PET Imaging of the Human Nicotinic Cholinergic Pathway in Atherosclerosis

Imaging Cholinergic Receptors in the Brain by Positron Emission Tomography

Cholinergic receptors represent a promising class of diagnostic and therapeutic targets due to their significant involvement in cognitive decline associated with neurological disorders and neurodegenerative diseases as well as cardiovascular impairment. Positron emission tomography (PET) is a noninvasive molecular imaging tool that has helped to shed light on the roles these receptors play in disease development and their diverse functions throughout the central nervous system (CNS). In recent years, there has been a notable advancement in the development of PET probes targeting cholinergic receptors. The purpose of this review is to provide a comprehensive overview of the recent progress in the development of these PET probes for cholinergic receptors with a specific focus on ligand structure, radiochemistry, and pharmacology as well as in vivo performance and applications in neuroimaging. The review covers the structural design, pharmacological properties, radiosynthesis approaches, and preclinical and clinical evaluations of current state-of-the-art PET probes for cholinergic receptors.

The Availability of the α7-Nicotinic Acetylcholine Receptor in Early Identification of Vulnerable Atherosclerotic Plaques: A Study Using a Novel 18F-Label Radioligand PET

Background: It has been confirmed that the α7-nicotinic acetylcholine receptor (α7nAChR) is an important target for identifying vulnerable atherosclerotic plaques. Previously, we successfully designed and synthesized a series of ¹⁸F-labeled PET molecular probes targeting α7nAChR, which are mainly used in the diagnosis of Alzheimer's disease. Based on the characteristics of α7nAChR in blood vessels, we have firstly screened for a suitable novel ¹⁸F-labeled PET molecular probe ([¹⁸F]YLF-DW), with high selectivity for α7nAChR over α4β2nAChR and a good effect for the imaging of atherosclerotic animal models, to effectively identify vulnerable atherosclerotic plaques at an early stage. Meanwhile, we compared it with the “gold standard” pathological examination of atherosclerosis, to verify the reliability of [¹⁸F]YLF-DW in early diagnosis of atherosclerosis.

Methods: The vulnerable atherosclerotic plaques model of ApoE-/-mice were successfully established. Then based on the methods of 3D-QSAR and molecular docking, we designed oxazolo[4,5-b] pyridines and fluorenone compounds, which are targeted at α7nAChR. Through further screening, a novel alpha7 nicotinic acetylcholine receptor radioligand ([¹⁸F]YLF-DW) was synthesized and automatically ¹⁸F-labeled using a Stynthra RNplus module. Subsequently, we employed [¹⁸F]YLF-DW for the targeting of α7nAChR in atherosclerotic plaques and control group, using a micro-PET/CT respectively. After imaging, the mice were sacrificed by air embolism and the carotid arteries taken out for making circular sections. The paraffin embedded specimens were sectioned with 5 μm thickness and stained with oil red. After staining, immunohistochemistry experiment was carried out to verify the effect of micro-PET/CT imaging.

Results: The micro-PET/CT imaging successfully identified the vulnerable atherosclerotic plaques in the carotid arteries of ApoE-/-mice; whereas, no signal was observed in normal control mice. In addition, compared with the traditional imaging agent [¹⁸F]FDG, [¹⁸F]YLF-DW had a significant effect on the early plaques imaging of carotid atherosclerosis. The results of oil red staining and immunohistochemistry also showed early formations of carotid plaques in ApoE-/-mice and provided pathological bases for the evaluation of imaging effect.

Conclusion: We innovated to apply the novel molecular probe ([¹⁸F]YLF-DW) to the identification of vulnerable atherosclerotic plaques in carotid arteries, to detect atherosclerosis early inflammatory response and provide powerful input for the early diagnosis of atherosclerotic lesions, which may play an early warning role in cardiovascular acute events.

Expression levels of the α7 nicotinic acetylcholine receptor in the brains of patients with Alzheimer's disease and their effect on synaptic proteins in SH-SY5Y cells

Alzheimer's disease (AD) is a chronic neurodegenerative, and abnormal aggregation of the neurotoxic β amyloid (Aβ) peptide is an early event in AD. The present study aimed to determine the correlation between the nicotinic acetylcholine receptor α7 subunit (α7 nAChR) and Aβ in the brains of patients with AD, and to investigate whether the increased expression levels of the α7 nAChR could alter the neurotoxicity of Aβ. The expression levels of α7 nAChR and Aβ in the brains of patients with AD and healthy brains were analyzed using immunofluorescence. Moreover, SH‑SY5Y cells were used to stably overexpress or silence α7 nAChR expression levels, prior to the treatment with or without 1 µmol/l Aβ1‑42 oligomer (AβO). The mRNA and protein expression levels of α7 nAChR, synaptophysin (SYP), postsynaptic density of 95 kDa (PSD‑95) and synaptosomal‑associated protein of 25 kDa (SNAP‑25) were subsequently analyzed using reverse transcription‑quantitative PCR and western blotting. In addition, the concentration of acetylcholine (ACh) and the activity of acetylcholinesterase (AChE) were analyzed using spectrophotometry, while the cell apoptotic rate was determined using flow cytometry. The expression of Aβ in the brains of patients with AD was found to be significantly increased, whereas the expression of α7 nAChR was significantly decreased compared with the healthy control group. In vitro, the expression levels of α7 nAChR were significantly increased or decreased following the overexpression or silencing of the gene, respectively. Consistent with these observations, the mRNA and protein expression levels of SYP, PSD‑95 and SNAP‑25 were also significantly increased following the overexpression of α7 nAChR and decreased following the genetic silencing of the receptor. In untransfected or negative control cells, the expression levels of these factors and the apoptotic rate were significantly reduced following the exposure to AβO, which was found to be attenuated by α7 nAChR overexpression, but potentiated by α7 nAChR RNA silencing. However, no significant differences were observed in either the ACh concentration or AChE activity following transfection. Collectively, these findings suggested that α7 nAChR may protect the brains of patients with AD against Aβ, as α7 nAChR overexpression increased the expression levels of SYP, SNAP‑25 and PSD‑95, and attenuated the inhibitory effect of Aβ on the expression of these synaptic proteins and cell apoptosis. Overall, this indicated that α7 nAChR may serve an important neuroprotective role in AD.

Vascular Inflammation in Subclinical Atherosclerosis Detected by Hybrid PET/MRI

BACKGROUND

Atherosclerosis is a chronic inflammatory disease, but data on arterial inflammation at early stages is limited.

OBJECTIVES

The purpose of this study was to characterize vascular inflammation by hybrid ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography/magnetic resonance imaging (PET/MRI).

METHODS

Carotid, aortic, and ilio-femoral ¹⁸F-FDG PET/MRI was performed in 755 individuals (age 40 to 54 years; 83.7% men) with known plaques detected by 2-/3-dimensional vascular ultrasound and/or coronary calcification in the PESA (Progression of Early Subclinical Atherosclerosis) study. The authors evaluated the presence, distribution, and number of arterial inflammatory foci (increased ¹⁸F-FDG uptake) and plaques with or without inflammation (coincident ¹⁸F-FDG uptake).

RESULTS

Arterial inflammation was present in 48.2% of individuals (24.4% femorals, 19.3% aorta, 15.8% carotids, and 9.3% iliacs) and plaques in 90.1% (73.9% femorals, 55.8% iliacs, and 53.1% carotids). ¹⁸F-FDG arterial uptakes and plaques significantly increased with cardiovascular risk factors (p < 0.01). Coincident ¹⁸F-FDG uptakes were present in 287 of 2,605 (11%) plaques, and most uptakes were detected in plaque-free arterial segments (459 of 746; 61.5%). Plaque burden, defined by plaque presence, number, and volume, was significantly higher in individuals with arterial inflammation than in those without (p < 0.01). The number of plaques and ¹⁸F-FDG uptakes showed a positive albeit weak correlation (r = 0.25; p < 0.001).

CONCLUSIONS

Arterial inflammation is highly prevalent in middle-aged individuals with known subclinical atherosclerosis. Large-scale multiterritorial PET/MRI allows characterization of atherosclerosis-related arterial inflammation and demonstrates ¹⁸F-FDG uptake in plaque-free arterial segments and, less frequently, within plaques. These findings suggest an arterial inflammatory state at early stages of atherosclerosis. (Progression of Early Subclinical Atherosclerosis [PESA]; NCT01410318).

Activation of the monocytic α7 nicotinic acetylcholine receptor modulates oxidative stress and inflammation-associated development of coronary artery spasm via a p38 MAP-kinase signaling-dependent pathway

OBJECTIVE

Smoking and high-sensitivity C-reactive protein (hs-CRP) are risk factors for coronary artery spasm (CAS), which is characterized by the increased interleukin-6 (IL-6) level and monocyte counts; however, limited data are available regarding the role of cigarette-embedded nicotine in the modulation of monocytic inflammatory activity in CAS.

APPROACH

We investigated and elucidated the putative roles and associations of nicotine, monocytic IL-6, α7 nicotinic acetylcholine receptor (α7-nAChR), and CRP in CAS development.

RESULTS

We demonstrated that a significantly increased α7-nAChR (p = 0.001) and IL-6 (p = 0.0036) messenger RNA (mRNA) expression in the serum of patients with CAS. Serum hs-CRP levels exhibited a strong positive correlation with the monocytic mRNA expression of α7-nAChR (r = 0.71, p < 0.001) and IL-6 (r = 0.49, p = 0.006). The α7-nAChR and IL-6 expression levels of the CAS group were also positively correlated (r = 0.63, p < 0.001). Compared with the untreated controls, THP-1 cells and patient-derived monocytes treated with different concentrations of CRP displayed significantly increased expression levels of α7-nAChR mRNA and protein (p = 0.0054), in a dose-dependent manner. We also demonstrated that compared with the IL-6 expression elicited by CRP alone (p = 0.0489), the CRP-induced rise in monocytic IL-6 mRNA and protein expression in the presence of nicotine (p = 0.0002), is mediated by α7-nAChR activation and the deregulation of the human p38 mitogen-activated protein kinases (MAPK) signaling pathway.

CONCLUSIONS

Our data demonstrate that the elevated monocytic IL-6 and α7-nAChR mRNA and protein expression levels are associated with the interaction between nicotine and CRP positively modulates CAS development. Our study suggests the potential role of α7-nAChR mRNA and/or protein expression as a diagnostic biomarker for CAS.

Venom based neural modulators

Different types of neuronal nicotinic acetylcholine receptors (nAChRs) are expected to occur in vivo, most structure-activity relationship studies have been carried out for just a few neuronal subtypes. The present review enlightens current aspects of venom modulators of nAChRs. Important electronic databases such as PubMed or Google scholar were explored for the collection of latest studies in the field. Clinical and basic research has shown that cholinergic receptors play a role in several disorders of the nervous system such as chronic pain, Alzheimers disease and addiction to nicotine, alcohol and drugs. Unfortunately, the lack of selective modulators for each subtype of nAChR makes their pharmacological characterization difficult, which has slowed the development of therapeutic nAChR modulators with high selectivity and absence of off-target side-effects. Animal venoms have proven to be an excellent natural source of bioactive molecules with activity against ion channels. The present review concludes that the presence of small-molecule nAChR modulators in spider venoms support the use of venoms as a potential source of novel modulators.

Potential of α7 nicotinic acetylcholine receptor PET imaging in atherosclerosis

Atherosclerotic events are usually acute and often strike otherwise asymptomatic patients. Although multiple clinical risk factors have been associated with atherosclerosis, as of yet no further individual prediction can be made as to who will suffer from its consequences based on biomarker analysis or traditional imaging methods like CT, MRI or angiography. Previously, non-invasive imaging with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) PET was shown to potentially fill this niche as it offers high sensitive detection of metabolic processes associated with inflammatory changes in atherosclerotic plaques. However, ¹⁸F-FDG PET imaging of arterial vessels suffers from non-specificity and has still to be proven to reliably identify vulnerable plaques, carrying a high risk of rupture. Therefore, it may be regarded only as a secondary marker for monitoring treatment effects and it does not offer alternative treatment options or direct insight in treatment mechanisms. In this review, an overview is given of the current status and the potential of PET imaging of inflammation and angiogenesis in atherosclerosis in general and special emphasis is given to imaging of α7 nicotinic acetylcholine receptors (α7 nAChRs). Due to the gaps that still exist in our understanding of atherogenesis and the limitations of the available PET tracers, the search continues for a more specific radioligand, able to differentiate between stable atherosclerosis and plaques prone to rupture. The potential role of the α7 nAChR as imaging marker for plaque vulnerability is explored. Today, strong evidence exists that nAChRs are involved in the atherosclerotic disease process. They are suggested to mediate the deleterious effects of the major tobacco component, nicotine, a nAChR agonist. Mainly based on in vitro data, α7 nAChR stimulation might increase plaque burden via increased neovascularization. However, in animal studies, α7 nAChR manipulation appears to reduce plaque size due to its inhibitory effects on inflammatory cells. Thus, reliable identification of α7 nAChRs by in vivo imaging is crucial to investigate the exact role of α7 nAChR in atherosclerosis before any therapeutic approach in the human setting can be justified. In this review, we discuss the first experience with α7 nAChR PET tracers and developmental considerations regarding the "optimal" PET tracer to image vascular nAChRs.

SPECT and PET imaging of angiogenesis and arteriogenesis in pre-clinical models of myocardial ischemia and peripheral vascular disease

Purpose

The extent of neovascularization determines the clinical outcome of coronary artery disease and other occlusive cardiovascular disorders. Monitoring of neovascularization is therefore highly important. This review article will elaborately discuss preclinical studies aimed at validating new nuclear angiogenesis and arteriogenesis tracers. Additionally, we will briefly address possible obstacles that should be considered when designing an arteriogenesis radiotracer.

Methods

A structured medline search was the base of this review, which gives an overview on different radiopharmaceuticals that have been evaluated in preclinical models.

Results

Neovascularization is a collective term used to indicate different processes such as angiogenesis and arteriogenesis. However, while it is assumed that sensitive detection through nuclear imaging will facilitate translation of successful therapeutic interventions in preclinical models to the bedside, we still lack specific tracers for neovascularization imaging. Most nuclear imaging research to date has focused on angiogenesis, leaving nuclear arteriogenesis imaging largely overlooked.

Conclusion

Although angiogenesis is the process which is best understood, there is no scarcity in theoretical targets for arteriogenesis imaging.