Endothelin receptors on cultured fetal rat diencephalic glia

The potential of endothelin antagonism as a therapeutic approach

Endothelin (ET) is a pivotal physiological regulator of blood pressure through its effects on blood vessels, heart, lung and kidneys, and the ET system can be overactive in disorders such as pulmonary hypertension, heart failure and renal disease. Such observations stimulated interest among scientists and pharmaceutical companies that have set up high-throughput screens to search for antagonists of ET receptors. The emerging compounds have been tested in animals with exciting results, leading to great hope that such inhibitors could be translated into human drugs with desirable therapeutic activities and few side effects. This review will describe the most relevant results obtained in experimental animals in a wide variety of disease conditions and focus on the data of selected compounds that have been employed in clinical trials.

New therapeutics that antagonize endothelin: promises and frustrations

The discovery of endothelin--a highly potent endogenous vasoconstrictor - in 1988 has led to considerable efforts to develop antagonists of endothelin receptors that could have therapeutic potential in disorders including hypertension, heart failure and renal diseases. However, in general, the results of trials in humans have not mirrored the highly promising effects in animal disease models. Here, we discuss preclinical and clinical results with endothelin antagonists, and consider possible approaches to fully realizing the potential of endothelin antagonism.

Endothelin-evoked Release of Arachidonic Acid from Mouse Astrocytes in Primary Culture

In striatal astrocytes, receptors for the vasoactive peptide endothelin (ET) are associated with several intracellular signalling pathways: ET-1 increases the breakdown of phosphoinositides, induces a sustained influx of Ca2+ and inhibits the isoproterenol-induced formation of cAMP (Marin et al., J. Neurochem., 56, 1270 - 1275, 1991). In the present study, it will be shown that ET-1 and ET-3 markedly stimulate the release of arachidonic acid (AA) from cultured astrocytes from the mouse striatum (EC50=3 and 7 nM for ET-1 and ET-3, respectively), mesencephalon and cerebral cortex. The ET-1-evoked release of AA probably resulted from the activation of a phospholipase A2, since it required extracellular Ca2+ and was prevented by mepacrine but not by RHC 80267, an inhibitor of diacylglycerol lipase. The ET-1-induced release of AA was shown to be partially mediated by a guanine nucleotide-binding protein sensitive to pertussis toxin but not to cholera toxin. A cAMP-dependent process is not involved since the ET-1-evoked release of AA was not affected when cells were incubated with either isoproterenol or 8-bromo-cAMP. The ET-1-evoked release of AA could be mimicked by the co-application of a calcium ionophore and a protein kinase C activator. However, staurosporine, a potent inhibitor of protein kinase C, which blocked the release of AA induced by the combined application of ionomycin and phorbol 12-myristate 12-acetate (PMA), was without effect on the ET-1-evoked response, indicating that protein kinase C is not directly involved in the ET-1-induced release of AA. Furthermore, the responses induced by ET-1 and by PMA were found to be additive. These results suggest that (1) ET-1 receptors are coupled to the release of AA by a mechanism independent of both protein kinase C activation and the adenylate cyclase pathway, possibly via the activation of phospholipase A2, (2) different mechanisms (or different phospholipase A2 subtypes) are involved in the control of AA release in astrocytes.

Astrocytes in sensory circumventricular organs of the rat brain express functional binding sites for endothelin

Sensory circumventricular organs bordering the anterior third cerebral ventricle, the subfornical organ and the organum vasculosum laminae terminalis, lack blood-brain barrier characteristics and are therefore accessible to circulating peptides like endothelins. Astrocytes of the rat subfornical organ and organum vasculosum laminae terminalis additionally showed immunocytochemical localization of endothelin-1/endothelin-3-like peptides, possibly acting as circumventricular organ-intrinsic modulators. Employing [125I]endothelin-1 as radioligand, quantitative autoradiography demonstrated specific binding sites throughout the rat organum vasculosum laminae terminalis and subfornical organ, and competitive displacement studies revealed expression of both ET(A) and ET(B) receptor subtypes for either circumventricular organ. ET(B) receptor binding prevailed for the whole brain and ET(A) receptors could be labelled in the peripheral vascular system. To characterize endothelin-specific receptors in astrocytes of both circumventricular organs, alterations in the intracellular calcium concentration due to endothelin-1/endothelin-3 stimulation were studied in primary culture of subfornical organ and organum vasculosum laminae terminalis cells obtained from early postnatal rat pups. Endothelin-1 and endothelin-3 induced Ca(2+) transients in 9-13% of either subfornical organ or organum vasculosum laminae terminalis astrocytes, respectively, and some glial cells (subfornical organ: 2%, organum vasculosum laminae terminalis: 5%) responded to both endothelin analogues. The antagonistic action of BQ123 specific for ET(A) receptors (74% of all astrocytes tested), and the pronounced responsiveness to the ET(B) receptor agonist [4Ala]ET-1 (subfornical organ: 27%, organum vasculosum laminae terminalis: 35%) demonstrated glial expression of both endothelin receptor subtypes. Agonist-induced elevations in the intracellular calcium concentration proved to be independent of extracellular Ca(2+). In summary, the results indicate that endothelin(s) interact(s) with circumventricular organ astrocytes. Competitive receptor binding techniques using brain tissue sections as well as a fura-2 loaded primary cell culture system of the subfornical organ and organum vasculosum laminae terminalis demonstrate glial expression of functional ET(A) and ET(B) receptors, with calcium as intracellular messenger emerging primarily from intracellular stores. Endothelin(s) of both circulating and circumventricular organ-intrinsic origin may afferently transfer information important for cardiovascular homeostasis to circumventricular organs serving as "windows to the brain".

Expression of lysophosphatidic acid receptor in rat astrocytes: mitogenic effect and expression of neurotrophic genes

Lysophosphatidic acid (LPA) is a phospholipid mediator with a variety of biological activities. It remains unknown, however, which cells in the brain express the LPA receptor. The present study was undertaken to identify cells in the rat brain expressing functional LPA receptors, and to explore biological roles of LPA in these cells. We found that the LPA receptor was most dominantly expressed in rat astrocytes, determined by LPA-induced Ca2+ imaging, and by Northern blot analyses. LPA induced a mitogenic response and expression of immediate early genes in astrocytes, through pertussis-toxin sensitive G-protein(s). LPA also stimulated the expression of various cytokine genes, including nerve growth factor, interleukin (IL)-1beta, IL-3 and IL-6. Thus, astrocytes are the major target of LPA in the brain. We propose that LPA may play important roles in neuronal development, gliosis and wound-healing process in the brain.

ETA and ETB receptor antagonists synergistically increase extracellular endothelin-1 levels in primary rat astrocyte cultures

Astrocytes produce and bind endothelins (ETs), suggesting that these cells have ET autoregulatory and eliminatory functions. To further investigate these functions in primary rat astrocytes, ET-1 levels in the cell culture media (RIA/HPLC) and intracellular content of ET-1 mRNA (RT PCR) were measured under basal and stimulated (thrombin, 2.2 U/ml) conditions in the presence and absence of ETA and ETB selective antagonists (BQ123 or LU135252, and BQ788, respectively). Neither basal nor stimulated ET-1 levels in astrocyte media were influenced by ETA or ETB antagonists alone, but were significantly increased by a combination of both. ir ET-3 levels were not affected by antagonist treatment. Exogenous ET-1, added to the cultures, was rapidly cleared from the supernatant; this clearance was markedly inhibited by a combination of BQ123 and BQ788. ET-1 mRNA levels were not altered by any treatment. To conclude, in primary rat astrocyte cultures, extracellular ET-1 is cleared by binding to ET-receptors, apparently involving both, ETA and ETB sites. Thus, a blockade of the astrocytic ET eliminatory function as a consequence of the in vivo application of non-selective ET receptor antagonists may lead to increased extracellular ET levels in the brain.

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.

Endothelin-3 reduces C-type natriuretic peptide-induced cyclic GMP formation in C6 glioma cells

The effect of endothelin-3 (ET-3) on C-type natriuretic peptide (CNP)-induced guanosine 3',5'-cyclic monophosphate (cGMP) was examined in C6 glioma cells, CNP-induced cGMP formation was both time- and dose-dependent, with an EC50 value of about 10 nM. While ET-3 and phorbol 12-myristate 13-acetate (PMA) had no effect on basal cGMP production, both compounds were potent inhibitors of CNP-induced cGMP formation, with IC50 values of approximately 10 and 2 nM, respectively. Although protein kinase C (PKC) inhibitors had no effect on basal cGMP formation, Ro 31-8220, a PKC inhibitor, reversed the ET-3 inhibition on CNP-induced cGMP formation by 63% and that of PMA almost completely. Our findings suggest that stimulation of cGMP formation by CNP in C6 glioma cells is negatively modulated by PKC activation, and that the inhibitory action of ET-3 on CNP-stimulated cGMP formation is mediated partly by PKC.

Expression of endothelin-B receptors by glia in vivo is increased after CNS injury in rats, rabbits, and humans

Previous studies have demonstrated that neonatal cultures of astrocytes express functional endothelin (ET) receptors. To determine if similar ET receptors are expressed by adult glia we used 125I-ET-1 to examine the expression of ET receptors both in vivo in the normal and transected optic nerves of the rabbit and rat and in vitro in cultures of astrocytes, microglia, or oligodendrocytes. Additionally, we examined the expression of ET receptors in the human optic nerve. Moderate levels of ET(B) receptors were identified in the rabbit and rat forebrain, whereas in the normal rabbit, rat, and human optic nerves a low density of ET(B) receptors was observed, mainly in association with glial fibrillary acidic protein + (GFAP+) astrocytes. After unilateral optic nerve transection, or damage to the retina, the density of glial ET(B) receptors in the optic nerve is significantly increased in all species examined. Thus, at 7 days posttransection there is a significant increase in ET(B) receptors, and by 90 days posttransection the density of ET(B) receptors in the rabbit or rat optic nerve was among the highest of any area in the central nervous system (CNS). Primary cultures of astrocytes or microglia, but not oligodendrocytes, express 125I-ET-1 binding sites. These data demonstrate that in the normal CNS, astrocytes express low but detectable levels of ET(B) receptors, and, after CNS injury, both astrocytes and microglia express high levels of ET(B) receptors. ET(B) receptors provide a therapeutic target for regulating glial proliferation and the release of neurotrophic factors from glia that occur in response to neuronal injury.

Endothelins promote the activation of astrocytes in rat neostriatum through ET(B) receptors

The effects of endothelin (ET)-3 and an ET(B) receptor agonist on astrocytic activation in rat caudate putamen were examined by an immunohistochemical staining of glial fibrillary acidic protein (GFAP), a marker of reactive astrocytes. A single injection of 40 pmol ET-3 into rat caudate putamen increased the number of GFAP positive cells compared to that in the contralateral saline-injected side. Ala(1,3,11,15)-ET-1 (40 pmol), an ET(B) receptor agonist, also increased the number of striatal GFAP positive cells. The increases in GFAP positive cells were maximum (about 150% of the control side) in 1-2 weeks after injections of the ETs, and then reduced in 4 weeks. A continuous infusion of BQ788, an ET(B) receptor antagonist (23 nmol/day), into the lateral ventricle of the cerebrum antagonized the effect of Ala(1,3,11,15)-ET-1, while BQ788 also reduced the number of GFAP positive cells in saline-injected caudate putamen. Intrastriatal injection of 40 pmol Ala(1,3,11,15)-ET-1 did not affect the number of cells stained by B4 isolectin from Griffonia simplicifolia, which labels activated microglia/macrophages. Intraperitoneal administration of 5 mg/kg per day chloroquine and 0.2 mg/kg per day colchicine did not affect the action of Ala(1,3,11,15)-ET-1. These results suggest that activation of ET(B) receptors is involved in the induction of reactive astrocytes.

Immortalized schwann cells express endothelin receptors coupled to adenylyl cyclase and phospholipase C

Endothelins (ETs) are potent regulators of renal, cardiovascular and endocrine functions and act as neurotransmitters in the CNS. Here we report that immortalized Schwann cells express receptors for ETs and characterize some of the cellular events triggered by their activation. Specific binding of [125I]-ET-1 to Schwann cell membranes was inhibited by ET-1 and ETB-selective agonists ET-3, sarafotoxin 6c and [Ala1,3,11,15]-ET-1 with IC50cor values ranging between 2 and 20 nM. No competition was observed with the ETA receptor-selective antagonist BQ123. Incubation of [3H]-inositol pre-labeled Schwann cells with ET-1, ET-3 or sarafotoxin 6c elicited a concentration-dependent increase in the release of [P1 that reached a plateau at approximately 100 nM. The efficacy of [Ala1,3,11,15]-ET-1 (a linear peptide analog of ET-1) was half of that corresponding to ET-1. These stimulatory effects were partially blocked by pre-incubation with pertussis toxin. When Schwann cells were incubated in the presence of 100 nM ET-1 or ET-3 there was a significant inhibition of basal and isoproterenol-stimulated cAMP levels. The inhibitory effects of sarafotoxin 6c and [Ala1,3,11,15]-ET-1 on isoproterenol-stimulated cAMP levels were similar to that observed with ET-1. Pre-incubation with pertussis toxin completely prevented this effect. These observations indicate that immortalized Schwann cells express receptors for ET peptides (predominantly ETB) coupled to modulation of phospholipase C and adenylyl cyclase activities. The actions of ETs on Schwann cells provide a novel example of the influence of vascular factors on nerve function.

The role of endothelins in the paracrine control of the secretion and growth of the adrenal cortex

Endothelins (ETs) are a family of vasoactive peptides (ET-1, ET-2, and ET-3) mainly secreted by vascular endothelium and widely distributed in the various body systems, where they play major autocrine/paracrine regulatory functions, acting via two subtypes of receptors (ETA and ETB): Adrenal cortex synthesizes and releases ETS and expresses both ETA and ETB. Zona glomerulosa possesses both ETA and ETB, whereas zona fasciculata/reticularis is almost exclusively provided with ETB. ETS exert a strong mineralocorticoid and a less intense glucocorticoid secretagogue action, mainly via ETB receptors. ETS also appear to enhance the growth and steroidogenic capacity of zona glomerulosa and to stimulate its proliferative activity. This trophic action of ETS is likely to be mediated mainly by ETA receptors. The intraadrenal release of ETS undergoes a multiple regulation, with the rise in blood flow rate and the local release of nitric oxide being the main stimulatory factors. Data are also available that indicate that ETS may also have a role in the pathophysiology of primary aldosteronism caused by adrenal adenomas and carcinomas.

Atrial natriuretic peptide inhibits mitogen-activated protein kinase through the clearance receptor. Potential role in the inhibition of astrocyte proliferation

The modulation of the activity of mitogen-activated protein kinase (MAPK) by endogenous growth factors or growth inhibitors provides a potential means of regulating cell proliferation. We determined the effect of the endogenous anti-proliferative peptide, atrial natriuretic peptide (ANP), on the ability of MAPK to phosphorylate myelin basic protein. In astrocytes, MAPK activity was significantly stimulated (up to 3-fold) by three known glial mitogens, endothelin-3, platelet-derived growth factor, or phorbol 12-myristate 13-acetate. ANP inhibited by 55-70% the ability of each of these mitogens to activate MAPK. The effects of ANP were equipotent to those caused by C-ANP 4-23, a peptide that specifically binds to the natriuretic peptide clearance receptor. Additionally, both natriuretic peptides caused a 70-80% inhibition of the sodium vanadate-stimulated MAPK activity, complete inhibition of the okadaic acid-stimulated activity, and inhibition of the mitogen-stimulated phosphorylation of MAPK. To understand the potential mechanism by which the natriuretic peptides act, we found that both ANP and C-ANP inhibited the mitogen-stimulated activity of the immediate upstream kinase in the cascade, MAPK kinase (MEK). C-ANP also strongly inhibited the endothelin-3-, platelet-derived growth factor-, and phorbol 12-myristate 13-acetate-induced stimulation of DNA synthesis in the astrocytes, while both okadaic acid and sodium vanadate significantly reversed these anti-proliferative actions. Our results identify ANP as a peptide hormone that inhibits growth factor-stimulated MAPK. These data suggest that the ability of the natriuretic peptides to inhibit MAPK may be important for their anti-growth actions. This effect likely occurs via the inhibition of upstream kinase(s), including MEK, uniquely resulting from ligand binding to the natriuretic peptide clearance receptor.

Human astrocytoma U138MG cells express predominantly type-A endothelin receptor

Endothelin-1 (ET-1) binding to human astrocytoma U138MG cells was time-dependent, and bound [125I]ET-1 was difficult to dissociate. The B(max) and Kd values of [125I]ET-1 binding were 70 fmol/mg and 0.07 nM, respectively. Interestingly, different from other astrocytoma cells and astrocytes, the U138MG cells expressed predominantly ETA receptor as shown by RT-PCR results and binding studies. ET-1, FR139317, BQ123, PD142893 and Ro46-2005 inhibited specific [125I]ET-1 binding with Ki values of 0.10, 0.53, 4.3, 22, and 320 nM, respectively. ETB selective ligands ET-3 and IRL1620 were much less potent. The inhibitory effects of antagonists BQ123 and PD142893 on [125I]ET-1 binding diminished following the incubation time. ET-1 binding caused a modest stimulation in phosphatidylinositol hydrolysis with an EC50 value of 24 nM. In comparison to the human U373MG cells, ET-1-induced receptor internalization in U138MG cells was less efficient with 42% of bound ET-1 internalized after 30 min of incubation. These results imply that human astrocytoma cells/astrocytes are able to express either ETA or ETB receptor under different pathophysiological conditions.

Endothelin-induced cytoskeletal actin re-organization in cultured astrocytes: inhibition by C3 ADP-ribosyltransferase

We investigated signal transduction mechanisms of endothelin (ET) receptor-mediated actin re-organization of rat cultured astrocytes. Staining of filamentous actin (F-actin) showed that stress fibers were a prominent cytoskeletal actin structure in protoplasmic astrocytes. A treatment with 0.5 mM dibutyryl cAMP (DBcAMP) caused cytoplasmic retraction and disappearance of stress fibers of astrocytes. A subsequent addition of 1 nM ET-3 after the DBcAMP treatment expanded the cytoplasm and stimulated stress fiber formation. ET-1, sarafotoxin S6b, and [Ala1,3,11,16]-ET-1 had similar effects. Pre-treatment with 0.1 microgram/ml pertussis toxin (PTX) and chelation of cytosolic Ca2+ did not affect astrocytic stress fiber formation by ET-3. ET-3 stimulated stress fiber formation in stellate astrocytes induced by 50 microM ML-9, 20 microM W-7, and 5 microM cytochalasin B (CB). Cytoplasmic microinjection of C3ADP-ribosyltransferase of C. botulinum (C3 enzyme), which impairs the interaction between rho proteins and the effectors, prevented ET-3-induced stress fiber formation and cytoplasmic expansion in DBcAMP-and CB-treated cells. Effects of ET-1 and sarafotoxin on stress fiber formation were also prevented by C3 enzyme. On the other hand, injection of C3 enzyme did not affect increase in cytoplasmic Ca2+ levels induced by ET-3. These results suggest that rho proteins are involved in the ET receptor-mediated actin re-organization of astrocytes.

Endothelins are extracellular signals modulating cytoskeletal actin organization in rat cultured astrocytes

Effects of endothelin-3 on rapid morphological changes and cytoskeletal actin organization of rat cortical cultured astrocytes were examined. In serum-free medium, treatments with 1 mM dibutyryl cAMP and 5 microM cytochalasin B, an inhibitor of actin polymerization, caused astrocytic morphological changes with cytoplasmic retraction (stellation). Concurrent addition of 1 nM endothelin-3 prevented astrocytic stellation by dibutyryl cAMP and cytochalasin B. The inhibition of endothelin-3 on the astrocytic stellation was dose-dependent, where IC50 and maximal effective dose were about 50 pM and 0.1 nM, respectively. Endothelin-1 and sarafotoxin S6b prevented the cytochalasin B-induced stellation with similar potencies to endothelin-3. Endothelin-3 reversed the stellate morphology of cytochalasin B-treated cells. Sixty minutes after addition of endothelin-3, most cytochalasin B-treated astrocytes lost their apparent distinction between cell body and processes. Treatment with dibutyryl cAMP and cytochalasin B decreased actin content in a 0.5% Triton X-100-insoluble fraction (cytoskeletal fraction) of cultured astrocytes. Subsequent treatments with endothelin-3 for 2 h restored the decreased cytoskeletal actin to that of non-treated cells. Rhodamine-phalloidin staining showed that a prominent structure of organized filamentous actin in protoplasmic astrocytes is stress fibers. The astrocytic stress fibers disappeared after treatment with dibutyryl cAMP and cytochalasin B. Endothelin-3 stimulated reorganization of stress fibers both in the dibutyryl cAMP- and the cytochalasin B-treated astrocytes. These results suggest that endothelins are extracellular signals to regulate cytoskeletal actin organization of astrocytes.

Increased endothelin-1 in the rabbit model of middle cerebral artery occlusion

Endothelin-mediated vasoconstriction may theoretically aggravate ischemic neuronal damage. Although investigators have demonstrated that endothelins are produced by cerebral microvessel endothelial cells, astrocytes and neurons in vitro, whether endothelins are produced during cerebral ischemia is still unclear. The purpose of this study, therefore, was to measure endothelin-1 in brain tissue and plasma following middle cerebral artery occlusion and to examine the relationship between brain tissue and plasma endothelin-1 levels. The middle cerebral artery of rabbits was occluded for 2, 4 or 24 h. The amount of endothelin-1 in both brain tissue and plasma was determined by RIA. The results demonstrate that the concentrations of endothelin-1 in the ischemic brain tissue and plasma are both significantly increased after focal cerebral ischemia (P < 0.01). The data confirm that an acute and marked increase of endothelin-1 in brain tissue and plasma is associated with focal ischemic events. The possibility that endothelin-1 has a role in neuronal cell damage following focal ischemia warrants further attention.

Astrocyte growth is regulated by neuropeptides through Tis 8 and basic fibroblast growth factor

The important intracellular mechanisms of astrocyte growth are not well defined. Using an inhibitor of astrocyte proliferation, atrial natriuretic peptide (ANP), and the glial mitogen endothelin (ET-3), we sought a common pathway for growth regulation in these neural cells. In cultured fetal rat diencephalic astrocytes, ANP selectively and rapidly inhibited the Tis 8 immediate early gene and protein. After 4 h, ANP selectively inhibited the basic fibroblast growth factor (bFGF) gene and protein. ET-3 significantly stimulated both Tis 8 and bFGF mRNAs and protein, but also stimulated several other immediate early and growth factor/receptor genes. An antisense oligonucleotide to Tis 8 strongly prevented ET-stimulated thymidine incorporation, while the inhibitory action of ANP was enhanced. The Tis 8 antisense oligonucleotide also significantly reversed ET-stimulated bFGF transcription and enhanced the bFGF inhibition caused by ANP. In addition, an antisense oligonucleotide to bFGF significantly reversed the ET-stimulated thymidine incorporation and enhanced the ANP inhibition of DNA synthesis. The sequential modulation of Tis 8, followed by bFGF, provides a novel mechanism for both positive and negative regulation of astrocyte growth by endogenous neuropeptides.

Ventral mesencephalic and cortical transplants into the rat striatum display enhanced activity for neutral endopeptidase 24.11 ('enkephalinase'; CALLA)

A role for neutral endopeptidase 24.11 (NEP) in growth and development is supported by the demonstration that NEP hydrolyses and inactivates a number of peptide growth factors including atrial natriuretic peptide, endothelins, bombesin-like peptides, and opioid peptides, including the enkephalins. In the present study, suspensions of cells obtained from the ventral mesencephalon or cortex of rat embryos (ED14) were implanted into the striatum of the adult rat brain. Three to 15 weeks after transplantation the relative distribution of NEP-positive cellular elements was visualized histochemically. NEP staining in the transplants consistently appeared before NEP staining in the surrounding host striatum supporting a relative increase in NEP activity in the transplants. The NEP staining richly visualized cells of varying size and morphology which lacked the normal organization of the host striatum. The histochemical staining in the transplants and the surrounding host tissue was completely blocked by a 100 nM concentration of the selective NEP inhibitors phosphoramidon or JHF-26, supporting the exclusive localization of NEP by this method. NEP localization in the embryonic (ED14) cortex and ventral mesencephalon was also confirmed, suggesting one possible origin for the NEP-positive cells visualized in the transplants. Fluorescent double-labeling studies for NEP and glial fibrillary acidic protein (GFAP) or transforming growth factor alpha precursor (TGF alpha p) revealed the presence of rich glial labeling within the transplants for both GFAP and TGFap. NEP-labeled cells in the transplants were closely associated with glial elements, however, only occasional glial elements in the transplants stained for NEP; supporting a non-astrocytic localization for the NEP in the transplants. The marked enhancement of NEP staining in the transplants may have significance for controlling the rate or pattern of growth of the transplanted cells through inactivation of peptide growth factors produced by, or in response to, the transplants.

Endothelins modulate dibutyryl cAMP-induced stellation of cultured astrocytes

Effect of endothelin-3 (ET-3) on dibutyryl cAMP (DBcAMP)-induced stellation of rat cerebral cultured astrocytes was examined. Treatment with 1 mM DBcAMP, 10 microM forskolin, 100 microM isoproterenol and 500 nM phorbol 12-myristate 13-acetate changed protoplasmic cultured astrocytes into process-bearing ones. ET-3 (1 nM) completely prevented the astrocytic stellation induced by these agents. The effect of ET-3 showed a dose-dependence, where IC50 value and maximal effective dose were 49 pM and about 0.1 nM, respectively. ET-1 and sarafotoxin (SRTX) S6b prevented the DBcAMP-induced astrocytic stellation with potencies similar to that of ET-3. ET-3 (1 nM) did not affect the cAMP accumulation after DBcAMP treatment in cultured astrocytes. Stellate astrocytes were reversed to the protoplasmic type cells by addition of 1 nM ET-3 in the presence of DBcAMP. ET-1 and SRTX similarly reversed the astrocytic stellation. ET-3 reversed the astrocytic stellation in the absence of extracellular Ca2+. Pre-loading of BAPTA-AM, a permeable Ca2+ chelator, on stellate astrocytes had no effect on the reversal by ET-3. ET-3 did not increase intracellular free Ca2+ concentration ([Ca2+]i) of most astrocytes tested at 0.1 nM. A high concentration (100 nM) of ET-3 increased astrocytic [Ca2+]i which was negated by Ca(2+)-free and BAPTA-AM loading. These results suggest that ETs modulate morphological changes in astrocytes through cAMP- and Ca(2+)-independent mechanisms.

Activation of protein kinase C attenuates the cyclic GMP responses to C-type natriuretic peptide in cultured mouse astrocytes

C-type natriuretic peptide (CNP), a recently discovered natriuretic peptide, has a potent stimulatory effect on cyclic GMP (cGMP) formation in cultured mouse astrocytes. Pretreatment of astrocytes with phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), attenuated CNP-induced cGMP responses in a dose-dependent manner, with a half-maximal inhibitory concentration of 6 nM, whereas the inactive phorbol ester analog, 4 alpha-phorbol 12,13-didecanoate, was without effect. In the presence of staurosporine, a PKC inhibitor, the inhibitory effect of PMA on CNP-stimulated cGMP production was reversed. These results suggest that PKC is an inhibitory modulator of CNP-stimulated cGMP responses in astrocytes and that CNP may interact with neuropeptides which stimulate PKC.