplasmic targets such as the mitogen-activated protein kinase signalling (MAPK) cascade. The inhibition of cardiac growth and hypertrophy may be an important but underappreciated action of the NP signalling system.

Presence of soluble and particulate guanylyl cyclase in the same hippocampal astrocytes

The localisation of particulate and soluble guanylyl cyclase was studied in hippocampal astrocytes. Counting the colocalisation of cGMP immunoreactivity with the astrocytic marker glial fibrillary acidic protein after stimulation of brain slices with sodium nitroprusside (0.1 mM) or atrial natriuretic peptide (100 nM), we were able to show that at least 67% of the hippocampal astrocytes contained both guanylyl cyclase isoforms. In addition, it was shown that a large number of atrial natriuretic peptide, brain-derived natriuretic peptide or sodium nitroprusside responsive cells contain the beta1-subunit of the soluble guanylyl cyclase. The results show that, in at least a subset of hippocampal astrocytes, soluble and particulate guanylyl cyclases are simultaneously present in the same cells.

Induction of type 2 deiodinase activity by cyclic guanosine 3',5'-monophosphate in cultured rat glial cells

We investigated the effects of cyclic guanosine 3',5'-monophosphate (cGMP) on type 2 iodothyronine deiodinase (D2) in cultured rat glial cells. Rat glial cells were cultured in Dulbecco's modified Eagle's medium supplemented with 15% fetal bovine serum. When cells were cultured in the presence of 8-bromo cGMP (8-Br cGMP), an analogue of cGMP, D2 activity was increased in a time- and concentration-dependent manner. Lineweaver-Burk plots revealed that the stimulation of D2 activity by 8-Br cGMP (10(-3) M) was associated with fivefold increase in maximum velocity but without a significant change in Michaelis-Menten constant, suggesting that cGMP increases D2 activity via new enzyme synthesis. Both atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) are well known to increase the intracellular cGMP level via their guanylate cyclase-linked receptors in rat glial cells. In the present study, ANP (10(-6) M) and CNP (10(-6) M) significantly increased the D2 activity in rat glial cells (1.9-fold [ANP] or 2.3-fold [CNP] compared with control activity, respectively). Northern blot analysis demonstrated that D2 mRNA level increased in the presence of 8-Br cGMP (10(-3) M), and reached a plateau (six-fold) after 4 hours of incubation. The increment of D2 mRNA level by 8-Br cGMP was comparable with the increase of the D2 activity by this agent. Our data suggest that cGMP induces rat D2 activity, at least in part, at the pretranslational level, and that ANP and CNP increase D2 activity most likely via their guanylate cyclase-linked receptors in rat glial cells.

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.

Region-specific developmental patterns of atrial natriuretic factor- and nitric oxide-activated guanylyl cyclases in the postnatal frontal rat brain

In the rat central nervous system, cyclic GMP can be produced by two isoforms of guanylyl cyclase: a cytosolic isoform, which is activated by nitric oxide, and a membrane-bound isoform, activated by atrial natriuretic factor. We studied the development of guanylyl cyclase activity upon maturation of the rat forebrain from postnatal days 4 to 24, using a combined immunocytochemical and biochemical approach. Atrial natriuretic factor-activated particulate guanylyl cyclase activity was found to decrease in the frontal cortex, in the lateral septum and in the piriform cortex upon maturation. A transient expression of atrial natriuretic factor-sensitive guanylyl cyclase activity was observed at postnatal day 8 in the caudate putamen complex, whereas an increase was observed in the lateral olfactory tract from postnatal days 8 to 24. Biochemical and immunocytochemical studies using the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester, or the inhibitor of soluble guanylyl cyclase 1H-[1,2,4]oxadiazolo[4,3-a]quinaloxin-1-one, indicated high levels of endogenous nitric oxide release at postnatal days 4 and 8. This activity decreased strongly in all brain areas examined. From postnatal day 8 onwards, atrial natriuretic factor-responsive cyclic GMP-immunoreactive cells could be characterized as astrocytes, with the exception of those in the the lateral olfactory tract, where the myelinated fibers became cyclic GMP producing. Furthermore, our results on activation of both guanylyl cyclases at postnatal day 8 leads to the suggestion that both isoforms might be found in the same cells. This study shows that there are pronounced differences between various frontal brain areas in the development of the responsiveness of both the particulate and soluble isoforms of guanylyl cyclase, and lends further support to the hypothesis that natriuretic peptides have a role in neuronal growth and plasticity of the rat brain.

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.

Endothelin-3 attenuates the cyclic GMP responses to C-type natriuretic peptide in cultured mouse astrocytes

The effect of endothelin-3 (ET-3) on cyclic GMP (cGMP) responses to C-type natriuretic peptide (CNP) was studied in primary cultures of mouse astrocytes. Attenuation of CNP-stimulated cGMP formation by ET-3 was time-dependent, with maximum inhibition achieved at 30 min of preincubation. ET-3 suppressed cGMP production in response to 10 nM CNP in a dose-dependent fashion, with an IC50 of 0.04 nM and a maximal inhibitory concentration of 1 microM, which led to a 66% reduction of the cGMP increment from 45.0 +/- 4.2 pmol/mg protein to 15.4 +/- 2.6 pmol/mg protein. ET-1, ET-2, and ET-3 were equipotent in suppressing the CNP-induced cGMP response, suggesting that this effect was mediated by ETB receptors. Staurosporine, Ro 31-8220, calcium-free medium, nifedipine, verapamil, lanthanum, thapsigargin, BAPTA, W7, calmidazolium, U-73122, neomycin, quinacrine, wortmannin, herbimycin-A, okadaic acid, and sodium orthovanadate failed to block the effect of ET-3. Cycloheximide (100 microM), however, partially but significantly reversed the inhibitory effect of ET-3 on CNP-induced cGMP from 48.2 to 73.3% of the control value. The results support the premise that ET-3 and CNP interact within the central nervous system. The data also suggest that cGMP accumulation in mouse astrocytes is mediated by activation of certain kinases through as yet undefined mechanisms and not by protein kinase C, increased intracellular calcium, or other second messenger pathways such as phospholipases A2, C, D, tyrosine kinase, or protein phosphatases.

Localization of natriuretic peptide-activated guanylate cyclase mRNAs in the rat brain

Physiological actions of atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) are elaborated by membrane-bound natriuretic peptide receptors (NPRs). These receptors possess intracellular guanylate cyclase domains that mobilize cyclic guanosine monophosphate upon binding of peptide. Two distinct NPR subtypes have been described in brain: the NPR-A selectively binds ANP, whereas NPR-B exhibits high affinity for CNP. To define further the potential domains of ANP and CNP action in brain, the present study used in situ hybridization histochemistry to map NPR-A and NPR-B mRNA-expressing cell populations. Significant levels of neuronal NPR-A mRNA expression were observed only in the mitral cell layer of the olfactory bulb, medial habenula, subfornical organ, and area postrema. Expression of NPR-A mRNA was observed in forebrain white matter tracts, suggesting synthesis in glial cells. In contrast, NPR-B mRNA was widely expressed throughout the neuraxis. In the telencephalon, signal was abundant throughout limbic cortex and neocortex, olfactory bulb, hippocampus, and amygdala. Intense NPR-B mRNA hybridization was observed in preoptic-hypothalamic neuroendocrine circuits and in motor nuclei of cranial nerves. Intermediate expression of NPR-B mRNA was observed in brainstem nuclei controlling autonomic function. Labeling for NPR-B but not NPR-A mRNA was observed in pituicytes in the neural lobe of the pituitary and in scattered cells of the anterior pituitary. These results suggest that CNP is the primary biologically active natriuretic peptide in brain. In contrast with NPR-B, NPR-A appears to be expressed largely in restricted cell populations containing high levels of ANP and in circumventricular organs. These data implicate the NPR-A in autoregulation of ANP neurons and central registration of cardiac ANP release.

Binding of CNP-22 and CNP-53 to cultured mouse astrocytes and effects on cyclic GMP

We examined the binding characteristics of the two endogenous forms of C-type natriuretic peptide (CNP-22 and CNP-53) and their effects on cyclic GMP (cGMP) accumulation in primary cultures of mouse astrocytes. CNP-22 and CNP-53 competitively inhibited the specific binding of [125I][Tyr0]CNP, with an IC50 value of 32 and 37 pM, respectively. They also induced cGMP production in a dose-dependent and similar fashion, with an EC50 of 32 nM and maximal cGMP responses of 189.6 +/- 21.6 pmol/mg protein for CNP-22, and 170.3 +/- 18.7 pmol/mg protein for CNP-53, respectively. The effect of CNP-53 could not be explained by conversion to CNP-22, because HPLC analysis did not show significant proteolytic conversion by astrocytes during the incubation. Our results suggest that CNP-53 could, in concert with other natriuretic peptides, have a neuromodulatory function and thereby contribute to the central regulation of hemodynamic and fluid homeostasis.

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.