Angiotensin II increases differentiation of dopaminergic neurons from mesencephalic precursors via angiotensin type 2 receptors

Non-HLA angiotensin-type-1 receptor autoantibodies mediate the long-term loss of grafted neurons in Parkinson's disease models

BACKGROUND

Clinical trials have provided evidence that transplants of dopaminergic precursors, which may be replaced by new in vitro stem cell sources, can integrate into the host tissue, and alleviate motor symptoms in Parkinson´s disease (PD). In some patients, deterioration of graft function occurred several months after observing a graft-derived functional improvement. Rejection of peripheral organs was initially related to HLA-specific antibodies. However, the role of non-HLA antibodies is now considered also relevant for rejection. Angiotensin-II type-1 receptor autoantibodies (AT1-AA) act as agonists of the AT1 receptors. AT1-AA are the non-HLA antibodies most widely associated with graft dysfunction or rejection after transplantation of different solid organs and hematopoietic stem cells. However, it is not known about the presence and possible functional effects of AT1-AA in dopaminergic grafts, and the effects of treatment with AT1 receptor blockers (ARBs) such as candesartan on graft survival.

METHODS

In a 6-hydroxydopamine PD rat model, we studied the short-term (10 days)- and long-term (3 months) effects of chronic treatment with the ARB candesartan on survival of grafted dopaminergic neurons and microglial graft infiltration, as well as the effects of dopaminergic denervation and grafting on serum and CSF AT1-AA levels. The expression of AT1 receptors in grafted neurons was determined by laser capture microdissection.

RESULTS

At the early period post-grafting, the number of grafted dopaminergic neurons that survived was not significantly different between treated and untreated hosts (i.e., control rats and rats treated with candesartan), probably because, just after grafting, other deleterious factors are predominant for dopaminergic cell death, such as mechanical trauma, lack of growth factors/nutrients and ischemia. However, several months post-grafting, we observed a significantly higher number of surviving dopaminergic neurons and a higher density of striatal dopaminergic terminals in the candesartan-treated group. For several months, grafted rats showed blood and cerebrospinal fluid levels of AT1-AA higher than normal controls, and also higher AT1-AA levels than non-grafted parkinsonian rats.

CONCLUSIONS

The results suggest the use of ARBs such as candesartan in PD patients, particularly before and after dopaminergic grafts, and the need to monitor AT1-AA levels in PD patients, particularly in those candidates for dopaminergic grafting.

SARS-CoV-2-Mimicking Pseudoviral Particles Accelerate α-Synuclein Aggregation In Vitro

Since the SARS-CoV-2 virus started spreading worldwide, evidence pointed toward an impact of the infection on the nervous system. COVID-19 patients present neurological manifestations and have an increased risk of developing brain-related symptoms in the long term. In fact, evidence in support of the neuroinvasive potential of SARS-CoV-2 has emerged. Considering that viral parkisonism was observed as a consequence of encephalopathies caused by viral infections, it has been already suggested that COVID-19 could affect the dopaminergic neurons and contribute to neurodegeneration in Parkinson's disease (PD), by promoting the formation of amyloid fibrils constituted by the PD-related protein α-synuclein. Here, we observe not only that SARS-CoV-2 viral spike protein and nucleocapsid protein can alone promote α-synuclein aggregation but also that the spike protein organization in a corona shape on the viral envelope may be crucial in triggering fast amyloid fibrils formation, thus possibly contributing to PD pathogenesis.

The Angiotensin AT2 Receptor: From a Binding Site to a Novel Therapeutic Target

Discovered more than 30 years ago, the angiotensin AT2 receptor (AT2R) has evolved from a binding site with unknown function to a firmly established major effector within the protective arm of the renin-angiotensin system (RAS) and a target for new drugs in development. The AT2R represents an endogenous protective mechanism that can be manipulated in the majority of preclinical models to alleviate lung, renal, cardiovascular, metabolic, cutaneous, and neural diseases as well as cancer. This article is a comprehensive review summarizing our current knowledge of the AT2R, from its discovery to its position within the RAS and its overall functions. This is followed by an in-depth look at the characteristics of the AT2R, including its structure, intracellular signaling, homo- and heterodimerization, and expression. AT2R-selective ligands, from endogenous peptides to synthetic peptides and nonpeptide molecules that are used as research tools, are discussed. Finally, we summarize the known physiological roles of the AT2R and its abundant protective effects in multiple experimental disease models and expound on AT2R ligands that are undergoing development for clinical use. The present review highlights the controversial aspects and gaps in our knowledge of this receptor and illuminates future perspectives for AT2R research. SIGNIFICANCE STATEMENT: The angiotensin AT2 receptor (AT2R) is now regarded as a fully functional and important component of the renin-angiotensin system, with the potential of exerting protective actions in a variety of diseases. This review provides an in-depth view of the AT2R, which has progressed from being an enigma to becoming a therapeutic target.

Common Inflammatory Mechanisms in COVID-19 and Parkinson's Diseases: The Role of Microbiome, Pharmabiotics and Postbiotics in Their Prevention

In the last decade, metagenomic studies have shown the key role of the gut microbiome in maintaining immune and neuroendocrine systems. Malfunction of the gut microbiome can induce inflammatory processes, oxidative stress, and cytokine storm. Dysfunction of the gut microbiome can be caused by short-term (virus infection and other infectious diseases) or long-term (environment, nutrition, and stress) factors. Here, we reviewed the inflammation and oxidative stress in neurodegenerative diseases and coronavirus infection (COVID-19). Here, we reviewed the renin-angiotensin-aldosterone system (RAAS) involved in the processes of formation of oxidative stress and inflammation in viral and neurodegenerative diseases. Moreover, the coronavirus uses ACE2 receptors of the RAAS to penetrate human cells. The coronavirus infection can be the trigger for neurodegenerative diseases by dysfunction of the RAAS. Pharmabiotics, postbiotics, and next-generation probiotics, are considered as a means to prevent oxidative stress, inflammatory processes, neurodegenerative and viral diseases through gut microbiome regulation.

The Renin-Angiotensin System Modulates Dopaminergic Neurotransmission: A New Player on the Scene

Parkinson’s disease (PD) is an extrapyramidal disorder characterized by neuronal degeneration in several regions of the peripheral and central nervous systems. It is the second most frequent neurodegenerative disease after Alzheimer’s. It has become a major health problem, affecting 1% of the world population over 60 years old and 3% of people beyond 80 years. The main histological findings are intracellular Lewy bodies composed of misfolded α-synuclein protein aggregates and loss of dopaminergic neurons in the central nervous system. Neuroinflammation, apoptosis, mitochondrial dysfunction, altered calcium homeostasis, abnormal protein degradation, and synaptic pathobiology have been put forward as mechanisms leading to cell death, α-synuclein deposition, or both. A progressive loss of dopaminergic neurons in the substantia nigra late in the neurodegeneration leads to developing motor symptoms like bradykinesia, tremor, and rigidity. The renin–angiotensin system (RAS), which is involved in regulating blood pressure and body fluid balance, also plays other important functions in the brain. The RAS is involved in the autocrine and paracrine regulation of the nigrostriatal dopaminergic synapses. Dopamine depletion, as in PD, increases angiotensin II expression, which stimulates or inhibits dopamine synthesis and is released via AT1 or AT2 receptors. Furthermore, angiotensin II AT1 receptors inhibit D1 receptor activation allosterically. Therefore, the RAS may have an important modulating role in the flow of information from the brain cortex to the basal ganglia. High angiotensin II levels might even aggravate neurodegeneration, activating the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex, which leads to increased reactive oxygen species production.

Contributions by the Brain Renin-Angiotensin System to Memory, Cognition, and Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease characterized by progressive neuron losses in memory-associated brain structures that rob patients of their dignity and quality of life. Five drugs have been approved by the FDA to treat AD but none modify or significantly slow disease progression. New therapies are needed to delay the course of this disease with the ultimate goal of preventing neuron losses and preserving memory functioning. In this review we describe the renin-angiotensin II (AngII) system (RAS) with specific regard to its deleterious contributions to hypertension, facilitation of neuroinflammation and oxidative stress, reduced cerebral blood flow, tissue remodeling, and disruption of memory consolidation and retrieval. There is evidence that components of the RAS, AngIV and Ang(1-7), are positioned to counter such damaging influences and these systems are detailed with the goal of drawing attention to their importance as drug development targets. Ang(1-7) binds at the Mas receptor, while AngIV binds at the AT4 receptor subtype, and these receptor numbers are significantly decreased in AD patients, accompanied by declines in brain aminopeptidases A and N, enzymes essential for the synthesis of AngIV. Potent analogs may be useful to counter these changes and facilitate neuronal functioning and reduce apoptosis in memory associated brain structures of AD patients.

Interaction between Angiotensin Type 1, Type 2, and Mas Receptors to Regulate Adult Neurogenesis in the Brain Ventricular-Subventricular Zone

The renin-angiotensin system (RAS), and particularly its angiotensin type-2 receptors (AT2), have been classically involved in processes of cell proliferation and maturation during development. However, the potential role of RAS in adult neurogenesis in the ventricular-subventricular zone (V-SVZ) and its aging-related alterations have not been investigated. In the present study, we analyzed the role of major RAS receptors on neurogenesis in the V-SVZ of adult mice and rats. In mice, we showed that the increase in proliferation of cells in this neurogenic niche was induced by activation of AT2 receptors but depended partially on the AT2-dependent antagonism of AT1 receptor expression, which restricted proliferation. Furthermore, we observed a functional dependence of AT2 receptor actions on Mas receptors. In rats, where the levels of the AT1 relative to those of AT2 receptor are much lower, pharmacological inhibition of the AT1 receptor alone was sufficient in increasing AT2 receptor levels and proliferation in the V-SVZ. Our data revealed that interactions between RAS receptors play a major role in the regulation of V-SVZ neurogenesis, particularly in proliferation, generation of neuroblasts, and migration to the olfactory bulb, both in young and aged brains, and suggest potential beneficial effects of RAS modulators on neurogenesis.

Four main therapeutic keys for Parkinson's disease: A mini review

OBJECTIVES

Parkinson's disease (PD) is characterized by motor and cognitive dysfunctions. The progressive degeneration of dopamine-producing neurons that are present in the substantia nigra pars compacta (SNpc) has been the main focus of study and PD therapies since ages.

MATERIALS AND METHODS

In this manuscript, a systematic revision of experimental and clinical evidence of PD-associated cell process was conducted.

RESULTS

Classically, the damage in the dopaminergic neuronal circuits of SNpc is favored by reactive oxidative/nitrosative stress, leading to cell death. Interestingly, the therapy for PD has only focused on avoiding the symptom progression but not in finding a complete reversion of the disease. Recent evidence suggests that the renin-angiotensin system imbalance and neuroinflammation are the main keys in the progression of experimental PD.

CONCLUSION

The progression of neurodegeneration in SNpc is due to the complex interaction of multiple processes. In this review, we analyzed the main contribution of four cellular processes and discussed in the perspective of novel experimental approaches.

Angiotensin II induces oxidative stress and upregulates neuroprotective signaling from the NRF2 and KLF9 pathway in dopaminergic cells

Nuclear factor-E2-related factor 2 (NRF2) is a transcription factor that activates the antioxidant cellular defense in response to oxidative stress, leading to neuroprotective effects in Parkinson's disease (PD) models. We have previously shown that Angiotensin II (AngII) induces an increase in reactive oxygen species (ROS) via AngII receptor type 1 and NADPH oxidase (NOX), which may activate the NRF2 pathway. However, controversial data suggest that AngII induces a decrease in NRF2 signaling leading to an increase in oxidative stress. We analyzed the effect of AngII and the dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA) in culture and in vivo, and examined the effects on the expression of NRF2-related genes. Treatment of neuronal cell lines Mes23.5, N27 and SH-SY5Y with AngII, 6-OHDA or a combination of both increased ROS production and reduced cell viability. Simultaneously, these treatments induced an increase in expression in the NRF2-regulated genes heme oxygenase 1 (Hmox1), NAD(P)H quinone dehydrogenase 1 (Nqo1) and Kruppel like factor 9 (Klf9). Moreover, overexpression of KLF9 transcription factor caused a reduction in the production of ROS induced by treatment with AngII or 6-OHDA and improved the survival of these neuronal cells. Rats treated with AngII, 6-OHDA or a combination of both also showed an increased expression of NRF2 related genes and KLF9. In conclusion, our data indicate that AngII induces a damaging effect in neuronal cells, but also acts as a signaling molecule to activate NRF2 and KLF9 neuroprotective pathways in cellular and animal models of PD.

Renin-angiotensin system as a potential target for new therapeutic approaches in Parkinson's disease

INTRODUCTION

Currently, available therapies for Parkinson's disease (PD) are symptomatic. Therefore, the search for neuroprotective drugs remains a top priority. Areas covered: In this review, the potential symptomatic or disease-modifying effect of drugs targeting the Renin-Angiotensin System (RAS) in PD will be explored. Expert opinion: The importance of nigrostriatal local RAS has only begun to be unraveled in the last decades. On one hand, there is a complex feedback cycle between RAS and dopamine (DA). On the other hand, RAS affects dopaminergic neurons vulnerability. Neuroprotective effects in animal PD models have been shown for the angiotensin-converting enzyme (ACE) inhibitors captopril and perindopril, and the AT1 receptor antagonists losartan, candesartan and telmisartan. These effects appear to be mediated by a reduction in the overproduction of reactive oxygen species. In a proof-of-concept, randomized, double-blind, crossover study in PD patients, perindopril enhanced the effect of levodopa without inducing dyskinesias. There has not been any clinical trial exploring the neuroprotective effect of RAS drugs, but one cohort study in hypertensive patients suggested a protective effect of ACE inhibitors on PD risk. RAS is a promising target for symptomatic and neuroprotective therapies in PD. Further studies in PD animal models and patients are warranted.

Brain Renin-Angiotensin System and Microglial Polarization: Implications for Aging and Neurodegeneration

Microglia can transform into proinflammatory/classically activated (M1) or anti-inflammatory/alternatively activated (M2) phenotypes following environmental signals related to physiological conditions or brain lesions. An adequate transition from the M1 (proinflammatory) to M2 (immunoregulatory) phenotype is necessary to counteract brain damage. Several factors involved in microglial polarization have already been identified. However, the effects of the brain renin-angiotensin system (RAS) on microglial polarization are less known. It is well known that there is a “classical” circulating RAS; however, a second RAS (local or tissue RAS) has been observed in many tissues, including brain. The locally formed angiotensin is involved in local pathological changes of these tissues and modulates immune cells, which are equipped with all the components of the RAS. There are also recent data showing that brain RAS plays a major role in microglial polarization. Level of microglial NADPH-oxidase (Nox) activation is a major regulator of the shift between M1/proinflammatory and M2/immunoregulatory microglial phenotypes so that Nox activation promotes the proinflammatory and inhibits the immunoregulatory phenotype. Angiotensin II (Ang II), via its type 1 receptor (AT1), is a major activator of the NADPH-oxidase complex, leading to pro-oxidative and pro-inflammatory effects. However, these effects are counteracted by a RAS opposite arm constituted by Angiotensin II/AT2 receptor signaling and Angiotensin 1–7/Mas receptor (MasR) signaling. In addition, activation of prorenin-renin receptors may contribute to activation of the proinflammatory phenotype. Aged brains showed upregulation of AT1 and downregulation of AT2 receptor expression, which may contribute to a pro-oxidative pro-inflammatory state and the increase in neuron vulnerability. Several recent studies have shown interactions between the brain RAS and different factors involved in microglial polarization, such as estrogens, Rho kinase (ROCK), insulin-like growth factor-1 (IGF-1), tumor necrosis factor α (TNF)-α, iron, peroxisome proliferator-activated receptor gamma, and toll-like receptors (TLRs). Metabolic reprogramming has recently been involved in the regulation of the neuroinflammatory response. Interestingly, we have recently observed a mitochondrial RAS, which is altered in aged brains. In conclusion, dysregulation of brain RAS plays a major role in aging-related changes and neurodegeneration by exacerbation of oxidative stress (OS) and neuroinflammation, which may be attenuated by pharmacological manipulation of RAS components.

Menopause and Parkinson's disease. Interaction between estrogens and brain renin-angiotensin system in dopaminergic degeneration

The neuroprotective effects of menopausal hormonal therapy in Parkinson's disease (PD) have not yet been clarified, and it is controversial whether there is a critical period for neuroprotection. Studies in animal models and clinical and epidemiological studies indicate that estrogens induce dopaminergic neuroprotection. Recent studies suggest that inhibition of the brain renin-angiotensin system (RAS) mediates the effects of estrogens in PD models. In the substantia nigra, ovariectomy induces a decrease in levels of estrogen receptor-α (ER-α) and increases angiotensin activity, NADPH-oxidase activity and expression of neuroinflammatory markers, which are regulated by estrogen replacement therapy. There is a critical period for the neuroprotective effect of estrogen replacement therapy, and local ER-α and RAS play a major role. Astrocytes play a major role in ER-α-induced regulation of local RAS, but neurons and microglia are also involved. Interestingly, treatment with angiotensin receptor antagonists after the critical period induced neuroprotection.

Dopamine Receptors and Neurodegeneration

Dopamine (DA) is one of the major neurotransmitters and participates in a number of functions such as motor coordination, emotions, memory, reward mechanism, neuroendocrine regulation etc. DA exerts its effects through five DA receptors that are subdivided in 2 families: D1-like DA receptors (D1 and D5) and the D2-like (D2, D3 and D4). All DA receptors are widely expressed in the central nervous system (CNS) and play an important role in not only in physiological conditions but also pathological scenarios. Abnormalities in the DAergic system and its receptors in the basal ganglia structures are the basis Parkinson's disease (PD), however DA also participates in other neurodegenerative disorders such as Huntington disease (HD) and multiple sclerosis (MS). Under pathological conditions reorganization of DAergic system has been observed and most of the times, those changes occur as a mechanism of compensation, but in some cases contributes to worsening the alterations. Here we review the changes that occur on DA transmission and DA receptors (DARs) at both levels expression and signals transduction pathways as a result of neurotoxicity, inflammation and in neurodegenerative processes. The better understanding of the role of DA receptors in neuropathological conditions is crucial for development of novel therapeutic approaches to treat alterations related to neurodegenerative diseases.

Angiotensin II type 2 receptor signaling affects dopamine levels in the brain and prevents binge eating disorder

INTRODUCTION

Binge eating disorder (BED) is associated with dopaminergic activation as food reward, resulting in metabolism-related disorders. Stimulation of angiotensin type 2 (AT2) receptor is reported to inhibit dopamine synthesis. We investigated the possible roles of AT2 receptor-mediated dopamine regulation in the pathogenesis of BED.

MATERIALS AND METHODS

Male C57BL/6 mice, type 2 diabetic (KKAy) mice and AT2 receptor-null (AT2KO) mice at eight weeks old were treated with AT2 receptor agonist, compound 21 (C21) or saline for two weeks. Mice were subjected to fasting for two days followed by re-feeding for seven days.

RESULTS

Treatment with C21 attenuated the rebound proportion of body weight, food intake and water intake in KKAy mice, but not in C57BL/6 and AT2KO mice. Dopamine concentration in the striatum was further increased by fasting in KKAy and AT2KO mice. Administration of C21 significantly attenuated this fasting-induced increase in dopamine level only in KKAy mice. Dopamine receptor D1, D2 expression in the substantia nigra were markedly lower in KKAy mice compared with C57BL/6 mice, while administration of C21 increased their expression in KKAy mice.

CONCLUSIONS

Our study suggests that AT2 receptor stimulation may be a new therapeutic approach to improve eating disorder associated with dopamine resistance.

Loss of angiotensin II receptor expression in dopamine neurons in Parkinson's disease correlates with pathological progression and is accompanied by increases in Nox4- and 8-OH guanosine-related nucleic acid oxidation and caspase-3 activation

Background

In rodent models of Parkinson’s disease (PD), dopamine neuron loss is accompanied by increased expression of angiotensin II (AngII), its type 1 receptor (AT1), and NADPH oxidase (Nox) in the nigral dopamine neurons and microglia. AT1 blockers (ARBs) stymie such oxidative damage and neuron loss. Whether changes in the AngII/AT1/Nox4 axis contribute to Parkinson neuropathogenesis is unknown. Here, we studied the distribution of AT1 and Nox4 in dopamine neurons in two nigral subregions: the less affected calbindin-rich matrix and the first-affected calbindin-poor nigrosome 1 of three patients, who were clinically asymptomatic, but had nigral dopamine cell loss and Braak stages consistent with a neuropathological diagnosis of PD (prePD). For comparison, five clinically- and neuropathologically-confirmed PD patients and seven age-matched control patients (AMC) were examined.

Results

AT1 and Nox4 immunoreactivity was noted in dopamine neurons in both the matrix and the nigrosome 1. The total cellular levels of AT1 in surviving dopamine neurons in the matrix and nigrosome 1 declined from AMC>prePD>PD, suggesting that an AngII/AT1/Nox4 axis orders neurodegenerative progression. In this vein, the loss of dopamine neurons was paralleled by a decline in total AT1 per surviving dopamine neuron. Similarly, AT1 in the nuclei of surviving neurons in the nigral matrix declined with disease progression, i.e., AMC>prePD>PD. In contrast, in nigrosome 1, the expression of nuclear AT1 was unaffected and similar in all groups. The ratio of nuclear AT1 to total AT1 (nuclear + cytoplasmic + membrane) in dopamine neurons increased stepwise from AMC to prePD to PD. The proportional increase in nuclear AT1 in dopamine neurons in nigrosome 1 of prePD and PD patients was accompanied by elevated nuclear expression of Nox4, oxidative damage to DNA, and caspase-3-mediated cell loss.

Conclusions

Our observations are consistent with the idea that AngII/AT1/Nox4 axis-mediated oxidative stress gives rise to the dopamine neuron dysfunction and loss characteristic of the neuropathological and clinical manifestations of PD and suggest that the chance for a neuron to survive increases in association with lower total as well as nuclear AT1 expression. Our results support the need for further evaluation of ARBs as disease-modifying agents in PD.

The development of small molecule angiotensin IV analogs to treat Alzheimer's and Parkinson's diseases

Alzheimer's (AD) and Parkinson's (PD) diseases are neurodegenerative diseases presently without effective drug treatments. AD is characterized by general cognitive impairment, difficulties with memory consolidation and retrieval, and with advanced stages episodes of agitation and anger. AD is increasing in frequency as life expectancy increases. Present FDA approved medications do little to slow disease progression and none address the underlying progressive loss of synaptic connections and neurons. New drug design approaches are needed beyond cholinesterase inhibitors and N-methyl-d-aspartate receptor antagonists. Patients with PD experience the symptomatic triad of bradykinesis, tremor-at-rest, and rigidity with the possibility of additional non-motor symptoms including sleep disturbances, depression, dementia, and autonomic nervous system failure. This review summarizes available information regarding the role of the brain renin-angiotensin system (RAS) in learning and memory and motor functions, with particular emphasis on research results suggesting a link between angiotensin IV (AngIV) interacting with the AT4 receptor subtype. Currently there is controversy over the identity of this AT4 receptor protein. Albiston and colleagues have offered convincing evidence that it is the insulin-regulated aminopeptidase (IRAP). Recently members of our laboratory have presented evidence that the brain AngIV/AT4 receptor system coincides with the brain hepatocyte growth factor/c-Met receptor system. In an effort to resolve this issue we have synthesized a number of small molecule AngIV-based compounds that are metabolically stable, penetrate the blood-brain barrier, and facilitate compromised memory and motor systems. These research efforts are described along with details concerning a recently synthesized molecule, Dihexa that shows promise in overcoming memory and motor dysfunctions by augmenting synaptic connectivity via the formation of new functional synapses.

Angiotensin AT2 receptor stimulation inhibits activation of NADPH oxidase and ameliorates oxidative stress in rotenone model of Parkinson's disease in CATH.a cells

Oxidative stress has long been considered as a major contributing factor in the pathogenesis of Parkinson's disease (PD). The brain has an independent local renin-angiotensin system (RAS). Angiotensin II (Ang II) activates NADPH-dependent oxidases, which are a major source of superoxide and are upregulated in major aging-related diseases such as hypertension and neurodegenerative disease. In this study, we firstly examined whether CGP42112, an AT2 receptor (AT2R) agonist, may exert direct protective effects on the rotenone-induced CATH.a cell injury in vitro. We used CATH.a cell line to evaluate changes in cultured dopaminergic neuron levels of superoxide dismutase (SOD), glutathione (GSH) and reactive oxygen species (ROS). We also evaluated expression of NADPH oxidase, AT1 and AT2 receptors in treated with phosphate buffer saline (PBS), rotenone, Ang II, AT2R agonist CGP42112, or AT2R antagonist PD123319, alone and combined (n=6, each group). Quantitative reverse transcriptase PCR (qRT-PCR) and western blot were used to determine messenger RNA (mRNA) and protein levels of the AT1, AT2 receptors and NADPH oxidase. ROS generation was determined by the dichlorodihydrofluorescein diacetate fluorescent probe assay. The levels of SOD and GSH were measured by using available kits. In our study, CGP42112 (100nM) significantly reduced rotenone-induced oxidative stress and elevated the total SOD activity and GSH level. In addition, CGP42112 significantly increased AT2R expression and attenuated Ang II-induced NADPH oxidase activation, and these effects were completely abolished by the AT2R antagonist, PD123319 (1μM). Our results suggest that CGP42112 attenuates rotenone-induced oxidative stress in CATH.a neuron via activating AT2R and suppressing NADPH oxidase expression.

Rho Kinase and Dopaminergic Degeneration

The small GTP-binding protein Rho plays an important role in several cellular functions. RhoA, which is a member of the Rho family, initiates cellular processes that act on its direct downstream effector Rho-associated kinase (ROCK). ROCK inhibition protects against dopaminergic cell death induced by dopaminergic neurotoxins. It has been suggested that ROCK inhibition activates neuroprotective survival cascades in dopaminergic neurons. Axon-stabilizing effects in damaged neurons may represent another mechanism of neuroprotection of dopaminergic neurons by ROCK inhibition. However, it has been shown that microglial cells play a crucial role in neuroprotection by ROCK inhibition and that activation of microglial ROCK mediates major components of the microglial inflammatory response. Additional mechanisms such as interaction with autophagy may also contribute to the neuroprotective effects of ROCK inhibition. Interestingly, ROCK interacts with several brain factors that play a major role in dopaminergic neuron vulnerability such as NADPH-oxidase, angiotensin, and estrogen. ROCK inhibition may provide a new neuroprotective strategy for Parkinson’s disease. This is of particular interest because ROCK inhibitors are currently used against vascular diseases in clinical practice. However, it is necessary to develop more potent and selective ROCK inhibitors to reduce side effects and enhance the efficacy.

Brain renin-angiotensin system and dopaminergic cell vulnerability

Although the renin-angiotensin system (RAS) was classically considered as a circulating system that regulates blood pressure, many tissues are now known to have a local RAS. Angiotensin, via type 1 receptors, is a major activator of the NADPH-oxidase complex, which mediates several key events in oxidative stress (OS) and inflammatory processes involved in the pathogenesis of major aging-related diseases. Several studies have demonstrated the presence of RAS components in the basal ganglia, and particularly in the nigrostriatal system. In the nigrostriatal system, RAS hyperactivation, via NADPH-oxidase complex activation, exacerbates OS and the microglial inflammatory response and contributes to progression of dopaminergic degeneration, which is inhibited by angiotensin receptor blockers and angiotensin converting enzyme (ACE) inhibitors. Several factors may induce an increase in RAS activity in the dopaminergic system. A decrease in dopaminergic activity induces compensatory upregulation of local RAS function in both dopaminergic neurons and glia. In addition to its role as an essential neurotransmitter, dopamine may also modulate microglial inflammatory responses and neuronal OS via RAS. Important counterregulatory interactions between angiotensin and dopamine have also been observed in several peripheral tissues. Neurotoxins and proinflammatory factors may also act on astrocytes to induce an increase in RAS activity, either independently of or before the loss of dopamine. Consistent with a major role of RAS in dopaminergic vulnerability, increased RAS activity has been observed in the nigra of animal models of aging, menopause and chronic cerebral hypoperfusion, which also showed higher dopaminergic vulnerability. Manipulation of the brain RAS may constitute an effective neuroprotective strategy against dopaminergic vulnerability and progression of Parkinson's disease.

Inhibition of the microglial response is essential for the neuroprotective effects of Rho-kinase inhibitors on MPTP-induced dopaminergic cell death

Several recent studies have shown that activation of the RhoA/Rho-associated kinase (ROCK) pathway is involved in the MPTP-induced dopaminergic cell degeneration and possibly in Parkinson's disease. ROCK inhibitors have been suggested as candidate neuroprotective drugs for Parkinson's disease. However, the mechanism responsible for the increased survival of dopaminergic neurons after treatment with ROCK inhibitors is not clear. We exposed primary (neuron-glia) mesencephalic cultures, cultures of the MES 23.5 dopaminergic neuron cell line and primary mesencephalic cultures lacking microglial cells to the dopaminergic neurotoxin MPP+ and the ROCK inhibitor Y-27632 in order to study the effects of ROCK inhibition on dopaminergic cell loss and the length of neurites of surviving dopaminergic neurons. In primary (neuron-glia) cultures, simultaneous treatment with MPP+ and the ROCK inhibitor significantly reduced the loss of dopaminergic neurons. In the absence of microglia, treatment with the ROCK inhibitor did not induce a significant reduction in the dopaminergic cell loss. Treatment with the ROCK inhibitor induced a significant decrease in axonal retraction in primary cultures with and without microglia and in cultures of the MES 23.5 neuron cell line. In conclusion, inhibition of microglial ROCK is essential for the neuroprotective effects of ROCK inhibitors against cell death induced by the dopaminergic neurotoxin MPP+. In addition, ROCK inhibition induced a direct effect against axonal retraction in surviving neurons. However, the latter effect was not sufficient to cause a significant increase in the survival of dopaminergic neurons after treatment with MPP+.

A Role for the Brain RAS in Alzheimer's and Parkinson's Diseases

The brain renin-angiotensin system (RAS) has available the necessary functional components to produce the active ligands angiotensins II (AngII), angiotensin III, angiotensins (IV), angiotensin (1-7), and angiotensin (3-7). These ligands interact with several receptor proteins including AT1, AT2, AT4, and Mas distributed within the central and peripheral nervous systems as well as local RASs in several organs. This review first describes the enzymatic pathways in place to synthesize these ligands and the binding characteristics of these angiotensin receptor subtypes. We next discuss current hypotheses to explain the disorders of Alzheimer's disease (AD) and Parkinson's disease (PD), as well as research efforts focused on the use of angiotensin converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs), in their treatment. ACE inhibitors and ARBs are showing promise in the treatment of several neurodegenerative pathologies; however, there is a need for the development of analogs capable of penetrating the blood-brain barrier and acting as agonists or antagonists at these receptor sites. AngII and AngIV have been shown to play opposing roles regarding memory acquisition and consolidation in animal models. We discuss the development of efficacious AngIV analogs in the treatment of animal models of AD and PD. These AngIV analogs act via the AT4 receptor subtype which may coincide with the hepatocyte growth factor/c-Met receptor system. Finally, future research directions are described concerning new approaches to the treatment of these two neurological diseases.

Dopamine-angiotensin interactions in the basal ganglia and their relevance for Parkinson's disease

Renin-angiotensin systems are known to act in many tissues, for example, the blood vessel wall or kidney, where a close interaction between angiotensin and dopamine has been demonstrated. Regulatory interactions between the dopaminergic and renin-angiotensin systems have recently been described in the substantia nigra and striatum. In animal models, dopamine depletion induces compensatory overactivation of the local renin-angiotensin system, which primes microglial responses and neuron vulnerability by activating NADPH-oxidase. Hyperactivation of the local renin-angiotensin system exacerbates the inflammatory microglial response, oxidative stress, and dopaminergic degeneration, all of which are inhibited by angiotensin receptor blockers and inhibitors of angiotensin-converting enzymes. In this review we provide evidence suggesting that the renin-angiotensin system may play an important role in dopamine's mediated neuroinflammation and oxidative stress changes in Parkinson's disease. We suggest that manipulating brain angiotensin may constitute an effective neuroprotective strategy for Parkinson's disease.

Nox4-generated superoxide drives angiotensin II-induced neural stem cell proliferation

Reactive oxygen species (ROS) have been reported to affect neural stem cell self-renewal and therefore may be important for normal development and may influence neurodegenerative processes when ROS activity is elevated. To determine if increasing production of superoxide, via activation of NADPH oxidase (Nox), increases neural stem cell proliferation, 100 nM angiotensin II (Ang II) - a strong stimulator of Nox - was applied to cultures of a murine neural stem cell line, C17.2. Twelve hours following a single treatment with Ang II, there was a doubling of the number of neural stem cells. This increase in neural stem cell numbers was preceded by a gradual elevation of superoxide levels (detected by dihydroethidium fluorescence) from the steady state at 0, 5, and 30 min and gradually increasing from 1 h to the maximum at 12 h, and returning to baseline at 24 h. Ang II-dependent proliferation was blocked by the antioxidant N-acetyl-L-cysteine. Confocal microscopy revealed the presence of two sources of intracellular ROS in C17.2 cells: (i) mitochondrial and (ii) extramitochondrial; the latter indicative of the involvement of one or more specific isoforms of Nox. Of the Nox family, mRNA expression for one member, Nox4, is abundant in neural stem cell cultures, and Ang II treatment resulted in elevation of the relative levels of Nox4 protein. SiRNA targeting of Nox4 mRNA reduced both the constitutive and Ang II-induced Nox4 protein levels and attenuated Ang II-driven increases in superoxide levels and stem cell proliferation. Our findings are consistent with our hypothesis that Ang II-induced proliferation of neural stem cells occurs via Nox4-generated superoxide, suggesting that an Ang II/Nox4 axis is an important regulator of neural stem cell self-renewal and as such may fine-tune normal, stress- or disease-modifying neurogenesis.

The Angiotensin II Type 2 Receptor in Brain Functions: An Update

Angiotensin II (Ang II) is the main active product of the renin-angiotensin system (RAS), mediating its action via two major receptors, namely, the Ang II type 1 (AT₁) receptor and the type 2 (AT₂) receptor. Recent results also implicate several other members of the renin-angiotensin system in various aspects of brain functions. The first aim of this paper is to summarize the current state of knowledge regarding the properties and signaling of the AT₂ receptor, its expression in the brain, and its well-established effects. Secondly, we will highlight the potential role of the AT₂ receptor in cognitive function, neurological disorders and in the regulation of appetite and the possible link with development of metabolic disorders. The potential utility of novel nonpeptide selective AT₂ receptor ligands in clarifying potential roles of this receptor in physiology will also be discussed. If confirmed, these new pharmacological tools should help to improve impaired cognitive performance, not only through its action on brain microcirculation and inflammation, but also through more specific effects on neurons. However, the overall physiological relevance of the AT₂ receptor in the brain must also consider the Ang IV/AT₄ receptor.

Importance of the brain Angiotensin system in Parkinson's disease

Parkinson's disease (PD) has become a major health problem affecting 1.5% of the world's population over 65 years of age. As life expectancy has increased so has the occurrence of PD. The primary direct consequence of this disease is the loss of dopaminergic (DA) neurons in the substantia nigra and striatum. As the intensity of motor dysfunction increases, the symptomatic triad of bradykinesia, tremors-at-rest, and rigidity occur. Progressive neurodegeneration may also impact non-DA neurotransmitter systems including cholinergic, noradrenergic, and serotonergic, often leading to the development of depression, sleep disturbances, dementia, and autonomic nervous system failure. L-DOPA is the most efficacious oral delivery treatment for controlling motor symptoms; however, this approach is ineffective regarding nonmotor symptoms. New treatment strategies are needed designed to provide neuroprotection and encourage neurogenesis and synaptogenesis to slow or reverse this disease process. The hepatocyte growth factor (HGF)/c-Met receptor system is a member of the growth factor family and has been shown to protect against degeneration of DA neurons in animal models. Recently, small angiotensin-based blood-brain barrier penetrant mimetics have been developed that activate this HGF/c-Met system. These compounds may offer a new and novel approach to the treatment of Parkinson's disease.

Expression of angiotensinogen and receptors for angiotensin and prorenin in the monkey and human substantia nigra: an intracellular renin-angiotensin system in the nigra

We have previously obtained in rodents a considerable amount of data suggesting a major role for the brain renin–angiotensin system (RAS) in dopaminergic neuron degeneration and potentially in Parkinson’s disease. However, the presence of a local RAS has not been demonstrated in the monkey or the human substantia nigra compacta (SNc). The present study demonstrates the presence of major RAS components in dopaminergic neurons, astrocytes and microglia in both the monkey and the human SNc. Angiotensin type 1 and 2 and renin–prorenin receptors were located at the surface of dopaminergic neurons and glial cells, as expected for a tissular RAS. However, angiotensinogen and receptors for angiotensin and renin–prorenin were also observed at the cytoplasm and nuclear level, which suggests the presence of an intracrine or intracellular RAS in monkey and human SNc. Although astrocytes and microglia were labeled for angiotensin and prorenin receptors in the normal SNc, most glial cells appeared less immunoreactive than the dopaminergic neurons. However, our previous studies in rodent models of PD and studies in other animal models of brain diseases suggest that the RAS activity is significantly upregulated in glial cells in pathological conditions. The present results together with our previous findings in rodents suggest a major role for the nigral RAS in the normal functioning of the dopaminergic neurons, and in the progression of the dopaminergic degeneration.

Genetic polymorphism of angiotensin I-converting enzyme (ACE), but not angiotensin II type I receptor (ATr1), has a gender-specific role in panic disorder

AIMS

Angiotensins were shown to have some role in the development of panic disorder (PD). In this study, we aimed to determine the frequency of polymorphisms in two angiotensin-related genes, angiotensin I-converting enzyme (ACE) and angiotensin II type I receptor (ATr1), in a sample of Turkish patients with PD and to evaluate their association with PD development.

METHODS

Polymerase chain reaction and restriction fragment length polymorphism was used to analyze ATr1 A1166C polymorphism, and only polymerase chain reaction was used to analyze functional ACE insertion/deletion polymorphism in 123 patients with PD and in 169 similarly aged disease-free controls.

RESULTS

 There was no significant difference in the genotype distribution between PD patients and controls for each polymorphism (P>0.05). Allele frequency of ACE insertion/deletion was borderline statistically significant between the groups (P=0.055; odds ratio: 1.39; 95% confidence interval: 0.99-1.95), and allele frequency of ATr1 A1166C was not significantly different between the groups (P=0.32; odds ratio: 0.81; 95% confidence interval: 0.53-1.22).

CONCLUSION

This study suggests that polymorphisms of ACE I/D and ATr1 A1166C are not associated with risk of PD in Turkish patients. However, in ACE insertion/deletion polymorphism, the insertion allele was found to be more frequent in the male subgroup of patients (χ²=4.61, P=0.032) than in controls, suggesting a potential male-specific role of the less active ACE insertion allele in the pathogenesis of PD.

Location of prorenin receptors in primate substantia nigra: effects on dopaminergic cell death

Angiotensin II acts via angiotensin type 1 receptors and is a major inducer of inflammation and oxidative stress. Local renin-angiotensin systems play a major role in the development of age-related disorders in several tissues. These processes are delayed, but not totally abolished, by blockade of angiotensin signaling. A specific receptor for renin and its precursor prorenin has recently been identified. We previously showed that neurotoxin-induced dopaminergic (DA) cell loss is decreased by inhibition of angiotensin receptors, but the location and functional effects of prorenin receptor (PRR) in the brain, including the DA system, are unknown. In the substantia nigra of Macaca fascicularis and in rat primary mesencephalic cultures, double immunofluorescence analysis revealed PRR immunoreactivity in neurons (including DA neurons) and microglia, but not in astrocytes. Administration of the PRR blocker, handle region peptide, led to a significant decrease in 6-hydroxydopamine-induced DA cell death in the cultures,whereas administration of renin with simultaneous blockade of angiotensin receptors led to an increase in 6-hydroxydopamine-induced cell death. These results suggest that active agent angiotensin II-independent PRR intracellular signaling may contribute to exacerbation of DA cell death in vivo. Therefore, potential neuroprotective strategies for DA neurons in Parkinson disease should address both angiotensin and PRR signaling.

Associations between the angiotensin-converting enzyme insertion/deletion polymorphism and monoamine metabolite concentrations in cerebrospinal fluid

Angiotensin II has been suggested to influence central dopamine and serotonin turnover. Since the angiotensin-converting enzyme (ACE) plays a key role in angiotensin regulation by converting inactive angiotensin I to active angiotensin II, we hypothesised that the functional insertion/deletion (I/D) polymorphism in the ACE gene, which has previously been suggested to be associated with, depression and panic disorder, may influence monoamine activity. A well-established technique for assessing brain monoamine turnover in humans is to measure concentrations of monoamine metabolites in the cerebrospinal fluid (CSF). We thus investigated possible associations between the ACE I/D polymorphism and CSF monoamine metabolite concentrations in a population of healthy male subjects. After having found such an association between the ACE I/D polymorphism and CSF levels of the dopamine metabolite homovanillic acid and the serotonin metabolite 5-hydroxyindoleacetic acid in this sample, I carriers displaying lower levels, we tried to replicate this observation in a population of violent male offenders from which also both CSF and DNA were available. Also in this sample, the same associations were found. Our results suggest that the ACE I/D polymorphism may play a role in the modulation of serotonergic and dopaminergic turnover in men.

The renin-angiotensin system in retinal health and disease: Its influence on neurons, glia and the vasculature

Renin-Angiotensin System is classically recognized for its role in the control of systemic blood pressure. However, the retina is recognized to have all the components necessary for angiotensin II formation, suggestive of a role for Angiotensin II in the retina that is independent of the systemic circulation. The most well described effects of Angiotensin II are on the retinal vasculature, with roles in vasoconstriction and angiogenesis. However, it is now emerging that Angiotensin II has roles in modulation of retinal function, possibly in regulating GABAergic amacrine cells. In addition, Angiotensin II is likely to have effects on glia. Angiotensin II has also been implicated in retinal vascular diseases such as Retinopathy of Prematurity and diabetic retinopathty, and more recently actions in choroidal neovascularizaiton and glaucoma have also emerged. The mechanisms by which Angiotensin II promotes angiogensis in retinal vascular diseases is indicative of the complexity of the RAS and the variety of cell types that it effects. Indeed, these diseases are not purely characterized by direct effects of Angiotensin II on the vasculature. In retinopathy of prematurity, for example, blockade of AT1 receptors prevents pathological angiogenesis, but also promotes revascularization of avascular regions of the retina. The primary site of action of Angiotensin II in this disease may be on retinal glia, rather than the vasculature. Indeed, blockade of AT1 receptors prevents glial loss and promotes the re-establishment of normal vessel growth. Blockade of RAS as a treatment for preventing the incidence and progression of diabetic retinopathy has also emerged based on a series of studies in animal models showing that blockade of the RAS prevents the development of a variety of vascular and neuronal deficits in this disease. Importantly these effects may be independent of actions on systemic blood pressure. This has culminated recently with the completion of several large multi-centre clinical trials that showed that blockade of the RAS may be of benefit in some at risk patients with diabetes. With the emergence of novel compounds targeting different aspects of the RAS even more effective ways of blocking the RAS may be possible in the future.

The role of the central renin-angiotensin system in Parkinson's disease

Since the discovery of a renin-angiotensin system (RAS) in the brain, several studies have linked this central RAS to neurological disorders such as ischaemia, Alzheimer's disease and depression. In the last decade, evidence has accumulated that the central RAS might also play a role in Parkinson's disease. Although the exact cause of this progressive neurodegenerative disorder of the basal ganglia remains unidentified, inflammation and oxidative stress have been suggested to be key factors in the pathogenesis and the progression of the disease. Since angiotensin II is a pro-inflammatory compound that can induce the production of reactive oxygen species due to activation of the NADPH-dependent oxidase complex, this peptide might contribute to dopaminergic cell death. In this review, three different strategies to interfere with the pathogenesis or the progression of Parkinson's disease are discussed. They include inhibition of the angiotensin-converting enzyme, blockade of the angiotensin II type 1 receptor and stimulation of the angiotensin II type 2 receptor.

The inflammatory response in the MPTP model of Parkinson's disease is mediated by brain angiotensin: relevance to progression of the disease

The neurotoxin MPTP reproduces most of the biochemical and pathological hallmarks of Parkinson's disease. In addition to reactive oxygen species (ROS) generated as a consequence of mitochondrial complex I inhibition, microglial NADPH-derived ROS play major roles in the toxicity of MPTP. However, the exact mechanism regulating this microglial response remains to be clarified. The peptide angiotensin II (AII), via type 1 receptors (AT1), is one of the most important inflammation and oxidative stress inducers, and produces ROS by activation of the NADPH-oxidase complex. Brain possesses a local angiotensin system, which modulates striatal dopamine (DA) release. However, it is not known if AII plays a major role in microglia-derived oxidative stress and DA degeneration. The present study indicates that in primary mesencephalic cultures, DA degeneration induced by the neurotoxin MPTP/MPP(+) is amplified by AII and inhibited by AT1 receptor antagonists, and that protein kinase C, NADPH-complex activation and microglial activation are involved in this effect. In mice, AT1 receptor antagonists inhibited both DA degeneration and early microglial and NADPH activation. The brain angiotensin system may play a key role in the self-propelling mechanism of Parkinson's disease and constitutes an unexplored target for neuroprotection, as previously reported for vascular diseases.

HMG-CoA reductase inhibitor simvastatin inhibits proinflammatory cytokine production from murine mast cells

BACKGROUND

Statins inhibit 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, a key rate-limiting enzyme in the mevalonate pathway. Accumulating data suggest that statins exhibit anti-inflammatory effects on a number of experimental models including experimental autoimmune encephalomyelitis and antigen-induced allergic airway inflammation. However, the mechanism underlying the anti-inflammatory effect of statins is still largely unknown. In this study, we examined the effect of a representative statin, simvastatin, on proinflammatory cytokine production from murine mast cells.

METHODS

Bone marrow-derived mast cells (BMMCs) were stimulated with lipopolysaccharide (LPS) in the presence or absence of simvastatin, and TNF-alpha and IL-6 production from BMMCs was evaluated at mRNA and protein levels. The effect of simvastatin on the expression of tristetraprolin, an RNA-binding protein that promotes decay of TNF-alpha mRNA, was evaluated.

RESULTS

Incubation of BMMCs with simvastatin resulted in the inhibition of LPS-induced TNF-alpha production at both mRNA and protein levels. Simvastatin also inhibited IL-6 production from LPS-stimulated BMMCs. However, simvastatin did not enhance the expression of tristetraprolin.

CONCLUSIONS

Simvastatin inhibits the production of TNF-alpha and IL-6 from activated mast cells in part by inhibiting de novo synthesis of their transcripts and the inhibition may account for the anti-inflammatory effect of simvastatin.

Brain angiotensin enhances dopaminergic cell death via microglial activation and NADPH-derived ROS

Angiotensin II (AII) plays a major role in the progression of inflammation and NADPH-derived oxidative stress (OS) in several tissues. The brain possesses a local angiotensin system, and OS and inflammation are key factors in the progression of Parkinson's disease. In rat mesencephalic cultures, AII increased 6-OHDA-induced dopaminergic (DA) cell death, generation of superoxide in DA neurons and microglial cells, the expression of NADPH-oxidase mRNA, and the number of reactive microglial cells. These effects were blocked by AII type-1 (AT1) antagonists, NADPH inhibitors, or elimination of glial cells. DA degeneration increased angiotensin converting enzyme activity and AII levels. In rats, 6-OHDA-induced dopaminergic cell loss and microglial activation were reduced by treatment with AT1 antagonists. The present data suggest that AII, via AT1 receptors, increases the dopaminergic degeneration process by amplifying the inflammatory response and intraneuronal levels of OS, and that glial cells play a major role in this process.

Effects of GABA and GABA receptor inhibition on differentiation of mesencephalic precursors into dopaminergic neurons in vitro

Neurotransmitters have been shown to control CNS neurogenesis, and GABA-mediated signaling is thought to be involved in the regulation of nearly all key developmental stages. Generation of dopaminergic (DA) neurons from stem/precursor cells for cell therapy in Parkinson's disease has become a major focus of research. However, the possible effects of GABA on generation of DA neurons from proliferating neurospheres of mesencephalic precursors have not been studied. In the present study, GABA(A), and GABA(B) receptors were found to be located in DA cells. Treatment of cultures with GABA did not cause significant changes in generation of DA cells from precursors. However, treatment with the GABA(A) receptor antagonist bicuculline (10(-5) M) led to a significant increase in the number DA cells, and treatment with the GABA(B) receptor antagonist CGP 55845 (10(-5) M) to a significant decrease. Simultaneous treatment with bicuculline and CGP 55845 did not induce significant changes. Apoptotic cell death studies and bromodeoxyuridine immunohistochemistry indicated that the aforementioned differences in generation of DA neurons are not due to changes in survival or proliferation of DA cells, but rather to increased or decreased differentiation of mesencephalic precursors towards the DA phenotype. The results suggest that these effects are exerted via GABA receptors located on DA precursors, and are not an indirect consequence of effects on the serotonergic or glial cell population. Administration of GABA(A) receptor antagonists in the differentiation medium may help to obtain higher rates of DA neurons for potential use in cell therapy for Parkinson's disease.

Nicotine modulates the renin-angiotensin system of cultured neurons and glial cells from cardiovascular brain areas of Wistar Kyoto and spontaneously hypertensive rats

Considering the importance of the renin-angiotensin system (RAS) for the central control of blood pressure and that nicotine increases the probability of development of hypertension associated to genetic predisposition, our aims are (1) to determine RAS in cultured neurons and glia from the brainstem and hypothalamus of spontaneously hypertensive (SHR) and Wistar Kyoto (WKY) rats; (2) to analyze the possibility of nicotine to interact with brain RAS; and (3) to hypothesize any contribution of nicotine and RAS to the development of neurogenic hypertension. This study demonstrated physiological differences in RAS between cultured neuronal and glial cells from the brainstem and hypothalamus of SHR and WKY neonate rats. Our study also featured evidences of direct modulation of the RAS by nicotine in neurons and glia of brainstem and hypothalamus, which seems to be differential between the two rat strains. Such modulation gives us a clue about the mechanisms possibly involved in the genesis of neurogenic hypertension in vivo, for example, increase in angiotensin II type 1 receptor binding and decrease in angiotensin-converting enzyme 2. In conclusion, we demonstrated that neuronal and glial RAS from the brainstem and hypothalamus of SHR differ from WKY rats and nicotine differentially modulates the brain RAS in SHR and WKY.

Serotonin decreases generation of dopaminergic neurons from mesencephalic precursors via serotonin type 7 and type 4 receptors

Inductive signals mediating the differentiation of neural precursors into serotonergic (5-HT) or dopaminergic neurons have not been clarified. We have recently shown that in cell aggregates obtained from rat mesencephalic precursors, reduction of serotonin levels induces a marked increase in generation of dopaminergic neurons. In the present study we treated rat neurospheres with antagonists of the main subtypes of 5-HT receptors, 5-HT transport inhibitors, or 5-HT receptor agonists, and studied the effects on generation of dopaminergic neurons. Cultures treated with Methiothepin (5-HT(1,2,5,6,7) receptor antagonist), the 5-HT(4) receptor antagonist GR113808;67:00-.or the 5-HT(7) receptor antagonist SB 269970 showed a significant increase in generation of dopaminergic cells. Treatment with the 5-HT(1B/1D) antagonist GR 127935, the 5-HT(2) antagonist Ritanserin, the 5-HT transporter inhibitor Fluoxetine, the dopamine and norepinephrine transport inhibitor GBR 12935, or with both inhibitors together, or 5-HT(4) or 5-HT(7) receptor agonists induced significant decreases in generation of dopaminergic cells. Cultures treated with WAY100635 (5-HT(1A) receptor antagonist), the 5-HT(3) receptor antagonist Ondasetron, or the 5-HT(6) receptor antagonist SB 258585 did not show any significant changes. Therefore, 5-HT(4) and 5-HT(7) receptors are involved in the observed serotonin-induced decrease in generation of dopaminergic neurons from proliferating neurospheres of mesencephalic precursors. 5-HT(4) and 5-HT(7) receptors were found in astrocytes and serotonergic cells using double immunolabeling and laser confocal microscopy, and the glial receptors appeared to play a major role.

Angiotensin Receptor Type 1 Blockade in Astroglia Decreases Hypoxia-Induced Cell Damage and TNF Alpha Release

The present study investigated the role of angiotensin receptors (AT-R) in the survival and inflammatory response of astroglia upon hypoxic injury. Exposure of rat astroglial primary cultures (APC) to hypoxic conditions (HC) led to decreased viability of the cells and to a 3.5-fold increase in TNF-alpha release. AT-R type1 (AT1-R) antagonist losartan and its metabolite EXP3174 decrease the LDH release (by 36 +/- 9%; 45 +/- 6%) from APC under HC. Losartan diminished TNF-alpha release (by 40 +/- 15%) and the number of TUNEL-cells by 204 +/- 38% under HC, alone and together with angiotensin II (ATII), while EXP3174 was dependent on ATII for its effect on TNF-alpha. The AT2-R antagonist, PD123.319, did not influence the release of LDH and TNF-alpha under normoxic (NC) and HC. These data suggest that AT1-R may decrease the susceptibility of astrocytes to hypoxic injury and their propensity to release TNF-alpha. AT1-R antagonists may therefore be of therapeutic value during hypoxia-associated neurodegeneration.

Angiotensin type 1 receptor antagonist losartan, reduces MPTP-induced degeneration of dopaminergic neurons in substantia nigra

Background

Recent attention has focused on understanding the role of the brain-renin-angiotensin-system (RAS) in stroke and neurodegenerative diseases. Direct evidence of a role for the brain-RAS in Parkinson's disease (PD) comes from studies demonstrating the neuroprotective effect of RAS inhibitors in several neurotoxin based PD models. In this study, we show that an antagonist of the angiotensin II (Ang II) type 1 (AT₁) receptor, losartan, protects dopaminergic (DA) neurons against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity both in primary ventral mesencephalic (VM) cultures as well as in the substantia nigra pars compacta (SNpc) of C57BL/6 mice (Fig. 1).

Results

In the presence of exogenous Ang II, losartan reduced MPP⁺ (5 μM) induced DA neuronal loss by 72% in vitro. Mice challenged with MPTP showed a 62% reduction in the number of DA neurons in the SNpc and a 71% decrease in tyrosine hydroxylase (TH) immunostaining of the striatum, whereas daily treatment with losartan lessened MPTP-induced loss of DA neurons to 25% and reduced the decrease in striatal TH⁺ immunostaining to 34% of control.

Conclusion

Our study demonstrates that the brain-RAS plays an important neuroprotective role in the MPTP model of PD and points to AT₁ receptor as a potential novel target for neuroprotection.

Angiotensin II Type-2 Receptor Stimulation Prevents Neural Damage by Transcriptional Activation of Methyl Methanesulfonate Sensitive 2

The molecular mechanisms of the contribution of angiotensin II type-1 receptor blockers to neuronal protection are still unclear. Here, we investigated the effect of angiotensin II type-2 (AT2) receptor stimulation on neurons and cognitive function involving a new neuroprotective factor, methyl methanesulfonate sensitive 2 (MMS2). Angiotensin II treatment of neurospheres enhanced their differentiation and increased MMS2 expression. Knockdown of the MMS2 gene by small interference RNA (siRNA) significantly reduced the number of neurospheres, with loss of sphere formation. An angiotensin II type-1 receptor blocker, valsartan, enhanced such neurosphere differentiation and MMS2 induction, whereas an AT2 receptor antagonist, PD123319, inhibited them. After mice underwent permanent middle cerebral artery occlusion, AT2 receptor mRNA expression was significantly increased in the ischemic side of the brain. Passive avoidance rate to evaluate cognitive function was significantly impaired in AT2 receptor null (Agtr2-) mice compared with wild-type mice. Treatment with valsartan prevented the cognitive decline in wild-type mice, but this effect was weaker in Agtr2- mice. In ischemic brain regions, MMS2 was increased in wild-type mice, but not in Agtr2- mice. Valsartan also enhanced MMS2 expression to a greater degree in wild-type mice. Finally, intracerebroventricular administration of MMS2 siRNA showed more impaired avoidance rate after middle cerebral artery occlusion compared with that in control siRNA-transfected mice. These findings experimentally support the clinical evidence and indicate a unique mechanism of the AT2 receptor in brain protection.

Reduction of dopaminergic degeneration and oxidative stress by inhibition of angiotensin converting enzyme in a MPTP model of parkinsonism

There is growing evidence indicating that oxidative stress is a key contributor to the pathogenesis and progression of Parkinson's disease. The brain, and particularly the basal ganglia, possesses a local rennin-angiotensin system. Angiotensin activates NAD(P)H-dependent oxidases, which are a major intracellular source of superoxide, and angiotensin converting enzyme inhibitors (ACEIs) have shown antioxidant properties. We treated mice with MPTP and the ACEI captopril to study the possible neuroprotective and antioxidant effects of the latter on the dopaminergic system. Pre-treatment with captopril induced a significant reduction in the MPTP-induced loss of dopaminergic neurons in the substantia nigra and a significant reduction in the loss of dopaminergic terminals in the striatum. Furthermore, captopril reduced the MPTP-induced increase in the levels of major oxidative stress indicators (i.e. lipid peroxidation and protein oxidation) in the ventral midbrain and the striatum. Captopril did not reduce striatal MPP(+) levels, MAO-B activity or dopamine transporter activity, which may reduce MPTP neurotoxicity. Our results suggest that angiotensin-converting enzyme inhibitors may be useful for treatment of Parkinson's disease, and that further investigation should focus on the neuroprotective capacity of these compounds.

Angiotensin type-1-receptor antagonists reduce 6-hydroxydopamine toxicity for dopaminergic neurons

Angiotensin II activates (via type 1 receptors) NAD(P)H-dependent oxidases, which are a major source of superoxide, and is relevant in the pathogenesis of several cardiovascular diseases and certain degenerative changes associated with ageing. Given that there is a brain renin-angiotensin system and that oxidative stress is a key contributor to Parkinson's disease, we investigated the effects of angiotensin II and angiotensin type 1 (AT(1)) receptor antagonists in the 6-hydroxydopamine model of Parkinson's disease. Rats subjected to intraventricular injection of 6-hydroxydopamine showed bilateral reduction in the number of dopaminergic neurons and terminals. Injection of angiotensin alone did not induce any significant effect. However, angiotensin increased the toxic effect of 6-hydroxydopamine. Rats treated with the AT(1) receptor antagonist ZD 7155 and then 6-hydroxydopamine (with or without exogenous administration of angiotensin) showed a significant reduction in 6-hydroxydopamine-induced oxidative stress (lipid peroxidation and protein oxidation) and dopaminergic degeneration. Dopaminergic degeneration was also reduced by the NAD(P)H inhibitor apocynin. Angiotensin may play a pivotal role, via AT(1) receptors, in increasing the oxidative damage of dopaminergic cells, and treatment with AT(1) antagonists may reduce the progression of Parkinson's disease.

Angiotensin-converting enzyme inhibition reduces oxidative stress and protects dopaminergic neurons in a 6-hydroxydopamine rat model of Parkinsonism

It is now established that the brain possesses a local renin-angiotensin system and that angiotensin II exerts multiple actions in the nervous system, including regulation of striatal dopamine release. Furthermore, angiotensin activates NADPH-dependent oxidases, which are a major source of superoxide, and angiotensin-converting enzyme inhibitors, commonly used in the treatment of hypertension and chronic heart failure, have shown antioxidant properties in several tissues. Oxidative stress is a key contributor to the pathogenesis and progression of Parkinson's disease. In the present study, we treated rats with intraventricular injections of the dopaminergic neurotoxin 6-hydroxydopamine and subcutaneous injections of the angiotensin-converting enzyme inhibitor Captopril to study the possible neuroprotective effect of the latter on the dopaminergic system and on 6-hydroxydopamine-induced oxidative stress. Rats treated with Captopril and 6-hydroxydopamine showed significantly less reduction in the number of dopaminergic neurons (i.e., immunoreactive to tyrosine hydroxylase) in the substantia nigra and in the density of striatal dopaminergic terminals than 6-hydroxydopamine-lesioned rats not treated with Captopril. In addition, Captopril reduced the levels of major oxidative stress indicators (i.e., lipid peroxidation and protein oxidation) in the ventral midbrain and the striatum of 6-hydroxydopamine-lesioned rats. Our results suggest that angiotensin-converting enzyme inhibitors may be useful for treatment of Parkinson's disease and that further investigation should focus on the neuroprotective capacity of these compounds.

Angiotensin II and interleukin-1 interact to increase generation of dopaminergic neurons from neurospheres of mesencephalic precursors

Cultures of rat mesencephalic precursors treated with interleukin-1beta or angiotensin II contained significantly more dopaminergic neurons than controls. However, simultaneous treatment with angiotensin II and interleukin-1beta did not induce any further increase. Treatment with the angiotensin type-2 receptor antagonist PD 123319 precluded both the angiotensin- and the interleukin-induced increase. The present results indicate that angiotensin type-2 receptors and interleukin-1 cooperate to induce the dopaminergic phenotype.

Angiotensin II protects cultured midbrain dopaminergic neurons against rotenone-induced cell death

In this study, we demonstrate that angiotensin II (Ang II) protects dopamine (DA) neurons from rotenone toxicity in vitro. Primary ventral mesencephalic (VM) cultures from E15 rats were grown for 5 days and then cultured in the presence of the mitochondrial complex I inhibitor, rotenone. Acute exposure (20 h) to 20 nM rotenone reduced the number of tyrosine hydroxylase-positive (TH+) neurons by 50 +/- 6% when compared to untreated cultures. Pre-treatment of VM cultures with 100 nM Ang II decreased TH+ neuronal loss to 25 +/- 10% at the 20-nM rotenone concentration. Ang II in the presence of the angiotensin type 1 receptor (AT1R) antagonist, losartan, was even more effective in protecting DA neurons showing a loss of only 13 +/- 4% at 20 nM rotenone. Conversely, the AT2R antagonist, PD123319, abolished the protective effects of Ang II. Furthermore, both the NMDA receptor antagonist, MK801, and the antioxidant, alpha-tocopheryl succinate (vitamin E analogue), prevented rotenone-induced toxicity. Here, we show that acute exposure of VM cultures to the pesticide rotenone leads to dopaminergic neuronal cell death and that angiotensin acting through the AT2 receptor protects dopamine neurons from rotenone toxicity.