Immunolocalisation of endothelin-1 in human brain

Endothelins in inflammatory neurological diseases

Endothelins were discovered more than thirty years ago as potent vasoactive compounds. Beyond their well-documented cardiovascular properties, however, the contributions of the endothelin pathway have been demonstrated in several neuroinflammatory processes and the peptides have been reported as clinically relevant biomarkers in neurodegenerative diseases. Several studies report that endothelin-1 significantly contributes to the progression of neuroinflammatory processes, particularly during infections in the central nervous system (CNS), and is associated with a loss of endothelial integrity at the blood brain barrier level. Because of the paucity of clinical trials with endothelin-1 antagonists in several infectious and non-infectious neuroinflammatory diseases, it remains an open question whether the 21 amino acid peptide is a mediator/modulator rather than a biomarker of the progression of neurodegeneration. This review focuses on the potential roles of endothelins in the pathology of neuroinflammatory processes, including infectious diseases of viral, bacterial or parasitic origin in which the synthesis of endothelins or its pharmacology have been investigated from the cell to the bedside in several cases, as well as in non-infectious inflammatory processes such as neurodegenerative disorders like Alzheimers Disease or central nervous system vasculitis.

Androgen alleviates neurotoxicity of β-amyloid peptide (Aβ) by promoting microglial clearance of Aβ and inhibiting microglial inflammatory response to Aβ

AIMS

Lower androgen level in elderly men is a risk factor of Alzheimer's disease (AD). It has been reported that androgen reduces amyloid peptides (Aβ) production and increases Aβ degradation by neurons. Activated microglia are involved in AD by either clearing Aβ deposits through uptake of Aβ or releasing cytotoxic substances and pro-inflammatory cytokines. Here, we investigated the effect of androgen on Aβ uptake and clearance and Aβ-induced inflammatory response in microglia, on neuronal death induced by Aβ-activated microglia, and explored underlying mechanisms.

METHODS

Intracellular and extracellular Aβ were examined by immunofluorescence staining and Western blot. Amyloid peptides (Aβ) receptors, Aβ degrading enzymes, and pro-inflammatory cytokines were detected by RT-PCR, real-time PCR, and ELISA. Phosphorylation of MAP kinases and NF-κB was examined by Western blot.

RESULTS

We found that physiological concentrations of androgen enhanced Aβ42 uptake and clearance, suppressed Aβ42 -induced IL-1β and TNFα expression by murine microglia cell line N9 and primary microglia, and alleviated neuronal death induced by Aβ42 -activated microglia. Androgen administration also reduced Aβ42 -induced IL-1β expression and neuronal death in murine hippocampus. Mechanistic studies revealed that androgen promoted microglia to phagocytose and degrade Aβ42 through upregulating formyl peptide receptor 2 and endothelin-converting enzyme 1c expression, and inhibited Aβ42 -induced pro-inflammatory cytokines expression via suppressing MAPK p38 and NF-κB activation by Aβ42 , in an androgen receptor independent manner.

CONCLUSION

Our study demonstrates that androgen promotes microglia to phagocytose and clear Aβ42 and inhibits Aβ42 -induced inflammatory response, which may play an important role in reducing the neurotoxicity of Aβ.

Aβ-degrading enzymes: potential for treatment of Alzheimer disease

There is increasing evidence that deficient clearance of β-amyloid (Aβ) contributes to its accumulation in late-onset Alzheimer disease (AD). Several Aβ-degrading enzymes, including neprilysin (NEP), insulin-degrading enzyme, and endothelin-converting enzyme reduce Aβ levels and protect against cognitive impairment in mouse models of AD. The activity of several Aβ-degrading enzymes rises with age and increases still further in AD, perhaps as a physiological response to minimize the buildup of Aβ. The age- and disease-related changes in expression of more recently recognized Aβ-degrading enzymes (e.g. NEP-2 and cathepsin B) remain to be investigated, and there is strong evidence that reduced NEP activity contributes to the development of cerebral amyloid angiopathy. Regardless of the role of Aβ-degrading enzymes in the development of AD, experimental data indicate that increasing the activity of these enzymes (NEP in particular) has therapeutic potential in AD, although targeting their delivery to the brain remains a major challenge. The most promising current approaches include the peripheral administration of agents that enhance the activity of Aβ-degrading enzymes and the direct intracerebral delivery of NEP by convection-enhanced delivery. In the longer term, genetic approaches to increasing the intracerebral expression of NEP or other Aβ-degrading enzymes may offer advantages.

Effect of chronic central endothelin-1 on hemodynamics and plasma vasopressin in conscious rats

OBJECTIVES

These studies were designed to test whether chronic central administration of endothelin-1 induces changes in systemic hemodynamics and plasma vasopressin similar to those observed with acute microinjections of endothelin-1.

METHODS

Sprague Dawley rats underwent sham denervation or sinoaortic denervation. Three days later, baseline mean arterial blood pressure, heart rate, and vasopressin were assessed in conscious rats. Then, a cannula was stereotaxically inserted into the lateral ventricle and attached to an osmotic minipump that delivered one of the following: (i) artificial cerebrospinal fluid; (ii) endothelin-1, 10 pmol/hour; (iii) BQ-123, 400 pmol/hour; or (iv) endothelin-1+BQ-123. Mean arterial blood pressure and heart rate were monitored daily and blood was obtained for plasma vasopressin on days 3 and 9. On day 10, the rats were euthanized, the hypothalami were removed, and vasopressin messenger ribonucleic acid content was assessed.

RESULTS

The pressor effect of intracerebroventricular endothelin-1 was similar in intact and sinoaortic denervation rats and was prevented by endothelin receptor A antagonism with BQ-123. Administration of BQ-123 alone resulted in a depressor and bradycardia in sinoaortically denervated rats. Chronic endothelin-1 administration did not change plasma vasopressin but resulted in a significant decrease in hypothalamic vasopressin messenger ribonucleic acid levels, which was reversed by endothelin receptor A inhibition.

DISCUSSION

Although the pressor effect of chronic central endothelin-1 is similar to that reported with acute endothelin-1, plasma vasopressin levels do not increase, at least in part, due to downregulation of hypothalamic vasopressin gene expression. Sinoaortic denervation increases endogenous central endothelin receptor A tone. Furthermore, these observations confirm that the pressor effect of central endothelin-1 is not mediated by plasma vasopressin.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

beta-Amyloid degradation and Alzheimer's disease

Extensive β-amyloid (Aβ) deposits in brain parenchyma in the form of senile plaques and in blood vessels in the form of amyloid angiopathy are pathological hallmarks of Alzheimer's disease (AD). The mechanisms underlying Aβ deposition remain unclear. Major efforts have focused on Aβ production, but there is little to suggest that increased production of Aβ plays a role in Aβ deposition, except for rare familial forms of AD. Thus, other mechanisms must be involved in the accumulation of Aβ in AD. Recent data shows that impaired clearance may play an important role in Aβ accumulation in the pathogenesis of AD. This review focuses on our current knowledge of Aβ-degrading enzymes, including neprilysin (NEP), endothelin-converting enzyme (ECE), insulin-degrading enzyme (IDE), angiotensin-converting enzyme (ACE), and the plasmin/uPA/tPA system as they relate to amyloid deposition in AD.

Endothelin evokes distinct calcium transients in neuronal and non-neuronal cells of rat olfactory mucosa primary cultures

The olfactory system is regulated by several nervous and hormonal factors, and there is a growing body of evidence that some of these modulations already take place in the olfactory mucosa (OM). We recently suggested that, among others, vasoactive peptides might play multifaceted roles in different OM cells. Here we studied the effect of the vasoconstrictive peptide endothelin (ET) in the rat OM. We identified different components of the ET system both in the olfactory mucosa and in long-term primary culture of OM cells, composed of olfactory sensory neurons (OSNs) lying on a blend of non-neuronal OM cells (nNCs). We demonstrated that ET receptors are differentially expressed on OM cells, and that ET might be locally matured by the endothelin-converting enzyme ECE-1 located in OSNs. Using calcium imaging, we showed that ET triggers robust dose-dependent Ca(2+) responses in most OM cells, which consist of a transient phase, followed, in nNCs, by a sustained plateau phase. All transient responses depended on intracellular calcium release, while the sustained plateau phase also depended on subsequent external calcium entry. Using both pharmacology and spotting lethal (sl/sl) mutant rats, lacking functional ET(B) receptors, we finally demonstrated that these effects of ET are mediated through ET(B) receptors in OSNs and ET(A) receptors in nNCs.The present study therefore identifies endothelin as a potent endogenous modulator of the olfactory mucosa; specific endothelin-mediated Ca(2+) signals may serve distinct signaling functions, and thereby suggest differential functional roles of endothelin in both neuronal and non-neuronal OM cells.

Effects of 4-hydroxy-nonenal and Amyloid-beta on expression and activity of endothelin converting enzyme and insulin degrading enzyme in SH-SY5Y cells

The cerebral accumulation of amyloid-beta (Abeta) is a consistent feature of and likely contributor to the development of Alzheimer's disease (AD). In addition to dysregulated production, increasing experimental evidence suggests reduced catabolism plays an important role in Abeta accumulation. Although endothelin converting enzyme (ECE) and insulin degrading enzyme (IDE) degrade and thus contribute to regulating the steady-state levels of Abeta, how these enzymes are regulated remain poorly understood. In this study, we investigated the effects of 4-hydroxy-nonenal (HNE) and Abeta on the expression and activity of ECE-1 and IDE in human neuroblastoma SH-SY5Y cells. Treatment with HNE or Abeta upregulated ECE-1 mRNA and protein, while IDE was unchanged. Although both ECE-1 and IDE were oxidized within 24 h of HNE or Abeta treatment, ECE-1 catalytic activity was elevated while IDE specific activity was unchanged. The results demonstrated for the first time that both ECE-1 and IDE are substrates of HNE modification induced by Abeta. In addition, the results suggest complex mechanisms underlying the regulation of their enzymatic activity.

Endothelin-1 as a neuropeptide: neurotransmitter or neurovascular effects?

Endothelin-1 (ET-1) is an endothelium-derived peptide that also possesses potent mitogenic activity. There is also a suggestion the ET-1 is a neuropeptide, based mainly on its histological identification in both the central and peripheral nervous system in a number of species, including man. A neuropeptide role for ET-1 is supported by studies showing a variety of effects caused following its administration into different regions of the brain and by application to peripheral nerves. In addition there are studies proposing that ET-1 is implicated in a number of neural circuits where its transmitter affects range from a role in pain and temperature control to its action on the hypothalamo-neurosecretory system. While the effect of ET-1 on nerve tissue is beyond doubt, its action on nerve blood flow is often ignored. Here, we review data generated in a number of species and using a variety of experimental models. Studies range from those showing the distribution of ET-1 and its receptors in nerve tissue to those describing numerous neurally-mediated effects of ET-1.

Expression and significance of endothelin 1 in spiral ganglion cells of guinea pig

OBJECTIVES

Endothelin 1 has many biological activities including actions in the nervous system. This study aimed to investigate the expression of the endothelin 1 in spiral ganglion cells of guinea pig and its significance in the auditory transmission.

METHODS

Healthy guinea pigs were sacrificed and cardiac perfused with saline followed by 4% paraformaldehyde. Temporal bones were removed and fixed, decalcified in 10% EDTA, embedded in paraffin block and serially sectioned in 5 microm thick slice. Rabbit anti-endothelin 1 polyclonal antibody was used as primary antibody to examine the expression of endothelin 1 in the spiral ganglion by immunohistochemistry.

RESULTS

Endothelin 1 expression was detected in spiral ganglion cells from the basal turn to the apical turn of the cochlea.

CONCLUSIONS

The endothelin 1 presents in spiral ganglions cells of the guinea pig and might play a role in the auditory transmission.

Localization of the endothelin system in human diffuse astrocytomas

BACKGROUND

Endothelin-1 (ET-1), a vasoconstrictor and mitogen, has recently been implicated in the pathogenesis of human glioblastoma, neuroblastoma, and meningioma. ET-1, formed by proteolysis of the propeptide big ET-1 by endothelin-converting enzyme-1 (ECE-1), mediates its cellular actions through ETA and ETB receptors. Because only immunoreactive ET-1 has been observed within human astrocytic tumor cells, the authors investigated the localization of the entire ET-1 system (ET-1 mRNA, ET-1, ECE-1, ETA and ETB receptors) in surgical samples of human diffuse astrocytomas WHO Grade II (n = 6).

METHODS

ET-1 mRNA expression was elucidated by in situ reverse transcriptase polymerase chain reaction (RT-PCR) using synthetic primers. Polyclonal antibodies were used to localize ET-1, ECE-1, ETA and ETB receptors by immunocytochemistry.

RESULTS

All ET components were detected in the six tumor samples. Intense (3+) cytoplasmic ET-1 mRNA labeling was observed in more than 75% of cells in all 6 astrocytomas. Up to 75% of tumor cells displayed intense ET-1 and ECE-1 immunolabeling distributed throughout their cytoplasm. Immunoreactive ETA and ETB receptors, observed in 25% to 75% of astrocytic tumor cells, were of moderate intensity. In addition, all components of the ET system were seen within endothelial cells of tumor blood vessels.

CONCLUSIONS

The presence of ET-1 mRNA, ECE-1, and ET-1 within tumor astrocytes suggests local ET synthesis and processing. The mitogenic and antiapoptotic properties of ET-1, as well as the vasodilatory signaling of ETB receptors, may promote tumorigenesis.

Cellular distribution of the endothelin system in the human brain

The vasoconstrictor endothelin-1 (ET-1) may also act as a neuropeptide. ET-1 is formed by the catalytic action of endothelin-converting enzyme-1 (ECE-1) on big ET-1 and its cellular actions are mediated via ET(A) and ET(B) receptors. Although localisation of these components in rodent brain has been extensively investigated, no single study has mapped their distribution in human brain. Here we describe the localisation of ET-1 mRNA, ET-1, ECE-1, ET(A) and ET(B) receptors within 24 human brain regions. In situ RT-PCR has previously detected ET-1 mRNA in 22 areas (excluding the post-central gyrus and pineal gland), and ET-1 immunoreactivity was visualised in cells of all regions. Using specific antibodies we have immunolocalised ECE-1 and ET(B) receptors in cells of 24 areas, and ET(A) receptors in nine regions (choroidal epithelial cells, neurones in the diencephalon, hippocampus, amygdaloid, dentate nucleus, Purkinje cells of the cerebellum, flocculo-nodular lobe and vermis). ET-1 mRNA, ET-1, ECE-1 and ET(B) receptors were observed in cortical pyramidal cells, neurones (brainstem, basal nuclei, thalamus, insula and claustrum, limbic region), cells in the anterior pituitary gland; nerve cell processes in the pars nervosa; pinealocytes and choroidal epithelial cells. Only ET-1 mRNA, ET-1, ECE-1, and ET(B) receptors were visualised in cerebral capillary endothelial cells. The presence of ET-1 mRNA, ECE-1 and ET-1 in 22 brain regions confirms ET expression and processing in human brain. The localisation of ET-1 and ET(B) receptors suggests receptor-mediated action akin to a neurotransmitter role for ET-1.