A Molecular Probe for the Detection of Polar Lipids in Live Cells

Neutral rhenium(I) tricarbonyl complexes with sulfur-donor ligands: anti-proliferative activity and cellular localization

Rhenium(I) tricarbonyl complexes are widely studied for their cell imaging properties and anti-cancer and anti-microbial activities, but the complexes with S-donor ligands remain relatively unexplored. A series of six fac-[Re(NN)(CO)3(SR)] complexes, where (NN) is 2,2'-bipyridyl (bipy) or 1,10-phenanthroline (phen), and RSH is a series of thiocarboxylic acid methyl esters, have been synthesized and characterized. Cellular uptake and anti-proliferative activities of these complexes in human breast cancer cell lines (MDA-MB-231 and MCF-7) were generally lower than those of the previously described fac-[Re(NN)(CO)3(OH2)]+ complexes; however, one of the complexes, fac-[Re(CO)3(phen)(SC(Ph)CH2C(O)OMe)] (3b), was active (IC50 ∼ 10 μM at 72 h treatment) in thiol-depleted MDA-MB-231 cells. Moreover, unlike fac-[Re(CO)3(phen)(OH2)]+, this complex did not lose activity in the presence of extracellular glutathione. Taken together these properties show promise for further development of 3b and its analogues as potential anti-cancer drugs for co-treatment with thiol-depleting agents. Conversely, the stable and non-toxic complex, fac-[Re(bipy)(CO)3(SC(Me)C(O)OMe)] (1a), predominantly localized in the lysosomes of MDA-MB-231 cells, as shown by live cell confocal microscopy (λex = 405 nm, λem = 470-570 nm). It is strongly localized in a subset of lysosomes (25 μM Re, 4 h treatment), as shown by co-localization with a Lysotracker dye. Longer treatment times with 1a (25 μM Re for 48 h) resulted in partial migration of the probe into the mitochondria, as shown by co-localization with a Mitotracker dye. These properties make complex 1a an attractive target for further development as an organelle probe for multimodal imaging, including phosphorescence, carbonyl tag for vibrational spectroscopy, and Re tag for X-ray fluorescence microscopy.

Synthesis and cellular uptake of neutral rhenium(I) morpholine complexes

Morpholine motifs have been used extensively as targeting moieties for lysosomes, primarily in fluorescence imaging agents. Traditionally these imaging agents are based on organic molecules which have several shortcomings including small Stokes shifts, short emission lifetimes, and susceptibility to photobleaching. To explore alternative lysosome targeting imaging agents we have used a rhenium based phosphorescent platform which has been previously demonstrated to have an improved Stokes shift, a long lifetime emission, and is highly photostable. Rhenium complexes containing morpholine substituted ligands were designed to accumulate in acidic compartments. Two of the three complexes prepared exhibited bright emission in cells, when incubated at low concentrations (20 μM) and were non-toxic at concentrations as high as 100 μM, making them suitable for live cell imaging. We show that the rhenium complexes are amenable to chemical modification and that the morpholine targeted derivatives can be used for live cell confocal fluorescence imaging of endosomes-lysosomes.

Analytical Techniques for Single-Cell Biochemical Assays of Lipids

Lipids are essential cellular components forming membranes, serving as energy reserves, and acting as chemical messengers. Dysfunction in lipid metabolism and signaling is associated with a wide range of diseases including cancer and autoimmunity. Heterogeneity in cell behavior including lipid signaling is increasingly recognized as a driver of disease and drug resistance. This diversity in cellular responses as well as the roles of lipids in health and disease drive the need to quantify lipids within single cells. Single-cell lipid assays are challenging due to the small size of cells (∼1 pL) and the large numbers of lipid species present at concentrations spanning orders of magnitude. A growing number of methodologies enable assay of large numbers of lipid analytes, perform high-resolution spatial measurements, or permit highly sensitive lipid assays in single cells. Covered in this review are mass spectrometry, Raman imaging, and fluorescence-based assays including microscopy and microseparations.

Synthesis and the photophysical and biological properties of tricarbonyl Re(I) diimine complexes bound to thiotetrazolato ligands

Twelve Re(I) tricarbonyl diimine (2,2'-bipyridine and 1,10-phenanthroline) complexes with thiotetrazolato ligands have been synthesised and fully characterised. Structural characterisation revealed the capacity of the tetrazolato ligand to bind to the Re(I) centre through either the S atom or the N atom with crystallography revealing most complexes being bound to the N atom. However, an example where the Re(I) centre is linked via the S atom has been identified. In solution, the complexes exist as an equilibrating mixture of linkage isomers, as suggested by comparison of their NMR spectra at room temperature and 373 K, as well as 2D exchange spectroscopy. The complexes are photoluminescent in fluid solution at room temperature, with emission either at 625 or 640 nm from the metal-to-ligand charge transfer excited states of triplet multiplicity, which seems to be exclusively dependent on the nature of the diimine ligand. The oxygen-sensitive excited state lifetime decay ranges between 12.5 and 27.5 ns for the complexes bound to 2,2'-bipyrdine, or between 130.6 and 155.2 ns for those bound to 1.10-phenanthroline. Quantum yields were measured within 0.4 and 1.5%. The complexes were incubated with human lung (A549), brain (T98g), and breast (MDA-MB-231) cancer cells, as well as with normal human skin fibroblasts (HFF-1), revealing low to moderate cytotoxicity, which for some compounds exceeded that of a standard anti-cancer drug, cisplatin. Low cytotoxicity combined with significant cellular uptake and photoluminescence properties provides potential for their use as cellular imaging agents. Furthermore, the complexes were assessed in disc diffusion and broth microdilution assays against methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus (VRE), Escherichia coli (E. coli), and Pseudomonas aeruginosa (P. aeruginosa) bacterial strains, which revealed negligible antibacterial activity in the dark or after irradiation.

Rhenium(I) conjugates as tools for tracking cholesterol in cells

Cholesterol is vital to control membrane integrity and fluidity, but is also a precursor to produce steroid hormones, bile acids, and vitamin D. Consequently, altered cholesterol biology has been linked to many diseases, including metabolic syndromes and cancer. Defining the intracellular pools of cholesterol and its trafficking within cells is essential to understand both normal cell physiology and mechanisms of pathogenesis. We have synthesized a new cholesterol mimic (ReTEGCholestanol), comprising a luminescent rhenium metal complex and a cholestanol targeting unit, linked using a tetraethylene glycol (TEG) spacer. ReTEGCholestanol demonstrated favourable imaging properties and improved water solubility when compared to a cholesterol derivative, and structurally related probes lacking the TEG linker. A non-malignant and three malignant prostate cell lines were used to characterize the uptake and intracellular distribution of ReTEGCholestanol. The ReTEGCholestanol complex was effectively internalized and mainly localized to late endosomes/lysosomes in non-malignant PNT1a cells, while in prostate cancer cells it also accumulated in early endosomes and multivesicular bodies, suggesting disturbed cholesterol biology in the malignant cells. The ReTEGCholestanol is a novel imaging agent for visualizing endosomal uptake and trafficking, which may be used to define cholesterol related biology including membrane integration and altered lipid trafficking/processing.

Fluorescence Microscopy-An Outline of Hardware, Biological Handling, and Fluorophore Considerations

Fluorescence microscopy has become a critical tool for researchers to understand biological processes at the cellular level. Micrographs from fixed and live-cell imaging procedures feature in a plethora of scientific articles for the field of cell biology, but the complexities of fluorescence microscopy as an imaging tool can sometimes be overlooked or misunderstood. This review seeks to cover the three fundamental considerations when designing fluorescence microscopy experiments: (1) hardware availability; (2) amenability of biological models to fluorescence microscopy; and (3) suitability of imaging agents for intended applications. This review will help equip the reader to make judicious decisions when designing fluorescence microscopy experiments that deliver high-resolution and informative images for cell biology.

A Bimodal Fluorescence-Raman Probe for Cellular Imaging

Biochemical changes in specific organelles underpin cellular function, and studying these changes is crucial to understand health and disease. Fluorescent probes have become important biosensing and imaging tools as they can be targeted to specific organelles and can detect changes in their chemical environment. However, the sensing capacity of fluorescent probes is highly specific and is often limited to a single analyte of interest. A novel approach to imaging organelles is to combine fluorescent sensors with vibrational spectroscopic imaging techniques; the latter provides a comprehensive map of the relative biochemical distributions throughout the cell to gain a more complete picture of the biochemistry of organelles. We have developed NpCN1, a bimodal fluorescence-Raman probe targeted to the lipid droplets, incorporating a nitrile as a Raman tag. NpCN1 was successfully used to image lipid droplets in 3T3-L1 cells in both fluorescence and Raman modalities, reporting on the chemical composition and distribution of the lipid droplets in the cells.

Neutral Re(I) Complex Platform for Live Intracellular Imaging

Luminescent metal complexes are a valuable platform for the generation of cell imaging agents. However, many metal complexes are cationic, a factor that can dominate the intracellular accumulation to specific organelles. Neutral Re(I) complexes offer a more attractive platform for the development of bioconjugated imaging agents, where charge cannot influence their intracellular distribution. Herein, we report the synthesis of a neutral complex (ReAlkyne), which was used as a platform for the generation of four carbohydrate-conjugated imaging agents via Cu(I)-catalyzed azide-alkyne cycloaddition. A comprehensive evaluation of the physical and optical properties of each complex is provided, together with a determination of their utility as live cell imaging agents in H9c2 cardiomyoblasts. Unlike their cationic counterparts, many of which localize within mitochondria, these neutral complexes have localized within the endosomal/lysosomal network, a result consistent with examples of dinuclear carbohydrate-appended neutral Re(I) complexes that have been reported. This further demonstrates the utility of these neutral Re(I) complex imaging platforms as viable imaging platforms for the development of bioconjugated cell imaging agents.

Spectroscopic and Molecular Docking Study of the Interaction between Neutral Re(I) Tetrazolate Complexes and Bovine Serum Albumin

Re(I) complexes have potential in biomedical sciences as imaging agents, diagnostics and therapeutics. Thus, it is crucial to understand how Re(I) complexes interact with carrier proteins, like serum albumins. Here, two neutral Re(I) complexes were used (fac-[Re(CO)₃ (1,10-phenanthroline)L], in which L is either 4-cyanophenyltetrazolate (1) or 4-methoxycarbonylphenyltetrazole ester (2), to study the interactions with bovine serum albumin (BSA). Spectroscopic measurements, calculations of thermodynamic and Förster resonance energy transfer parameters, as well as molecular modelling, were performed to study differential binding between BSA and complex 1 and 2. Induced-fit docking combined with quantum-polarised ligand docking were employed in what is believed to be a first for a Re(I) complex as a ligand for BSA. Our findings provide a basis for other molecular interaction studies and suggest that subtle functional group alterations at the terminal region of the Re(I) complex have a significant impact on the ability of this class of compounds to interact with BSA.

Detecting metabolic differences in fetal and adult sheep adipose and skeletal muscle tissues

The primary metabolic pathway required to produce ATP differs as a result of tissue type, developmental stage and substrate availability. We utilized molecular and histological techniques to define the metabolic status in foetal and adult, adipose and skeletal muscle tissues. Redox ratios of these tissues were also determined optically by two-photon microscopy. Adult perirenal adipose tissue had a higher optical redox ratio than fetal perirenal adipose tissue, which aligned with glycolysis being used for ATP production; whereas adult skeletal muscle had a lower optical redox ratio than fetal skeletal muscle, which aligned with oxygen demanding oxidative phosphorylation activity being utilized for ATP production. We have compared traditional molecular and microscopy techniques of metabolic tissue characterization with optical redox ratios to provide a more comprehensive report on the dynamics of tissue metabolism.

Photophysical and biological investigation of phenol substituted rhenium tetrazolato complexes

The synthesis, structural and photophysical characterisation of four tricarbonyl rhenium(i) complexes bound to 1,10-phenanthroline and a tetrazolato ancillary ligand are reported. The complexes are differentiated by the nature (hydroxy or methoxy) and position (meta or para) of the substituent attached to the phenyl ring in conjugation to the tetrazole ring. The complexes exhibit phosphorescence emission from triplet charge transfer excited states, with the maxima around 600 nm, excited state lifetime decays in the 200-300 ns range, and quantum yield values of 4-6% in degassed acetonitrile solutions. The nature and position of the substituent does not significantly affect the photophysical properties, which remain unchanged even after deprotonation of the hydroxide group on the phenol ring. The interpretation of the photophysical data was further validated by resonance Raman spectroscopy and time-dependent density functional theory calculations. All the complexes are internalised within cells, albeit to variable degrees. As highlighted by a combination of flow cytometry and confocal microscopy, the species display diffuse cytoplasmic localisation except for the complex with the hydroxy functional group at the para position, which reveals lower accumulation in cells and more pronounced punctate staining. Overall, the complexes displayed low levels of cytotoxicity.

Development and mechanistic study of a microemulsion containing vitamin E TPGS for the enhancement of oral absorption of celecoxib

Purpose: The purpose of this study was to develop a microemulsion containing D-α-tocopheryl polyethylene glycol 1000 succinate (vitamin E TPGS) as a biodegradable surfactant to increase the oral absorption of celecoxib. Methods: This study investigated the intestinal absorption enhancement mechanism of this microemulsion by measuring transepithelial electrical resistance (TEER) values. This study also evaluated microemulsion particle-intestine interactions in terms of release and attachment processes using confocal laser scanning microscopy (CLSM). Results: The prepared microemulsion particles had a size of <300 nm with a neutral surface charge. The celecoxib-loaded microemulsion release kinetic was classified as the zero-order model. This vitamin E TPGS-based microemulsion significantly increased the in vitro intestinal absorption of celecoxib compared to celecoxib solution. The CLSM study suggested that microemulsion particles with entrapped drugs might attach to the intestinal epithelium before releasing the entrapped drug into tissues. The TEER value of the intestinal tissues treated with the celecoxib-loaded microemulsion was significantly decreased compared to the value before treatment, indicating an increase in drug transport via the paracellular pathway. The evaluation of intestinal tissue cytotoxicity using lactate dehydrogenase cytotoxicity assay suggested that the prepared celecoxib-loaded microemulsion was safe for oral route administration. Conclusions: The prepared celecoxib loaded microemulsion could increase the intestinal absorption of celecoxib compared to celecoxib solution. The intestinal absorption enhancement mechanism of this microemulsion resulted from the increase of the drug transport via the paracellular pathway.

Individual body size as a predictor of lipid storage in Baltic Sea zooplankton

The size structure of a zooplankton community is frequently used as a trait reflecting functional properties, including biochemical composition. Therefore, a shift in zooplankton body size can reflect shifts in the nutritional quality of zooplankton. In dominant Baltic copepods and cladocerans, neutral to polar lipid ratio (NL/PL ratio), a proxy for the mass-normalized lipid storage, was determined and related to individual body weight. A significant relationship between the NL/PL ratio and body weight was found; the latter was the strongest and the most significant predictor of the lipid storage capacity across different species and developmental stages. These findings provide support for using mean body weight in zooplankton community as a proxy for lipid storage capacity of zooplankton prey and justify applicability of zooplankton mean size as an indicator of nutritional conditions for Baltic zooplanktivores.

Lipid profiles of prostate cancer cells

Lipids are important cellular components which can be significantly altered in a range of disease states including prostate cancer. Here, a unique systematic approach has been used to define lipid profiles of prostate cancer cell lines, using quantitative mass spectrometry (LC-ESI-MS/MS), FTIR spectroscopy and fluorescent microscopy. All three approaches identified significant difference in the lipid profiles of the three prostate cancer cell lines (DU145, LNCaP and 22RV1) and one non-malignant cell line (PNT1a). Specific lipid classes and species, such as phospholipids (e.g., phosphatidylethanolamine 18:1/16:0 and 18:1/18:1) and cholesteryl esters, detected by LC-ESI-MS/MS, allowed statistical separation of all four prostate cell lines. Lipid mapping by FTIR revealed that variations in these lipid classes could also be detected at a single cell level, however further investigation into this approach would be needed to generate large enough data sets for quantitation. Visualisation by fluorescence microscopy showed striking variations that could be observed in lipid staining patterns between cell lines allowing visual separation of cell lines. In particular, polar lipid staining by a fluorescent marker was observed to increase significantly in prostate cancer lines cells, when compared to PNT1a cells, which was consistent with lipid quantitation by LC-ESI-MS/MS and FTIR spectroscopy. Thus, multiple technologies can be employed to either quantify or visualise changes in lipid composition, and moreover specific lipid profiles could be used to detect and phenotype prostate cancer cells.

A pH-Responsive Amphiphilic Hydrogel Based on Pseudopeptides and Poly(ethylene glycol) for Oral Delivery of Hydrophobic Drugs

Oral administration is a noninvasive and convenient drug delivery route most preferred by patients. However, poor stability in the gastrointestinal tract and low bioavailability of hydrophobic drugs has greatly limited their oral administration. To address this problem, we report a pH-responsive, amphiphilic hydrogel drug carrier based on a pseudopeptide poly(l-lysine isophthalamide) (PLP) and poly(ethylene glycol) (PEG). The hydrogels were prepared by a simple N-(3-(dimethylamino)propyl)-N'-ethyl carbodiimide hydrochloride (EDC)/N-hydroxysuccinimide (NHS) coupling reaction, and the cross-linking was confirmed by infrared spectroscopy and differential scanning calorimetry analyses. Because of the pH-responsive conformational alteration of PLP, the hydrogels were relatively hydrophobic and collapsed at acidic pH, but became hydrophilic and swollen at neutral pH. The amphiphilicity enabled the hydrogels to well retain and protect hydrophobic model drugs in the simulated gastric fluid, but efficiently release them in the simulated intestinal fluid. These results suggested that the pH-responsive amphiphilic hydrogels are promising candidates for oral delivery of hydrophobic drugs.

Luminescent protein staining with Re(i) tetrazolato complexes

Re(i) tricarbonyl diiimine complexes for the first time exploited as luminescent staining agents for SDS-PAGE.

Investigating Intracellular Localisation and Cytotoxicity Trends for Neutral and Cationic Iridium Tetrazolato Complexes in Live Cells

A family of five neutral cyclometalated iridium(III) tetrazolato complexes and their methylated cationic analogues have been synthesised and characterised. The complexes are distinguished by variations of the substituents or degree of π conjugation on either the phenylpyridine or tetrazolato ligands. The photophysical properties of these species have been evaluated in organic and aqueous media, revealing predominantly a solvatochromic emission originating from mixed metal-to-ligand and ligand-to-ligand charge transfer excited states of triplet multiplicity. These emissions are characterised by typically long excited-state lifetimes (∼hundreds of ns), and quantum yields around 5-10 % in aqueous media. Methylation of the complexes caused a systematic red-shift of the emission profiles. The behaviour and the effects of the different complexes were then examined in cells. The neutral species localised mostly in the endoplasmic reticulum and lipid droplets, whereas the majority of the cationic complexes localised in the mitochondria. The amount of complexes found within cells does not depend on lipophilicity, which potentially suggests diverse uptake mechanisms. Methylated analogues were found to be more cytotoxic compared to the neutral species, a behaviour that might to be linked to a combination of uptake and intracellular localisation.

Development of a novel microemulsion for oral absorption enhancement of all-trans retinoic acid

This study was aimed to develop a novel microemulsion that contained oleth-5 as a surfactant to enhance the oral absorption of all-trans retinoic acid (ATRA). The prepared microemulsion was evaluated for its particle size, shape, zeta potential, in vitro release, in vitro intestinal absorption, intestinal membrane cytotoxicity and stability. The obtained microemulsion was spherical in shape with a particle size of <200 nm and a negative surface charge. The in vitro release of the ATRA-loaded microemulsion was best fit with the zero-order model. This microemulsion significantly improved the intestinal absorption of ATRA. Confocal laser scanning microscopy analysis using a fluorescent dye-loaded microemulsion also confirmed the intestinal absorption result. The intestinal membrane cytotoxicity of the ATRA-loaded microemulsion did not differ from an edible oil (fish oil). Stability testing showed that the ATRA-loaded microemulsion was more stable at 25°C than 40°C.

Imaging and lipidomics methods for lipid analysis in metabolic and cardiovascular disease

Cardiometabolic diseases exhibit changes in lipid biology, which is important as lipids have critical roles in membrane architecture, signalling, hormone synthesis, homoeostasis and metabolism. However, Developmental Origins of Health and Disease studies of cardiometabolic disease rarely include analysis of lipids. This short review highlights some examples of lipid pathology and then explores the technology available for analysing lipids, focussing on the need to develop imaging modalities for intracellular lipids. Analytical methods for studying interactions between the complex endocrine and intracellular signalling pathways that regulate lipid metabolism have been critical in expanding our understanding of how cardiometabolic diseases develop in association with obesity and dietary factors. Biochemical methods can be used to generate detailed lipid profiles to establish links between lifestyle factors and metabolic signalling pathways and determine how changes in specific lipid subtypes in plasma and homogenized tissue are associated with disease progression. New imaging modalities enable the specific visualization of intracellular lipid traffic and distribution in situ. These techniques provide a dynamic picture of the interactions between lipid storage, mobilization and signalling, which operate during normal cell function and are altered in many important diseases. The development of methods for imaging intracellular lipids can provide a dynamic real-time picture of how lipids are involved in complex signalling and other cell biology pathways; and how they ultimately regulate metabolic function/homoeostasis during early development. Some imaging modalities have the potential to be adapted for in vivo applications, and may enable the direct visualization of progression of pathogenesis of cardiometabolic disease after poor growth in early life.

Lipids: Evergreen autofluorescent biomarkers for the liver functional profiling

Depending on their chemical nature, lipids can be classified in two main categories: hydrophilic, greatly contributing to membrane composition and subcellular organelle compartmentalization, and hydrophobic, mostly triglycerides, greatly enrolled in the storage and production of energy. In both cases, some lipid molecules can be involved as signaling agents in the regulation of metabolism and protective or damaging pathways in responses to harmful stimuli. These events could affect in particular the liver, because of its central role in the maintenance of lipid homeostasis. Lipids have been demonstrated to fluoresce, contributing to the overall emission signal of the liver tissue along with other endogenous fluorophores, relatable to energy metabolism and oxidative events. The mere estimation of the fluorescing lipid fraction in parallel with the other endogenous fluorophores, and with the common biochemical and histochemical biomarkers of tissue injury has been exploited to investigate the liver morpho- functional conditions in experimental hepatology. More interestingly, the fluorescing lipid fraction is greatly relatable to free fatty acids such as arachidonic, linoleic and linolenic acid, which are deserving increasing attention as precursors of products involved in several and complex signaling pathways. On these bases, the ability of autofluorescence to detect directly arachidonic acid and its balance with other unsaturated fatty acids may be exploited in the diagnosis and follow-up of fatty livers, helping to improve the personalization of the metabolic/ lipidomic profiling. This could also contribute to elucidate the role of the injuring factors in the choice of suitable donors, and in the set-up of preservation procedures in liver transplantation.

Recent development of luminescent rhenium(i) tricarbonyl polypyridine complexes as cellular imaging reagents, anticancer drugs, and antibacterial agents

This Perspective summarizes recent advances in the biological applications of luminescent rhenium(i) tricarbonyl polypyridine complexes.