Molecular Magnetic Resonance Imaging of Nitric Oxide in Biological Systems

Positron Emission Tomography of Nitric Oxide by a Specific Radical-Generating Dihydropyridine Tracer

Nitric oxide (NO) plays a pivotal role as a biological signaling molecule, presenting challenges in its specific detection and differentiation from other reactive nitrogen and oxygen species within living organisms. Herein, a 18F-labeled (fluorine-18, t1/2 = 109.7 min) small-molecule tracer dimethyl 4-(4-(4-[18F]fluorobutoxy)benzyl)-2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate ([18F]BDHP) is developed based on the dihydropyridine scaffold for positron emission tomography (PET) imaging of NO in vivo. [18F]BDHP exhibits a highly sensitive and efficient C-C cleavage reaction specifically triggered by NO under physiological conditions, leading to the production of a 18F-labeled radical that is readily retained within the cells. High uptakes of [18F]BDHP are found within and around NO-generating cells, such as macrophages treated with lipopolysaccharide or benzo(a)pyrene. MicroPET/CT imaging of arthritic animal model mice reveals distinct tracer accumulation in the arthritic legs, showcasing a higher distribution of NO compared with the control legs. In summary, a specific radical-generating dihydropyridine tracer with a unique radical retention strategy has been established for the marking of NO in real-time in vivo.

Wireless agents for brain recording and stimulation modalities

New sensors and modulators that interact wirelessly with medical modalities unlock uncharted avenues for in situ brain recording and stimulation. Ongoing miniaturization, material refinement, and sensitization to specific neurophysiological and neurochemical processes are spurring new capabilities that begin to transcend the constraints of traditional bulky and invasive wired probes. Here we survey current state-of-the-art agents across diverse realms of operation and evaluate possibilities depending on size, delivery, specificity and spatiotemporal resolution. We begin by describing implantable and injectable micro- and nano-scale electronic devices operating at or below the radio frequency (RF) regime with simple near field transmission, and continue with more sophisticated devices, nanoparticles and biochemical molecular conjugates acting as dynamic contrast agents in magnetic resonance imaging (MRI), ultrasound (US) transduction and other functional tomographic modalities. We assess the ability of some of these technologies to deliver stimulation and neuromodulation with emerging probes and materials that provide minimally invasive magnetic, electrical, thermal and optogenetic stimulation. These methodologies are transforming the repertoire of readily available technologies paired with compatible imaging systems and hold promise toward broadening the expanse of neurological and neuroscientific diagnostics and therapeutics.

Blood-Brain Barrier Permeable Photoacoustic Probe for High-Resolution Imaging of Nitric Oxide in the Living Mouse Brain

Alternations in the brain nitric oxide (NO) homeostasis are associated with a variety of neurodegeneration diseases; therefore, high-resolution imaging of NO in the brain is essential for understanding pathophysiological processes. However, currently available NO probes are unsuitable for this purpose due to their poor ability to cross the blood-brain barrier (BBB) or to image in deep tissues with spatial resolution. Herein, we developed a photoacoustic (PA) probe with BBB crossing ability to overcome this obstacle. The probe shows a highly selective ratiometric response toward NO, which enables the probe to image NO with micron resolution in the whole brain of living mice. Using three-dimensional PA imaging, we demonstrated that the probe could be used to visualize the detailed NO distribution in varying depth cross-sections (0-8 mm) of the living Parkinson's disease (PD) mouse brain. We also investigated the therapeutic properties of natural polyphenols in the PD mouse brain using the probe as an imaging agent and suggested the potential of the probe for screening therapeutic agents. This study provides a promising imaging agent for imaging of NO in the mouse brain with high resolution. We anticipate that these findings may open up new possibilities for understanding the biological functions of NO in the brain and the development of new imaging agents for the diagnosis and treatment of brain diseases.

A ratiometric near-infrared fluorescence/photoacoustic dual-modal probe with strong donor dithienopyrrole for in vivo nitric oxide detection

Integrating the imaging techniques of near-infrared fluorescence (NIRF) and photoacoustic (PA) can make up for each other and provide more useful medical information. Ratiometric imaging activated by disease-associated biomarkers can further augment imaging specificity. However, very few studies have employed the NIRF/PA dual-modal ratiometric imaging to improve the accuracy and specificity of disease diagnosis to date. In this paper, we present the synthesis of a nitric oxide (NO)-activated ratiometric NIRF/PA dual-modal nanoprobe RAPNP for in vivo NO imaging. The ratiometric imaging function was achieved jointly by a NO/acidity-responsive molecule DTP-BTDA and a nonresponsive fluorophore DTP-BBTD. In these fluorophores, the dithienopyrrole (DTP) moiety had strong electron-donating ability and imparted strong intramolecular charge transfer and relatively long emission wavelengths. The BTDA moiety in DTP-BTDA could be rapidly oxidized by NO under weak acidic environments, achieving the NIRF and PA signal activation. By using RAPNP as a contrast agent, we achieved the ratiometric detection of the endogenous NO in inflammatory bowel disease by NIRF/PA dual-modal imaging. This work provides the first case of the NIRF/PA dual-signal ratiometric probe for the real-time detection of NO in vivo.

Design and Characterization of Hyperpolarized 15N-BBCP as a H2O2-Sensing Probe

Hydrogen peroxide (H2O2) is a type of reactive oxygen species that regulates essential biological processes. Despite the central role of H2O2 in pathophysiological states, available molecular probes for assessing H2O2 in vivo are still limited. This work develops hyperpolarized 15N-boronobenzyl-4-cyanopyridinium (15N-BBCP) as a rationally designed molecular probe for detecting H2O2. The 15N-BBCP demonstrated favorable physicochemical and biochemical properties for H2O2 detection and dynamic nuclear polarization, allowing noninvasive detection of H2O2. In particular, 15N-BBCP and the products possessed long spin-lattice relaxation times and spectrally resolvable 15N chemical shift differences. The performance of hyperpolarized 15N-BBCP was demonstrated both in vitro and in vivo with time-resolved 15N-MRS. This study highlights a promising approach to designing a reaction-based 15N-labeled molecular imaging agent for detecting oxidative stress in vivo.

Design, synthesis and evaluation of 15N- and 13C-labeled molecular probes as hyperpolarized nitric oxide sensors

Nitric oxide (NO) is an important signaling molecule involved in a wide range of biological processes. Development of non-invasive, real-time detection of NO is greatly desired yet remains challenging. Here we report the design and development of novel 15N- and 13C-labeled NO-sensing probes for hyperpolarized nuclear magnetic resonance (HP-NMR) studies. These probes undergo selective and rapid reaction with NO to generate in situ AZO-products that can be monitored with distinguishable NMR signals as a read-out. This study also allows for a direct comparison of the 15N and 13C nuclei performances in hyperpolarized reaction-based probes. The simple and general SABRE-SHEATH hyperpolarization method works on the 15N- and 13C-NO-sensing probes. Measured long spin-lattice relaxation (T1) values, especially for 15N-NO probes, will allow for real-time reaction-based imaging of NO.

Probing nitric oxide signaling using molecular MRI

Wide field measurements of nitric oxide (NO) signaling could help understand and diagnose the many physiological processes in which NO plays a key role. Magnetic resonance imaging (MRI) can support particularly powerful approaches for this purpose if equipped with molecular probes sensitized to NO and NO-associated targets. In this review, we discuss the development of MRI-detectable probes that could enable studies of nitrergic signaling in animals and potentially human subjects. Major families of probes include contrast agents designed to capture and report integrated NO levels directly, as well as molecules that respond to or emulate the activity of nitric oxide synthase enzymes. For each group, we outline the relevant molecular mechanisms and discuss results that have been obtained in vitro and in animals. The most promising in vivo data described to date have been acquired using NO capture-based relaxation agents and using engineered nitric oxide synthases that provide hemodynamic readouts of NO signaling pathway activation. These advances establish a beachhead for ongoing efforts to improve the sensitivity, specificity, and clinical applicability of NO-related molecular MRI technology.

Redox-Responsive MRI Probes Based on First-Row Transition-Metal Complexes

The presence of multiple oxidation and spin states of first-row transition-metal complexes facilitates the development of switchable MRI probes. Redox-responsive probes capitalize on a change in the magnetic properties of the different oxidation states of the paramagnetic metal ion center upon exposure to biological oxidants and reductants. Transition-metal complexes that are useful for MRI can be categorized according to whether they accelerate water proton relaxation (T₁ or T₂ agents), induce paramagnetic shifts of ¹H or ¹⁹F resonances (paraSHIFT agents), or are chemical exchange saturation transfer (CEST) agents. The various oxidation state couples and their properties as MRI probes are summarized with a focus on Co(II)/Co(III) or Fe(II)/Fe(III) complexes as small molecules or as liposomal agents. Solution studies of these MRI probes are reviewed with an emphasis on redox changes upon treatment with oxidants or with enzymes that are physiologically important in inflammation and disease. Finally, we outline the challenges of developing these probes further for in vivo MRI applications.

Designing Protein-Based Probes for Sensing Biological Analytes with Magnetic Resonance Imaging

Genetically encoded sensors provide unique advantages for monitoring biological analytes with molecular and cellular-level specificity. While sensors derived from fluorescent proteins represent staple tools in biological imaging, these probes are limited to optically accessible preparations owing to physical curbs on light penetration. In contrast to optical methods, magnetic resonance imaging (MRI) may be used to noninvasively look inside intact organisms at any arbitrary depth and over large fields of view. These capabilities have spurred the development of innovative methods to connect MRI readouts with biological targets using protein-based probes that are in principle genetically encodable. Here, we highlight the state-of-the-art in MRI-based biomolecular sensors, focusing on their physical mechanisms, quantitative characteristics, and biological applications. We also describe how innovations in reporter gene technology are creating new opportunities to engineer MRI sensors that are sensitive to dilute biological targets.

Activity-Based NIR Bioluminescence Probe Enables Discovery of Diet-Induced Modulation of the Tumor Microenvironment via Nitric Oxide

Nitric oxide (NO) plays a critical role in acute and chronic inflammation. NO's contributions to cancer are of particular interest due to its context-dependent bioactivities. For example, immune cells initially produce cytotoxic quantities of NO in response to the nascent tumor. However, it is believed that this fades over time and reaches a concentration that supports the tumor microenvironment (TME). These complex dynamics are further complicated by other factors, such as diet and oxygenation, making it challenging to establish a complete picture of NO's impact on tumor progression. Although many activity-based sensing (ABS) probes for NO have been developed, only a small fraction have been employed in vivo, and fewer yet are practical in cancer models where the NO concentration is <200 nM. To overcome this outstanding challenge, we have developed BL₆₆₀-NO, the first ABS probe for NIR bioluminescence imaging of NO in cancer. Owing to the low intrinsic background, high sensitivity, and deep tissue imaging capabilities of our design, BL₆₆₀-NO was successfully employed to visualize endogenous NO in cellular systems, a human liver metastasis model, and a murine breast cancer model. Importantly, its exceptional performance facilitated two dietary studies which examine the impact of fat intake on NO and the TME. BL₆₆₀-NO provides the first direct molecular evidence that intratumoral NO becomes elevated in mice fed a high-fat diet, which became obese with larger tumors, compared to control animals on a low-fat diet. These results indicate that an inflammatory diet can increase NO production via recruitment of macrophages and overexpression of inducible nitric oxide synthase which in turn can drive tumor progression.

Functional dissection of neural circuitry using a genetic reporter for fMRI

The complex connectivity of the mammalian brain underlies its function, but understanding how interconnected brain regions interact in neural processing remains a formidable challenge. Here we address this problem by introducing a genetic probe that permits selective functional imaging of distributed neural populations defined by viral labeling techniques. The probe is an engineered enzyme that transduces cytosolic calcium dynamics of probe-expressing cells into localized hemodynamic responses that can be specifically visualized by functional magnetic resonance imaging. Using a viral vector that undergoes retrograde transport, we apply the probe to characterize a brain-wide network of presynaptic inputs to the striatum activated in a deep brain stimulation paradigm in rats. The results reveal engagement of surprisingly diverse projection sources and inform an integrated model of striatal function relevant to reward behavior and therapeutic neurostimulation approaches. Our work thus establishes a strategy for mechanistic analysis of multiregional neural systems in the mammalian brain.

Molecular fMRI of neurochemical signaling

Magnetic resonance imaging (MRI) is the most widely applied technique for brain-wide measurement of neural function in humans and animals. In conventional functional MRI (fMRI), brain signaling is detected indirectly, via localized activity-dependent changes in regional blood flow, oxygenation, and volume, to which MRI contrast can be readily sensitized. Although such hemodynamic fMRI methods are powerful tools for analysis of brain activity, they lack specificity for the many molecules and cell types that play functionally distinct roles in neural processing. A suite of techniques collectively known to as "molecular fMRI," addresses this limitation by permitting MRI-based detection of specific molecular processes in deep brain tissue. This review discusses how molecular fMRI is coming to be used in the study of neurochemical dynamics that mediate intercellular communication in the brain. Neurochemical molecular fMRI is a potentially powerful approach for mechanistic analysis of brain-wide function, but the techniques are still in early stages of development. Here we provide an overview of the major advances and results that have been achieved to date, as well as directions for further development.

Single-nanometer iron oxide nanoparticles as tissue-permeable MRI contrast agents

Magnetic nanoparticles are robust contrast agents for MRI and often produce particularly strong signal changes per particle. Leveraging these effects to probe cellular- and molecular-level phenomena in tissue can, however, be hindered by the large sizes of typical nanoparticle contrast agents. To address this limitation, we introduce single-nanometer iron oxide (SNIO) particles that exhibit superparamagnetic properties in conjunction with hydrodynamic diameters comparable to small, highly diffusible imaging agents. These particles efficiently brighten the signal in T ₁-weighted MRI, producing per-molecule longitudinal relaxation enhancements over 10 times greater than conventional gadolinium-based contrast agents. We show that SNIOs permeate biological tissue effectively following injection into brain parenchyma or cerebrospinal fluid. We also demonstrate that SNIOs readily enter the brain following ultrasound-induced blood-brain barrier disruption, emulating the performance of a gadolinium agent and providing a basis for future biomedical applications. These results thus demonstrate a platform for MRI probe development that combines advantages of small-molecule imaging agents with the potency of nanoscale materials.

Molecular characterization of the nitric oxide synthase gene and its immunomodulation of nitric oxide production in the triangle shell mussel (Hyriopsis cumingii)

Nitric oxide synthase (NOS) is a critical enzyme that catalyzes nitric oxide biosynthesis and orchestrates various immunological responses mediated by nitric oxide (NO) in host animals. In this study, the NOS gene was identified in the triangle shell mussel (Hyriopsis cumingii) (HcNOS). HcNOS was highly conserved in the characteristic gene structures of NOS. Phylogenetic analysis suggested that HcNOS was a typical invertebrate NOS. Further gene expression analysis, NOS activity assays and nitric oxide content measurements demonstrated the inducibility of HcNOS in responses to lipopolysaccharide (LPS) challenge and during tissue transplantation. Of note, mantle grafting induced a prolonged HcNOS/NO response, suggesting that through the HcNOS/NO system, multiple immunomodulators may play decisive roles in tissue grafting in triangle shell mussels. Thus, HcNOS appears to be a crucial player in responding to both bacterial infection and tissue transplantation.

Modeling NO Biotransport in Brain Using a Space-Fractional Reaction-Diffusion Equation

Nitric oxide (NO) is a small gaseous molecule that is involved in some critical biochemical processes in the body such as the regulation of cerebral blood flow and pressure. Infection and inflammatory processes such as those caused by COVID-19 produce a disequilibrium in the NO bioavailability and/or a delay in the interactions of NO with other molecules contributing to the onset and evolution of cardiocerebrovascular diseases. A link between the SARS-CoV-2 virus and NO is introduced. Recent experimental observations of intracellular transport of metabolites in the brain and the NO trapping inside endothelial microparticles (EMPs) suggest the possibility of anomalous diffusion of NO, which may be enhanced by disease processes. A novel space-fractional reaction-diffusion equation to model NO biotransport in the brain is further proposed. The model incorporates the production of NO by synthesis in neurons and by mechanotransduction in the endothelial cells, and the loss of NO due to its reaction with superoxide and interaction with hemoglobin. The anomalous diffusion is modeled using a generalized Fick’s law that involves spatial fractional order derivatives. The predictive ability of the proposed model is investigated through numerical simulations. The implications of the methodology for COVID-19 outlined in the section “Discussion” are purely exploratory.

Highly Sensitive D-A-D-Type Near-Infrared Fluorescent Probe for Nitric Oxide Real-Time Imaging in Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a common gastrointestinal inflammatory disease, affecting a huge number of people worldwide with increasing morbidity each year. Although the etiology of IBD has not been fully elucidated, it is understood to be closely related to upregulation of the production of NO. Herein, we first report a donor-acceptor-donor (D-A-D)-type near-infrared (NIR) fluorescent probe LS-NO for real-time detection of NO in IBD by harnessing the enhanced intramolecular charge transfer mechanism. LS-NO exhibited good water solubility, high photostability, and excellent NIR absorbance and emission at 700 and 750/800 nm, respectively. Moreover, it was able to sensitively and specifically detect exogenous and endogenous NO in the lysosomes of living cells. Notably, LS-NO enabled to noninvasively visualize NO generation in a lipopolysaccharide-induced IBD mouse model for 30 h, showing a two- to threefold higher NIR fluorescence intensity in the intestines and feces of IBD mice than normal mice. This work demonstrates that LS-NO is promising as a diagnosis agent for real-time detection of NO in IBD and may promote inflammatory stool examination simultaneously.

Metal-based environment-sensitive MRI contrast agents

Interactions of paramagnetic metal complexes with their biological environment can modulate their magnetic resonance imaging (MRI) contrast-enhancing properties in different ways, and this has been widely exploited to create responsive probes that can provide biochemical information. We survey progress in two rapidly growing areas: the MRI detection of biologically important metal ions, such as calcium, zinc, and copper, and the use of transition metal complexes as smart MRI agents. In both fields, new imaging technologies, which take advantage of other nuclei (¹⁹F) and/or paramagnetic contact shift effects, emerge beyond classical, relaxation-based applications. Most importantly, in vivo imaging is gaining ground, and the promise of molecular MRI is becoming reality, at least for preclinical research.