Immunohistochemical localization of renin in mouse brain

Role of neurons and glia in the CNS actions of the renin-angiotensin system in cardiovascular control

Despite tremendous research efforts, hypertension remains an epidemic health concern, leading often to the development of cardiovascular disease. It is well established that in many instances, the brain plays an important role in the onset and progression of hypertension via activation of the sympathetic nervous system. Further, the activity of the renin-angiotensin system (RAS) and of glial cell-mediated proinflammatory processes have independently been linked to this neural control and are, as a consequence, both attractive targets for the development of antihypertensive therapeutics. Although it is clear that the predominant effector peptide of the RAS, ANG II, activates its type-1 receptor on neurons to mediate some of its hypertensive actions, additional nuances of this brain RAS control of blood pressure are constantly being uncovered. One of these complexities is that the RAS is now thought to impact cardiovascular control, in part, via facilitating a glial cell-dependent proinflammatory milieu within cardiovascular control centers. Another complexity is that the newly characterized antihypertensive limbs of the RAS are now recognized to, in many cases, antagonize the prohypertensive ANG II type 1 receptor (AT1R)-mediated effects. That being said, the mechanism by which the RAS, glia, and neurons interact to regulate blood pressure is an active area of ongoing research. Here, we review the current understanding of these interactions and present a hypothetical model of how these exchanges may ultimately regulate cardiovascular function.

Distribution of cells expressing human renin-promoter activity in the brain of a transgenic mouse

Renin plays a critical role in fluid and electrolyte homeostasis by cleaving angiotensinogen to produce Ang peptides. Whilst it has been demonstrated that renin mRNA is expressed in the brain, the distribution of cells responsible for this expression remains uncertain. We have used a transgenic mouse approach in an attempt to address this question. A transgenic mouse, in which a 12.2 kb fragment of the human renin promoter was used to drive expression of Cre-recombinase, was crossed with the ROSA26-lac Z reporter mouse strain. Cre-recombinase mediated excision of the floxed stop cassette resulted in expression of the reporter protein, beta-galactosidase. This study describes the distribution of beta-galactosidase in the brain of the crossed transgenic mouse. In all cases where it was examined the reporter protein was co-localized with the neuronal marker NeuN. An extensive distribution was observed with numerous cells labeled in the somatosensory, insular, piriform and retrosplenial cortices. The motor cortex was devoid of labeled cells. Several other regions were labeled including the parts of the amygdala, periaqueductal gray, lateral parabrachial nucleus and deep cerebellar nuclei. Overall the distribution shows little overlap with those regions that are known to express receptors for the renin-angiotensin system in the adult brain. This transgenic approach, which demonstrates the distribution of cells which have activated the human renin promoter at any time throughout development, yields a unique and extensive distribution of putative renin-expressing neurons. Our observations suggest that renin may have broader actions in the brain and may indicate a potential for interaction with the (pro)renin receptor or production of a ligand for non-AT(1)/AT(2) receptors.

Centrally injected angiotensin II trans-synaptically activates angiotensin II-sensitive neurons in the anterior hypothalamic area of rats

Previously, we have demonstrated that pressure-ejected application of angiotensin II onto some neurons in the anterior hypothalamic area (AHA) of rats increases their firing rate. In contrast, pressure application of the angiotensin AT1 receptor antagonist losartan onto AHA neurons blocked the basal firing of the neurons. To investigate possible participation of these AHA neurons in the brain angiotensin system, we examined whether intracerebroventricular injection of angiotensin II results in an activation of angiotensin II-sensitive neurons in the AHA of rats. Intracerebroventricular injection of angiotensin II increased the firing rate of AHA angiotensin II-sensitive neurons. The angiotensin II-induced increase of unit firing in AHA neurons was abolished by pressure application of losartan onto the same neurons. In addition, the angiotensin II-induced increase of firing in AHA neurons was abolished by pressure application of N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W7), a calmodulin inhibitor, onto the same neurons. Pressure application of W7 onto AHA neurons affected neither the basal firing rate nor the increase in unit firing induced by pressure application of angiotensin II onto the same neurons. Intracerebroventricular injection of the cholinergic agonist carbachol did not affect the firing rate of angiotensin II-sensitive neurons in the AHA. These findings suggest that intracerebroventricular injection of angiotensin II activates AHA angiotensin II-sensitive neurons via angiotensinergic inputs to the neurons.

Tonic angiotensinergic inputs to neurons in the anterior hypothalamic area of rats

We have previously reported that microinjection of angiotensin II into the anterior hypothalamic area (AHA) produces a pressor response in rats and that the angiotensin AT1 receptor antagonist, losartan, similarly injected causes a depressor response in hypertensive rats. In this study, we examined whether endogenous angiotensins are involved in activation of neurons in the AHA. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Pressure-ejected application of angiotensin II and glutamate onto some neurons in the AHA increased their firing rate. The increase of unit firing induced by angiotensin II but not by glutamate was inhibited by losartan. Application of losartan alone inhibited the basal firing rate of angiotensin II-sensitive neurons in a concentration-dependent manner. Application of the angiotensin AT2 receptor antagonist, PD123319, did not affect the increase of unit firing induced by angiotensin II and the basal firing rate of angiotensin II-sensitive neurons. Pressure application of angiotensin I onto angiotensin II-sensitive neurons also increased firing rate and the increase of unit firing by angiotensin I was inhibited by the angiotensin converting enzyme inhibitor, captopril. Captopril alone inhibited the basal firing rate of angitensin II-sensitive neurons. Acetylcholine did not affect unit firing of angiotensin II-sensitive neurons, whereas it increased the firing rate of some angiotensin II-insensitive neurons in the AHA. Increases of blood pressure by intravenous phenylephrine completely inhibited the basal firing rate of angiotensin II-sensitive neurons. These findings suggest that some neurons in the AHA are tonically activated by endogenous angiotensins. It seems likely that newly synthesized angiotensins are used for the angiotensinergic transmission in the AHA.

Renin antisense injected intraventricularly decreases blood pressure in spontaneously hypertensive rats

Brain renin-angiotensin system plays an important role in blood pressure regulation and is suggested to play a role in the development and maintenance of hypertension. To test the hypothesis that brain renin may play a significant role in hypertension in spontaneously hypertensive rats (SHR), phosphorothioated antisense oligodeoxynucleotides targeted to renin mRNA were administered intracerebroventricularly in SHR. Administration of an antisense but not its sense oligodeoxynucleotide produced a prolonged duration of decrease in blood pressure. Intra-arterial administration of the antisense oligodeoxynucleotide at the same dose that decreased blood pressure when administered intraventricularly did not affect blood pressure. Furthermore, renin mRNA but not angiotensin AT1 receptor mRNA levels were decreased in the hypothalamus of the antisense oligodeoxynucleotide-treated rats. These results suggest that brain renin may play a significant role in hypertension in SHR.

Expression of renin in the rat liver

Renin is well-known to be a trigger enzyme in the renin-angiotensin system (RAS). In contrast to the classical RAS, the local RAS has recently been noted in several tissues. The local RAS has a function independent from that of the classical RAS, although its physiological principles are not well known. In the present study, we immunohistochemically demonstrated that hepatocytes in the rat express renin. No renin-immunoreactive cells were detected in rat liver at 0 min after death. At 15 min after death, a small number of renin-positive cells was demonstrated in the lamina hepatica, and they increased with time to the end of observation. Immunoreactivity for renin was scarce throughout the cytoplasm, sometimes condensed to below the cell membrane and around the intracellular granules. Histoplanimetrically, the values from 15 min to 120 min after death were significantly different from that at 0 min after death. Hybridohistochemistry revealed no hybrid signals throughout the liver at either 0 min or 30 min, although renin-immunoreactivity was clearly demonstrated in adjacent sections of liver at 30 min after death. In RT-PCR, the radioactivities in kidney and liver at 0 min after death were not different from those at 30 min after death, respectively. These results suggest the existence of hepatic renin in the rat.

Expression of renin in coagulating glands is regulated by testosterone

BACKGROUND

The presence of extrarenal or local renin-angiotensin system (RAS) has been noted in several tissues, although its functions have not yet been clarified. Renin from the coagulating gland (CG) is the most recently discovered local RAS and is a significant subject for investigation because large amounts of both mRNA and proteins are detected in this organ. Recently, it has been reported that testosterone influences renin synthesis in several extrarenal tissues, although it has no effect on intrarenal renin. Therefore, it is possible that CG renin is also regulated by testosterone.

METHODS

Forty-four male C57BL/6 mice, aged 3 wk to 6 mo, were used in studies on the ontogeny and androgen regulation of the RAS in the CG. The tissues were fixed with Bouin's solution and paraffin sections were stained with immunohistochemical methods using antirenin antiserum. In each immunostained section, the relative number of renin-containing cells in terminal portions of the CG were counted.

RESULTS

Immunoreactivity for renin was first detected at 6 wk after birth. After that time, the number of renin-containing cells gradually increased throughout the experiment. In adults, several patterns of renin immunoreactivity were demonstrated in almost all epithelial cells of CGs, specifically; (1) basolateral granular reaction, (2) diffuse immunoreactivity throughout the cytoplasm, and (3) restricted nuclear reaction. Excretory products of some terminal lumina were also found to be positive for renin. At 10 days after castration, renin-containing cells in ductal termini were decreased and remained at low levels until at 4 wk after castration. After testosterone injection, numerical values of renin-containing cells were high at 1 wk and then decreased at 2-3 wk.

CONCLUSION

It is suggested that CG renin of the mouse is expressed together with sexual maturation during development and that it depends on the testis, possibly the male sex hormone.

Co-existence of renin-like immunoreactivity in the rat maternal and fetal neocortex

Renin-like immunoreactive material was examined in maternal and fetal brain. A continuous layer of renin was localized in neocortex which begins in the fetal brain during gestation and continues throughout the animal's normal life.

An immunohistochemical study on the embryonic development of renin-containing cells in the mouse and pig

The prenatal occurrence and distribution of renin-containing (RC) cells were investigated immunohistochemically in mouse and pig embryos. The RC cells of the mouse embryo were first observed at the 13th day of gestation at the walls of the renal, the mesonephric, the adrenal, the abdominal arteries, the adrenal glands and the testis. As the gestation of the mouse progressed, the RC cells had a tendency to localize in areas of the vascular pole of the metanephric glomerulus. In pig, when CRL was 0.8-2.0 cm, RC cells first appeared at the ventral walls of the dorsal aorta, the omphalo-mesenteric (i.e., the cranial mesenteric), the mesonephric, the mesonephric afferent glomerular arteries/arterioles and the inside of the mesonephric glomerulus. As the length of the pig embryo increased, no renin-immunoreactivity could be demonstrated at the degenerated mesonephros, while in the metanephros marked immunoreactivities were found only at the terminal regions of intralobular arteries, i.e., afferent arterioles or the vascular pole of the glomerulus.

The renin-angiotensin system in the rat brain. Immunocytochemical localization of angiotensinogen in glial cells and neurons

The distribution of angiotensinogen containing cells was determined in the brain of rats using immunocytochemistry. Specific angiotensinogen immunoreactivity is demonstrated both in glial cells and neurons throughout the brain, except the neocortical and cerebellar territories. Positive neurons are easily and invariably detected in female brains, and haphazardly in male brain (sex hormone dependent). Angiotensinogen immunoreactivity in male brain neurons can be induced by water deprivation or binephrectomy in some areas and particularly in paraventricular nuclei. Finally, the highest concentrations of positive neurons are found in the anterior and lateral hypothalamus, preoptic area, amygdala and some well known nuclei of the mesencephalon and the brainstem. Our results confirm the wide distribution of angiotensinogen mRNA in the brain reported recently by Lynch et al. (1987). Thus the demonstration of angiotensinogen in neurons and glial cells allows a greater understanding of the biochemical and physiological data in accordance with multiple brain renin angiotensin systems.

Presence of renin in primary neuronal and glial cells from rat brain

Immunocytochemical and biochemical techniques have been utilized in the present study to characterize renin in brain cell cultures. With the use of renin-specific antibody, positive renin staining was seen in neuronal and in astrocytic glial cells using the peroxidase-antiperoxidase method. Renin concentration was pH-dependent with highest concentrations at 5.5, decreasing from pH 6.0 to 6.5. At pH 7.4 no renin was detectable in either glial or neuronal cells. The contribution of cathepsin D to the measured renin was about 10% at pH 5.5; 7% at pH 6.0 and 3% at pH 6.5. Comparison of glial with neuronal cells from WKY rats revealed significantly elevated renin at pH 5.5 in glial cells. No difference was seen between glial and neuronal renin levels in WKY rats at pH 6.0 and 6.5. At pH 5.5 and 6.0 renin was significantly increased in neuronal cells of SHR compared to WKY, whereas at pH 6.5 no difference was observed. The renin concentration in cells kept for 2 days in serum-free medium did not differ from those measured in cells kept in serum-containing medium. The generated peptide was identified as [Ile5]Angiotensin I on reversed-phase HPLC.

Cellular organization of the brain renin-angiotensin system

A model of intracellular Ang II formation (Figure 1) implies that angiotensinogen neurons exist and that CNS Ang II acts both as a neurotransmitter as well as a neurohormone. Such a mechanism is consistent with the immunocytochemical localization of a fraction of brain Ang II in neurosecretory vesicles. To date, several dozen peptide neurotransmitters and neurohormones have been studied. Those assigned to peptidergic systems follow the generalized pathway of biosynthesis shown in Figure 1. In peptidergic systems, a prohormone and all of its processing enzymes are synthesized in the rough endoplasmic reticulum of a cell and move into the Golgi apparatus (Figure 1: #1-3). In the Golgi the prohormone and processing enzymes are packaged into the same vesicle (#3). These secretory vesicles then migrate toward the plasma membrane, frequently via axonal or dendritic projections to terminals. Within these cytoplasmic vesicles and prior to release, the processing enzymes are activated (#4) and the prohormone enzymatically processed, yielding the active peptide (#5-6). Only then do the vesicles fuse with the plasma membrane (in a calcium-dependent process), releasing their contents (#7-8). Once released, the active peptide migrates across the extracellular space and interacts with specific cell surface receptors to initiate a response (#9). Finally, receptor-bound peptide degradation is initiated by receptor-mediated endocytosis (#10-11). For angiotensin peptides to be produced intracellularly, the cell must present only one secretory pathway for Golgi packaging of renin and angiotensinogen; otherwise current theories of protein sorting would predict that these two proteins would be segregated even if synthesized within the same cell. Small quantities of co-packaged renin and angiotensinogen occurring via "spill-over" between compartments seems an unsatisfactory process for a regulated hormone system. Figure 2, depicting an extracellular mechanism for producing Ang II in the brain, has also been proposed. The mechanism of extracellular angiotensin formation is consistent with the molecular information encoded within the component proteins, known mechanisms of protein secretio, well-defined systemic renin-angiotensin enzymatic cascades, and demonstration of all the components of the renin-angiotensin system in the extracellular compartments of the brain. This model (Figure 2) allows independently coordinated gene expression and synthesis of renin (#1R), angiotensinogen (#1A), and angiotensin-converting enzyme (# 1C) in the same or different cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of renin (EC 3.4.23) and converting enzyme (EC 3.4.15.1) in nerve endings of rat brain

Synaptosomes and lysosomes of rat brain were separated by differential centrifugation and a two-step gradient centrifugation with colloidal silica-gel (Percoll). The organelles were identified by the measurement of established marker-enzymes and by electronmicroscopy. Renin activity, measured by radioimmunoassay for angiotensin I (ANG I), was localized in the synaptosomes and cathepsin D-activity was found in the lysosomal fraction. Converting-enzyme activity was present in the renin-containing synaptosomes. Part of the brain renin activity could be activated by pre-incubation with trypsin. Affinity chromatography of an organelle-enriched brain fraction was carried out using a caseinyl-sepharose column and resulted in the separation of renin from cathepsin D activity; the renin peak was inhibited by antibodies raised against rat kidney renin. We conclude, that the formation of ANG I and its activation to angiotensin II (ANG II) by converting enzyme is possible in synaptosomes. This adds further evidence to an intraneuronal synthesis of ANG I and ANG II in the brain and is in support of previous results demonstrating an intraneuronal localization of the components of the brain renin-angiotensin system.

Angiotensin I-generating acid endopeptidase activity in neurosecretory vesicles isolated from bovine pituitary

Secretory vesicles purified from the neural and intermediate lobes of the bovine pituitary contain acidic endopeptidases which are capable of converting renin tetradecapeptide (RTD) substrate to Angiotensin I (AI). Preliminary characterization of the neurosecretory vesicle (NSV) endopeptidase showed that it had a pH optimum of 4.0, and unlike renin was inactive at pHs greater than 6.0. It is inhibited by 10(-6) M pepstatin A, but not by PMSF, leupeptin, PMBS, or the specific renin inhibitor H-142. This NSV endopeptidase differed from cathepsin D in that it was unable to degrade alpha-casein, but was quite active in generating AI from RTD (Vmax = 5 moles/g protein/hour). No enzyme activity that could convert AI to Angiotensin II could be detected in the NSVs suggesting that the acidic endopeptidase is involved in processing neurosecretory vesicle proteins other than those associated with the renin angiotensin system in the brain.

Increased concentrations of immunoreactive cathepsin D in supraoptic nucleus of the Brattleboro rat

The distribution of cathepsin D, an aspartyl endopeptidase, was measured in selected, discrete nuclei of the forebrain of the Brattleboro rat by means of microdissection and radioimmunoassay. The results indicate that cathepsin D is widely distributed, but in varying amounts among nuclear groups in this region of the brain. High concentrations were detected in the supraoptic and paraventricular nuclei. In studies of the vasopressin-deficient Brattleboro rat, an increased content of cathepsin D in the supraoptic nucleus was observed compared to the heterozygous control. No differences were detected between homozygous and heterozygous Brattleboro rats in the caudate, medial preoptic, suprachiasmatic or paraventricular nuclei or globus pallidus. These results raise the possibility that brain cathepsin D may be involved in the physiological events related to fluid homeostasis.

The distribution of cathepsin D in rat tissues determined by immunocytochemistry

The distribution of cathepsin D (CD) was surveyed in rat tissues by light microscopic immunocytochemistry. Although immunoreactive CD was detected in all tissues examined, there was a marked difference in the amount in the cytoplasm of different cell types of the same organ. In the retina large amounts of CD were present in the pigment epithelium, ganglion cells, and Müller cells. Moderate to large amounts of CD were also found in neuronal perikarya of the gastrointestinal tract and adrenal medulla; in macrophages in the lung, liver, and spleen; in some secretory cells of the submandibular and lacrimal glands; in parts of renal distal convoluted and collecting tubules; and in the surface transitional epithelium of the calyx, ureter, and urinary bladder. Other cells adjacent to cells containing large amounts of the enzyme had little or no detectable CD themselves. These included hepatocytes, the proximal tubular cells of the kidney, selected cells of the submandibular gland, cells of the zona glomerulosa of the adrenal cortex, and lymphocytes in lymphoid organs. The localization of CD indicates that its degradative effect is exerted preferentially in certain cell types and suggests that physiological influences on CD may have a variety of effects in different organs.

Brain renin

Although the brain contains cathepsins at high concentrations which exhibit a non-specific renin-like activity at acidic pH, the presence of specific renin in the brain has been demonstrated by characterizing its specific properties. Renin was separated from cathepsin by affinity chromatography on casein-Sepharose. Brain renin showed neutral pH optima for the reaction to generate angiotensin I. The presence of inactive prorenin was also found. The isoelectric points of brain renin were significantly lower differences from that of renal or plasma renin. Immunohistochemical studies demonstrated a wide-spread localization of renin in many different regions. Angiotensin II, the final product of the prohormone-to-hormone conversion reaction mediated by renin and angiotensin converting enzyme, was found to exist in the same cell as renin by immunohistochemical studies of brain sections and with cloned and cultured neuroblastoma cells. This is the first demonstration of the mechanism of peptide hormone formation in neuronal cells. Similar intracellular formation was demonstrated in gonadotrophs of adenohypophysis. Coexistence of renin and angiotensin II was demonstrated in some cells. Electrophysiological studies have shown that angiotensin II functions to disinhibit the inhibition of neuronal response to electrical stimuli in the hippocampus.

Brain renin: localization in rat brain synaptosomal fractions

The distribution of brain renin activity was determined in subcellular fractions of rat brain prepared by discontinuous density gradient centrifugation. The highest amounts of brain renin activity occurred in both the light and heavy synaptosomal fractions, while the activity of choline acetyltransferase was elevated only in the light synaptosomal fraction. These results indicate an intraneuronal localization of brain renin.

In vivo activity of purified mouse brain renin

Mouse brain renin and kidney renin were purified by a 3-step procedure: acetone powder extraction. Sephadex G-100 chromatography, and blue agarose affinity chromatography. The latter efficiently separated from cathepsin D-like acid protease activity. Mouse brain renin had an optimum of enzyme activity of pH 7.0. This differed from mouse kidney renin, which had an optimum at pH 8.5. In vitro, brain renin formed angiotensin I from rat plasma angiotensinogen and had no angiotensinase activity. Mouse brain renin was inhibited by monospecific antibodies raised against pure mouse submandibular gland renin. In vivo activity of the enzyme was tested by injection of brain renin into the lateral brain ventricle of rats. This resulted in the formation of angiotensin I from endogenous brain angiotensinogen, in the stimulation of water uptake, and in a long-lasting increase of arterial blood pressure. The latter could be blocked by the competitive angiotensin II receptor antagonist, saralasin. The results showed that brain renin is active under physiological conditions.

Immunocytochemical localization of cathepsin D in rat neural tissue

The localization of cathepsin D (CD) in normal adult rat neural tissue was determined with an indirect immunoperoxidase technique utilizing rabbit anti rat brain CD followed by a horseradish peroxidase conjugate of the Fab portion of goat antirabbit IgG. The immunoreactive enzyme protein was distributed predominantly in a granular pattern, presumably lysosomal, in neurons and choroid plexus epithelium. Smaller amounts were detected in oligodendrocytes and ependymal cells. The neuronal localization included the perikaryon and its processes, was widely distributed, and displayed a range of staining intensities in different anatomical areas. Immunoreactive CD was heavily concentrated in brain stem and spinal cord motoneurons, large neurons of the caudate nucleus, neurons of several nuclear groups, especially the paraventricular and supraoptic, in the hypothalamus, and neurons of superior cervical and dorsal root ganglia. CD was also readily detected in brain stem sensory neurons, pyramidal cells of the hippocampus, inferior olive and Purkinje cells, but was absent or present in very small quantities in the granule cells of the cerebellar cortex, the more superficial layers of the neocortex, and smaller neurons of the caudate nucleus. This distribution suggests that CD may have a major role in specific chemical events in neural functions and peptidergic pathways and could be involved in the alterations of certain neural structures in disease states.

Angiotensin II in the hippocampus. A histochemical and electrophysiological study

The presence of the renin angiotensin system in the hippocampus is shown by immunohistochemistry. Intra- and extra-cellular recordings revealed that angiotensin II and III excite CA 1 pyramidal cells by disinhibition. The effect is antagonized by [Sar1, Thr8]-A II.