Cerebroprotective effects of RAS inhibitors: Beyond their cardio-renal actions

Abstract

Work on the brain renin–angiotensin system has been explored by various researchers and has led to elucidation of its basic physiologies and behavior, including its role in reabsorption and uptake of body fluid, blood pressure maintenance with angiotensin II being its prominent effector. Currently, this system has been implicated for its newly established effects, which are far beyond its cardio-renal effects accounting for maintenance of cerebral blood flow and cerebroprotection, seizure, in the etiology of Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, and bipolar disorder. In this review, we have discussed the distribution of angiotensin receptor subtypes in the central nervous system (CNS) together with enzymatic pathways leading to active angiotensin ligands and its interaction with angiotensin receptor 2 (AT2) and Mas receptors. Secondly, the use of angiotensin analogues (angiotensin converting enzyme inhibitors and AT1 and/or AT2 receptor blockers) in the treatment and management of the CNS disorders mentioned above has been discussed.

Brain renin–angiotensin system

Introduction

Existence of the brain renin–angiotensin system (RAS)/local RAS, despite its presence at the periphery, is claimed and confirmed by several studies.^1,2 These preliminary findings provided a boon and encouraged a number of investigators all over the world to scratch their heads to carry out their work on this proposed brain system. Consequently, further work on the brain RAS was carried out and led to elucidation of basic physiologies, including reabsorption and uptake of body fluid, blood pressure maintenance, and vasopressin release. ³ Angiotensin peptide, specifically angiotensin II (Ang II), induces cerebrovascular remodeling and promotes vascular inflammation and oxidative stress, resulting in dysregulation of cerebral blood flow (CBF).^4,5 Currently, this system has been implicated for its newly established effects, which are far beyond its cardio-renal effects, in several neurodegenerative disorders like Alzheimer’s disease (AD), Parkinson’s disease (PD), multiple sclerosis (MS, autoimmune demyelination), and bipolar disorder (BD) ⁶ . More recently, astrocyte senescence induced by Ang II has been associated with the neurodegenerative disease via increased superoxide production and generation of reactive oxygen species.^7,8 Therefore, establishing possible links between neuronal disease, stroke, cognitive dysfunction, and brain RAS is an utmost requirement for current cerebrovascular disorder.

Distribution of angiotensin receptors in the brain

Three different angiotensin receptors are known to be implicated in various physiologies of the central nervous system (CNS). Both AT1 and AT2 angiotensin receptors, which have equivalent potency and binding efficiency for Ang II, had been identified by the end of the 1980s. AT1 receptors (AT1R) are present more densely than AT2 receptors (AT2R) in brain areas regulating autonomic and hormonal response. In some specific brain areas, AT1R are induced under the influence of reproductive hormones in dopaminergic neurons which are not expressed normally. ⁹ In contrast, AT2R are upregulated under pathological conditions. AT2R are neuronally expressed in the thalamus, hypothalamus, and specific brainstem nuclei, as well as in motor and learning-associated areas. ¹⁰ Furthermore, AT2R are exclusively re-expressed in some pathological conditions, such as neuronal injury and vascular injury.^11,12 AT2R stimulation promotes axonal regeneration in the optic nerve and cell differentiation and regeneration in neuronal tissues. ¹³ AT2R have also been implicated in brain developmental processes; the presence of the AT2R gene in X chromosome is associated with mental retardation in humans. ¹⁴

Recently, AT4 receptors have been identified and characterized as having an identity like that of insulin-regulated aminopeptidase. ¹⁵ Therefore, they both share common competitive inhibitors, i.e. angiotensin AT4 ligand (Ang IV) and insulin-regulated aminopeptidase. ¹⁶ The regional distribution of angiotensin receptor subtypes in the central nervous subtypes is shown in Table 1.

Table 1.

Regional distribution of angiotensin receptor subtypes in the central nervous subtypes.^8,11,17–36

Area	Region	AT1	AT2	AT4
Area postrema		+	–	+
Amygdala	Basolateral nucleus	+	+	+
	Basomedial nucleus	+	–	+
	Central nucleus	+	+	+
	Medial nucleus	+	+	+
	Lateral nucleus	+	–	+
Caudate–putamen		+	–	+
Cerebellum		–	+	+
Cortex	Cingulate cortex	+	+	–
	Entorhinal cortex	+	–	+
	Frontal cortex	–	–	+
	Insular cortex	–	–	+
	Parietal cortex	–	–	+
	Piriform cortex	+	–	+
Globus pallidus		–	–	+
Hippocampus	CA1	+	+	+
	CA2	+	+	+
	CA3	+	+	+
Hypothalamus	Arcuate nucleus	+	–	+
	Dorsomedial hypothalamus	+	–	+
	Median eminence	+	–	–
	Paraventricular nucleus	+	+	+
	Preoptic nucleus	+	–	+
	Periventricular nucleus	+	–	–
	Suprachiasmatic nucleus	+	–	+
	Supraoptic nucleus	+	+	+
	Ventromedial hypothalamus	–	–	+
Lateral septal area		–	+	+
Medial septal area		+	–	+
Mesencephalon	Inferior colliculus	–	+	+
	Locus coeruleus	+	+	+
	Periaqueductal	–	–	+
	Pontine reticular nucleus	+	–	–
	Substantia nigra	+	+	+
	Superior colliculus	+	+	+
Thalamus	Anterior thalamus	+	+	+
	Centromedial thalamus	–	+	+
	Habenula	–	+	+
	Lateral geniculate nucleus	–	+	+
	Mediodorsal thalamus	–	+	+
	Medial geniculate nucleus	–	+	–
	Reticular nucleus	–	+	–
	Ventrolateral thalamus	–	–	+
	Ventroposterior thalamus	–	+	+

Angiotensins and neuronal disorders

Alzheimer’s disease

Accumulation of extracellular amyloid-β protein (Aβ) aggregates (both Aβ1–40 and Aβ1–42) and the intracellular neuritic plaques are considered as major pathological hallmarks of AD. ²⁶ Recently, investigators have identified a possible interconnection between learning, memory, and the brain RAS. ²⁷ Earlier, AT1R were known for mediating systemic blood pressure and had been considered as a major target for treating AD. Drugs falling under the category of angiotensin converting enzyme (ACE) inhibitors, i.e. captopril, enalapril, lisinopril, and perindopril, were the first drugs to be developed for the treatment of hypertension to date and structured correctly. These drugs act by producing a significant reduction in the conversion of Ang I to Ang II, ultimately leading to the decreased synthesis of Ang II which is associated with fall in blood pressure, decreased amount of acetylcholine (Ach) release, and elevated levels of substance P, a substrate of ACE which is reported to facilitate neprilysin activity, a well-known amyloid β degrading enzyme. ²⁸ However, many investigators have clearly indicated that Ang I concentration is increased with positive synthesis of Ang(1–9), Ang(1–7), Ang(2–7), and Ang(3–7) in different pathogenic conditions,^28–30 as shown in Figure 1. Further, the studies by Bartus et al. in 1982 and Barnes et al. in 1992 claimed that Ach release is hindered by elevated brain angiotensin levels,^31,32 which in turn hamper the normal learning and memory process, thus interfering with cognitive processing. This hypothesis is further supported by the fact that ACE inhibitors could assist the learning and memory process by reducing the synthesis of Ang II, thus removing a hindering barrier on Ach release. ³³ Therefore, ACE inhibitors appear to impact the central RAS together with their effect on the peripheral RAS. ³⁴ Memory-enhancing effects of captopril are more particularly on short-term memory, assessed through Y-maze, than on long-term memory. Results of this study demonstrated that ACE inhibitors had shown significant increase in both antioxidant enzyme levels, i.e. superoxide dismutase (SOD) and glutathione peroxidase, and depleted the levels of reactive oxygen species by reducing the levels of oxidative stress markers. As seen in rat hippocampus, angiotensin receptor blockers (losartan and PD-123177) have also shown similar effects on memory. ⁸ Decreased events or incidences of vascular dementia occurrence following hemorrhagic conditions or ischemic cerebrovascular accidents and improvement in cognitive function are correlated with the use of ACE inhibitors. ³⁵ Hu et al. reported that ACE inhibited extracellular accumulation of β-amyloid protein, deposition, and fibril formation. ³⁶ Normal rat cells were pre-incubated with β-amyloid, so consequently there was marked occurrence of β-amyloid-induced toxic effects on these rat cells; however, these effects could be suppressed with ACE. The application of an ACE inhibitor (lisinopril) has shown contrary results and was found to block this ACE protective influence. ³⁶ Supporting this hypothesis, Oba et al. found that captopril also had interfered with ACE inhibition of β-amyloid mediated toxic effects via cytotoxicity of rat cells. Angiotensin receptor blockers (ARBs) are associated with brain RAS inhibiting activity thus are involved in preventing onset of AD. ³⁷ Considering valsartan as an ARB, it was able to prevent onset of AD by attenuating oligomerization of amyloid-β protein into high-molecular weight oligomeric Aβ aggregates. ³⁶ Other ARBs which have been proven for their anti-amyloidogenic activity are telmisartan, losartan, candesartan, and olmesartan; hence, they all find there use in prevention of cognitive decline in AD.^38–42 An excessive level of brain Ang II activity negatively affects cognition and memory. Ang II, via AT1R stimulation, inhibits long-term potentiation in the hippocampus, which is indirectly linked with the inhibition of the cholinergic system, a major mechanism required for the maintenance of cognitive functions. ⁴³

Figure 1.

Release of angiotensinogen from liver; formation of various angiotensin ligands and its effect on AT2 receptor and Mas receptor in cerebroprotection.

Parkinson’s disease

Allen and his colleagues in 1992 established a relationship between the brain RAS and PD occurrence. ²¹ As described above, for the past two decades it has been shown that in addition to the presence of RAS at periphery, i.e. “classical” humoral RAS, there is the existence of a central RAS/second RAS or local RAS, including its presence in brain tissue.^44,45 It has been confirmed that any modulation in brain RAS with regard to its dopaminergic function may lead to PD. Therefore, the therapeutic interventions or strategies aimed to act via manipulation of brain RAS must be clarified and justified. In renal cells, both systems, i.e. dopamine and angiotensin systems, counterbalance each other for proper functioning.^46–48 It has been suggested that degeneration of dopaminergic neurons by 6-hydroxydopamine or 1-methyl-4-phenyl-1, 2,3,6-tetrahydropyridine (MPTP) caused a marked increase in AT receptor expression, which activate the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity, NF-kB signaling pathway for the production of free radicals, reactive oxygen species, and inflammatory mediators (chemokine, cytokines, and adhesion molecules).^49–51 However, these decreased levels of dopamine in substantia nigra can be compensated by the use of D1 and D2 agonists, which causes a decrease in expression of AT1 and an increase in dopamine level. ⁵² Further, it has been reported that treatment with ACE inhibitors (perindopril) has shown significant improvement in motor responses in a rat model. ⁵³ Combined receptor antagonism with losartan and PD123319 has shown beneficial effects on in vitro genetic model of PD, having raised content of α-synuclein and over-expression of the human neuroglioma H4 cell line. ⁵⁴ In PD, Ang II produced therapeutic action via AT1 R, which is considered as one of the most important known inducers of neuroinflammation and oxidative stress by activation of the reduced NADPH–oxidase complex. AT1 and AT2 receptors have been located in dopamine (DA) neurons, microglia, and astrocytes. ¹⁰ Evidence also suggests that DA cell loss induced by DA neurotoxins is further enhanced by Ang II via AT1, activation of the microglial NADPH-complex, and increased glial inflammatory response. These inflammatory responses and DA neuronal vulnerability were found to be inhibited by the AT1 antagonist candesartan.

Epilepsy

Earlier, in late 1980s it has been established and indicated that Ang II causes a marked increase in seizure threshold, stimulated by pentylenetetrazol, bicuculline, or picrotoxin-induced seizures in mice.^55,56 In more specific terms, upregulation of AT1R, and its mRNA expression in rat hippocampus and cortex, has been found in patients with temporal lobe epilepsy (TLE), whereas an increased expression of AT2R was seen only in the hippocampus without alteration in its mRNA levels. ⁵⁷ Ang IV analogues can be used for ameliorating the defects associated with TLE, i.e. dysfunctioning in metabolism of neurons, which is facilitated by neuronal glucose uptake through the glucose GLUT4. ⁷ Drug resistance in TLE can be investigated by the pilocarpine model. Elevated levels of dopamine as well as serotonin were seen in rat hippocampus via intracerebroventricular infusion of Ang IV or somatostatin-14 and protection against seizures induced by pilocarpine. However, these protective effects could be reversed or blocked by a somatostatin receptor antagonist, cyanamid 154806, supporting the fact that anticonvulsant effects produced by the Ang IV are primarily due to the activation of somatostatin receptor-2. ¹⁵

Stroke

Stroke is considered as one of the leading causes of death worldwide and associated with impaired quality of life as a result of neurological deficit. ⁵⁸ In a broader sense, this disease can be grouped into types: ischemic and hemorrhagic strokes. Ischemic stroke is a condition where the blood supply to the brain is reduced by a clot or by a blocked vessel, making oxygen and other nutrients unavailable for brain. ⁵⁹ However, hemorrhagic stroke, also known as cerebral hemorrhage, occurs when a blood vessel within the brain ruptures and releases blood. ⁶⁰ For the past few years, certain new advances have gained a significant importance and attention in emphasizing the role of the RAS in stoke. More recently, the ACE/Ang II/AT1R axis has been illustrated as the most beneficial therapeutic target, suggesting its possible involvement in stroke pathophysiology. Moreover, interaction between Ang-(1–7)/Mas modulates various signaling pathways, such as PI3K (phosphoinositide 3-kinase)/AKT and ERK (extracellular signal- regulated kinase) pathways comprising downstream effectors like NO, FOXO1 (forkhead box O1), and COX-2 (cyclo-oxygenase-2), ⁶¹ as shown in Figure 2. Peptide Ang II levels have been found to be increased in the cortex and hypothalamus and have been uniformly linked with deleterious effects following stroke. ⁶² ARBs have been found to show improvement in ischemic strokes as well as in hemorrhagic strokes by decreasing infarct size or by increasing blood supply to the brain region, in the former case, and decreasing its occurrence in the latter case.^63–70 Various studies have provided evidence that increase in AT2R expression and activation is directly related to its cerebroprotective action in stroke via increased differentiation and regeneration in neurons, thus delivering its neurotrophic action further. This neuroprotective and cerebroprotective action is blocked by selective AT receptor antagonists and ARBs, confirming its possible involvement in stroke.^71–73 As mentioned above, the ACE2/Ang-(1–7)/Mas axis is present in the brain and is considered as a newer therapeutic target for stroke therapy. This has been supported by the fact that activation of this axis opposes or blocks the actions of Ang II via AT1R and was found to be a promising approach for the treatment of hypertension, myocardial infarction, heart failure, and cancer.^74–81 In ischemic stroke, cerebroprotective effects produced by ACE2/Ang-(1–7)/Mas activation is attributed to its anti-inflammatory actions, as illustrated in Figure 2. Inflammation also acts as a central player in pathogenesis of both of ischemic stroke as well as hemorrhagic stroke; in the former case pro-inflammatory cytokines are released from astrocytes, microglia, smooth muscle cells, and endothelial cells.^82,83 Consequently there is increased expression of inducible nitric oxide synthase; however, in the latter case, inflammatory response occurs immediately after hemorrhage and peak levels are attained several days later in humans and animal models.^84–86 During this period, activation of microglia and further release of pro-inflammatory mediators may continue for 1 month. ⁸⁷ This data suggests that the ACE inhibitors together with the ARBs and ATR antagonists are used for the treatment of stroke and have gained much attraction as a potential therapeutic approach. Data obtained from several studies suggest that Ang II may have a cerebroprotective effect, mediated not merely via AT1R but also via AT2R. It has been thought that relative stimulation of AT2R signaling due to increased levels of Ang II accompanied by an ARB treatment might have a possible therapeutic advantage in preventing neurological damage.^45,88,89

Figure 2.

Role of Ang II in BD and in cognitive dysfunctions of AD and PD.

Multiple sclerosis (MS)/autoimmune demyelination

Inflammation, demyelination, and axon degeneration in the CNS are the major events seen and reported in MS, making it a complex autoimmune disease which is characterized by autoreactive immune cells such as T and B cells. Further, the role of T cell responses has been marked for RAS in an autoimmune disease like of MS. ⁹⁰ In addition, it has also been noticed that Ang II induced persistent CNS inflammation through upregulation of transforming growth factor (TGF-β) in an experimental autoimmune encephalomyelitis (EAE) mouse model. ⁹¹ Ang II receptors are expressed on macrophages and T cells, thus inhibiting ACE which produces Ang II which is seen in EAE, probably by the downregulation of AT1 receptor activation. ⁹⁰ Pathogenic condition associated with autoimmune demyelination involves infiltration of macrophages, CD4+ T cells, demyelination in the peripheral white matter of the spinal cord, elevated levels of renin in spleen and spinal cord, and increased expression of AT1aR and AT1bR as well as ACE and ACE2 in peritoneal macrophages and T cells, further causing worsening of the disease. ⁹² These devastating effects could be suppressed and prevented by the use of ACE inhibitors in the following manner. Firstly, by suppression of autoreactive Th1 and Th17 cells. Secondly, they promote CD4 positive FoxP3 positive Treg in an antigen-specific manner. ⁹³ The renin inhibitor aliskiren and AT1R antagonist losartan have also shown disease ameliorating effects, thus finding a potential therapeutic approach for the treatment of autoimmune demyelination.

Bipolar disorder

AT1R and AT2R are two angiotensin receptor subtypes that are involved in mediating the effects of Ang II via inducing G protein- and non-G protein-related signaling pathways, through MAP kinases (ERK1/2, JNK, p38MAPK), ⁹⁴ receptor tyrosine kinases (PDGF, EGFR, insulin receptor), and non-receptor tyrosine kinases (Src, JAK/STAT, focal adhesion kinase (FAK). The signaling pathways are mediated by AT1R and any alterations in these intracellular pathways have been directly or indirectly related to mental disorders. ⁹⁵ In ouabain-induced animal model of mania, tyrosine hydroxylase activation of the ERK1/2 signal pathway was observed. ⁹⁶ It has been well described that the tyrosine kinase signaling pathway regulates the activity of GSK3α/β, an intracellular pathway involved in BD pathophysiology, by its phosphorylation on tyrosine residues. Lithium, a mood stabilizer drug has been found as an effective approach for inhibiting this phosphorylation. ⁹⁶ Furthermore, the AT1R is also involved in mediating the generation and release of inflammatory markers through NAD(P)H oxidase activation is widely implicated in vascular inflammation and fibrosis, ⁹⁷ anxiety, and mood disorders. ⁹⁸ Any disturbance between the AT1R- and AT2R-triggered signals are known to cause or have been implicated in a number of diseases, one of which is hypertension, and mental disorders. ⁹⁹ Data obtained from clinical and pre-clinical studies have accredited the elevated levels of lipid peroxidation products and changes in antioxidant enzymes systems in BD.^99,100 In addition to this rise in tumor necrosis factor a (TNF-a), interleukin-1b (IL-1b) and interleukin 6 (IL-6) levels have also been noticed. ¹⁰¹ A brief pathway depicting the role of Ang II and the release of inflammatory mediators in the brain and their link in BD is shown in Figure 2. The above instances clearly indicate the role of brain RAS in governing the inflammatory and oxidative events in the brain and their contribution in BD pathophysiology. Thus, modulation of brain RAS could be a challenging strategy and may be considered as a beneficial add-on therapy for BD management. Earlier in 1980s, Zubenko and Nixon reported that the ACE inhibitor captopril has shown substantial improvement in mood in three depressed patients. ¹⁰² Research on ACE inhibitors and AT1R blockers against cognitive decline was further carried out and, consequently, their role in prevention, improvement, or even reversal of vascular dementias and AD has been well established. ¹⁰³ Hyperactivation of the hypothalamic–pituitary–adrenocortical (HPA) system/axis is indicated in the pathogenesis of depression and BD and it is assumed that ACE inhibitors have a profound influence on inhibiting the hyperactivity of the HPA axis.^104,105 As described earlier, increased levels of IL-1, IL-6, and TNF-a have been reported in BD and these increased levels of pro-inflammatory mediators activate the tryptophan and serotonin degrading enzyme, indoleamine 2–3 dioxygenase (IDO), in the plasma of bipolar patients. ¹⁰⁶ Valsartan, an AT1R blocker, exhibited neuroprotective effects via suppression of oxidative stress and mitochondrial injury. Moreover, numerous antioxidant drugs and mitochondrial modulators exhibit mood stabilizer properties.^107–109 Thus, ACE inhibitors and AT1R antagonists represent a potential target for treatment of BD and depression.

Conclusion

The role of the classical RAS has been well established and defined, involved in mediating and regulating cardiovascular functions like body water balance, resorption, etc. Existence of a RAS in the CNS has led to a better understanding and characterization of newer pharmacological and physiological functions, together with its relevance in various clinical manifestations. In this review we have discussed the disease states associated with the modulations in RAS, primarily those focusing on ACE inhibitors, ARBs, antagonists, and their therapeutic approaches. They have been proposed as an attractive measure in treating neuronal dysfunctions like AD, PD, epilepsy, stroke, MS, and BD. Therefore, they are claimed to be a preventive and useful therapeutic intervention. In stroke therapy, cerebroprotective effects are produced by ACE2/Ang-(1–7)/Mas activation. However, for better understanding of the treatment to be followed for epilepsy more investigations and further research should be required. Based on the enhanced cognition, improvement in cognitive performance is shown by ACE inhibitors competent at crossing the blood–brain barrier. Further, Ang II induces NAD(P)H oxidase activation, which causes an increase in oxidative stress, generation of reactive oxygen species, and release of pro-inflammatory mediators in several cerebrovascular diseases, thus providing evidence for the use of ACE inhibitors and AT1R blockers in BD and providing another tool for their use in other neurodegenerative diseases. From the findings discussed above it can be concluded that ACE inhibitors, ARBs, and antagonists may not modify the underlying cause of the disease, but they may normalize the pathological states or diseased condition, thus bringing a clinical benefit. Therefore, management of the CNS angiotensin system might be considered as one of the beneficial, novel therapeutic approaches in the treatment of neurological diseases and cognitive dysfunctions.