Cell-penetrating peptides as delivery vehicles for a protein-targeted terbium complex

Cell-Penetrating Peptides: A Powerful Tool for Targeted Drug Delivery

The cellular membrane hinders the effective delivery of therapeutics to targeted sites. Cellpenetrating peptide (CPP) is one of the best options for rapidly internalizing across the cellular membrane. CPPs have recently attracted lots of attention because of their excellent transduction efficiency and low cytotoxicity. The CPP-cargo complex is an effective and efficient method of delivering several chemotherapeutic agents used to treat various diseases. Additionally, CPP has become another strategy to overcome some of the current therapeutic agents' limitations. However, no CPP complex is approved by the US FDA because of its limitations and issues. In this review, we mainly discuss the cellpenetrating peptide as the delivery vehicle, the cellular uptake mechanism of CPPs, their design, and some strategies to synthesize the CPP complex via some linkers such as disulfide bond, oxime, etc. Here, we also discuss the recent status of CPPs in the market.

Bright and stable luminescent probes for target engagement profiling in live cells

The pace of progress in biomedical research directly depends on techniques that enable the quantitative interrogation of interactions between proteins and other biopolymers, or with their small-molecule ligands. Time-resolved Förster resonance energy transfer (TR-FRET) assay platforms offer high sensitivity and specificity. However, the paucity of accessible and biocompatible luminescent lanthanide complexes, which are essential reagents for TR-FRET-based approaches, and their poor cellular permeability have limited broader adaptation of TR-FRET beyond homogeneous and extracellular assay applications. Here, we report the development of CoraFluors, a new class of macrotricyclic terbium complexes, which are synthetically readily accessible, stable in biological media and exhibit photophysical and physicochemical properties that are desirable for biological studies. We validate the performance of CoraFluors in cell-free systems, identify cell-permeable analogs and demonstrate their utility in the quantitative domain-selective characterization of Keap1 ligands, as well as in isoform-selective target engagement profiling of HDAC1 inhibitors in live cells.

Lanthanide-based resonance energy transfer biosensors for live-cell applications

Lanthanide-based, Förster resonance energy transfer (LRET) biosensors enable sensitive, time-gated luminescence (TGL) imaging or multiwell plate analysis of protein-protein interactions (PPIs) in living mammalian cells. LRET biosensors are polypeptides that consist of an alpha-helical linker sequence sandwiched between a lanthanide complex-binding domain and a fluorescent protein (FP) with two interacting domains residing at each terminus. Interaction between the terminal affinity domains brings the lanthanide complex and FP in close proximity such that lanthanide-to-FP, LRET-sensitized emission is increased. A recent proof-of-concept study examined model biosensors that incorporated the affinity partners FKBP12 and the rapamycin-binding domain of m-Tor (FRB) as well as p53 (1-92) and HDM2 (1-128). The sensors contained an Escherichia coli dihydrofolate reductase (eDHFR) domain that binds with high selectivity and affinity to Tb(III) complexes coupled to the ligand trimethoprim (TMP). When cell lines that stably expressed the sensors were treated with TMP-Tb(III), TGL microscopy revealed dramatic differences (>500%) in donor- or acceptor-denominated, Tb(III)-to-GFP LRET ratios between open (unbound) and closed (bound) states of the biosensors. Much larger signal changes (>2500%) and Z'-factors of 0.5 or more were observed when cells were grown in 96-well or 384-well plates and analyzed using a TGL plate reader. In this chapter, we elaborate on the design and performance of LRET biosensors and provide detailed protocols to guide their use for live-cell microscopic imaging studies and high-throughput library screening.

Single-Chain Lanthanide Luminescence Biosensors for Cell-Based Imaging and Screening of Protein-Protein Interactions

Lanthanide-based, Förster resonance energy transfer (LRET) biosensors enabled sensitive, time-gated luminescence (TGL) imaging or multiwell plate analysis of protein-protein interactions (PPIs) in living cells. We prepared stable cell lines that expressed polypeptides composed of an alpha helical linker flanked by a Tb(III) complex-binding domain, GFP, and two interacting domains at each terminus. The PPIs examined included those between FKBP12 and the rapamycin-binding domain of m-Tor (FRB) and between p53 (1–92) and HDM2 (1–128). TGL microscopy revealed dramatic differences (>500%) in donor- or acceptor-denominated, Tb(III)-to-GFP LRET ratios between open (unbound) and closed (bound) states of the biosensors. We observed much larger signal changes (>2,500%) and Z′-factors of 0.5 or more when we grew cells in 96- or 384-well plates and analyzed PPI changes using a TGL plate reader. The modular design and exceptional dynamic range of lanthanide-based LRET biosensors will facilitate versatile imaging and cell-based screening of PPIs.

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Non-invasive, microscopic imaging or screening of protein-protein interactions

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Intracellular assembly of sensor polypeptides with luminescent Tb(III) complexes

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High dynamic range with time-gated detection of Tb(III)-to-GFP sensitized emission

TP10-Dopamine Conjugate as a Potential Therapeutic Agent in the Treatment of Parkinson's Disease

Parkinson's disease (PD) is a common progressive neurodegenerative disorder for which the current treatment is not fully satisfactory. One of the major drawbacks of current PD therapy is poor penetration of drugs across the blood-brain barrier (BBB). In recent years, cell-penetrating peptides (CPPs) such as Tat, SynB, or TP10 have gained great interest due to their ability to penetrate cell membranes and to deliver different cargos to their targets including the central nervous system (CNS). However, there is no data with respect to the use of CPPs as drug carriers to the brain for the treatment of PD. In the presented research, the covalent TP10-dopamine conjugate was synthesized and its pharmacological properties were characterized in terms of its ability to penetrate the BBB and anti-parkinsonian activity. The results showed that dopamine (DA) in the form of a conjugate with TP10 evidently gained access to the brain tissue, exhibited low susceptibility to O-methylation reaction by catechol- O-methyltransferase (lower than that of DA), possessed a relatively high affinity to both dopamine D₁ and D₂ receptors (in the case of D₁, a much higher than that of DA), and showed anti-parkinsonian activity (higher than that of l-DOPA) in the MPTP-induced preclinical animal model of PD. The presented results prove that the conjugation of TP10 with DA may be a good starting point for the development of a new strategy for the treatment of PD.

Lanthanide-based tools for the investigation of cellular environments

Biological probes constructed from lanthanides can provide a variety of readout signals, such as the luminescence of Eu(iii), Tb(iii), Yb(iii), Sm(iii) and Dy(iii), and the proton relaxation enhancement of Gd(iii) and Eu(ii). For numerous applications the intracellular delivery of the lanthanide probe is essential. Here, we review the methods for the intracellular delivery of non-targeted complexes (i.e. where the overall complex structure enhances cellular uptake), as well as complexes attached to a targeting unit (i.e. to a peptide or a small molecule) that facilitates delivery. The cellular applications of lanthanide-based supramolecules (dendrimers, metal organic frameworks) are covered briefly. Throughout, we emphasize the techniques that can confirm the intracellular localization of the lanthanides and those that enable the determination of the fate of the probes once inside the cell. Finally, we highlight methods that have not yet been applied in the context of lanthanide-based probes, but have been successful in the intracellular delivery of other metal-based probes.

Efficient route to pre-organized and linear polyaminopolycarboxylates: Cy-TTHA, Cy-DTPA and mono/di- reactive, tert-butyl protected TTHA/Cy-TTHA

Pre-organized polyaminopolycarboxylate chelators Cy-TTHA and Cy-DTPA were synthesized via modular five-step syntheses from commercially available starting materials in ~ 62% and 47% overall yields, respectively. Furthermore, strategies are reported for the efficient preparation of mono- and di-reactive, tert-butyl-protected TTHA/Cy-TTHA to selectively functionalize central chelators' carboxylic acids.

Brightly Luminescent and Kinetically Inert Lanthanide Bioprobes Based on Linear and Preorganized Chelators

The synthesis, photophysical properties, and kinetic stability of a series of water-soluble, highly emissive Tb(III) and Eu(III) complexes featuring triethylenetetraamine hexaacetic acid (TTHA) and cyclohexyl triethylenetetraamine hexaacetic acid (cyTTHA) chelator scaffolds and carbostyril sensitizers are reported. The unique and modular design of the chelators gives rise to striking quantum yields of emission in aqueous solutions (up to 54%) as well as the characteristic lanthanides' photophysical properties (long excited-state lifetimes, large effective Stokes shifts, and narrow emission peaks). Furthermore, the preorganized chelators (L3, L4, and L6) bind metal within minutes at ambient temperature yet exhibit substantial resistance to transchelation in the presence of a challenge solution (EDTA, 1 mM). Moreover, the Eu(III) complex of L4 remains stably luminescent in HeLa cells over hours, demonstrating the suitability of these compounds for live-cell imaging applications. Representative chelators suitable for derivatization and protein bioconjugation were also prepared that were functionalized with clickable azide and alkyne moieties, biotin, and trimethoprim (TMP). With exceptional long-wavelength brightness, enhanced kinetic inertness, and an adaptable synthetic route, the reported lanthanide complexes are promising probes and labels for time-gated bioanalysis, biosensing, and optical microscopy.

Transportan 10 improves the anticancer activity of cisplatin

The aim of this paper was to examine whether cell-penetrating peptides (CPPs) such as transportan 10 (TP10) or protein transduction domain (PTD4) may improve the anticancer activity of cisplatin (cPt). The complexes of TP10 or PTD4 with cPt were used in the experiments. They were carried out on two non-cancer (HEK293 (human embryonic kidney) and HEL299 (human embryo lung)) and two cancer (HeLa (human cervical cancer) and OS143B (human osteosarcoma 143B)) cell lines. Both complexes were tested (MTT assay) with respect to their anticancer or cytotoxic actions. TAMRA (fluorescent dye)-stained preparations were visualized in a fluorescence microscope. The long-term effect of TP10 + cPt and its components on non-cancer and cancer cell lines was observed in inverted phase contrast microscopy. In the MTT test (cell viability assay), the complex of TP10 + cPt produced a more potent effect on the cancer cell lines (HeLa, OS143B) in comparison to that observed after separate treatment with TP10 or cPt. At the same time, the action of the complex and its components was rather small on non-cancer cell lines. On the other hand, a complex of another CPP with cPt, i.e., PTD4 + cPt, was without a significant effect on the cancer cell line (OS143B). The images of the fluorescent microscopy showed TAMRA-TP10 or TAMRA-TP10 + cPt in the interior of the HeLa cells. In the case of TAMRA-PTD4 or TAMRA-PTD4 + cPt, only the first compound was found inside the cancer cell line. In contrast, none of the tested compounds gained access to the interior of the non-cancer cells (HEK293, HEL299). Long-term incubation with the TP10 + cPt (estimated by inverted phase contrast microscopy) lead to an enhanced action of the complex on cell viability (decrease in the number of cells and change in their morphology) as compared with that produced by each single agent. With regard to the tested CPPs, only TP10 improved the anticancer activity of cisplatin if both compounds were used in the form of a complex. Additionally, the complex was relatively safe for non-cancer cells. What is more, TP10 also produced an anticancer effect on HeLa and OS143B cell lines.

Evaluating the performance of time-gated live-cell microscopy with lanthanide probes

Probes and biosensors that incorporate luminescent Tb(III) or Eu(III) complexes are promising for cellular imaging because time-gated microscopes can detect their long-lifetime (approximately milliseconds) emission without interference from short-lifetime (approximately nanoseconds) fluorescence background. Moreover, the discrete, narrow emission bands of Tb(III) complexes make them uniquely suited for multiplexed imaging applications because they can serve as Förster resonance energy transfer (FRET) donors to two or more differently colored acceptors. However, lanthanide complexes have low photon emission rates that can limit the image signal/noise ratio, which has a square-root dependence on photon counts. This work describes the performance of a wide-field, time-gated microscope with respect to its ability to image Tb(III) luminescence and Tb(III)-mediated FRET in cultured mammalian cells. The system employed a UV-emitting LED for low-power, pulsed excitation and an intensified CCD camera for gated detection. Exposure times of ∼1 s were needed to collect 5-25 photons per pixel from cells that contained micromolar concentrations of a Tb(III) complex. The observed photon counts matched those predicted by a theoretical model that incorporated the photophysical properties of the Tb(III) probe and the instrument's light-collection characteristics. Despite low photon counts, images of Tb(III)/green fluorescent protein FRET with a signal/noise ratio ≥ 7 were acquired, and a 90% change in the ratiometric FRET signal was measured. This study shows that the sensitivity and precision of lanthanide-based cellular microscopy can approach that of conventional FRET microscopy with fluorescent proteins. The results should encourage further development of lanthanide biosensors that can measure analyte concentration, enzyme activation, and protein-protein interactions in live cells.

Transportan 10 improves the anticancer activity of cisplatin

The aim of this paper was to examine whether cell-penetrating peptides (CPPs) such as transportan 10 (TP10) or protein transduction domain (PTD4) may improve the anticancer activity of cisplatin (cPt). The complexes of TP10 or PTD4 with cPt were used in the experiments. They were carried out on two non-cancer (HEK293 (human embryonic kidney) and HEL299 (human embryo lung)) and two cancer (HeLa (human cervical cancer) and OS143B (human osteosarcoma 143B)) cell lines. Both complexes were tested (MTT assay) with respect to their anticancer or cytotoxic actions. TAMRA (fluorescent dye)-stained preparations were visualized in a fluorescence microscope. The long-term effect of TP10 + cPt and its components on non-cancer and cancer cell lines was observed in inverted phase contrast microscopy. In the MTT test (cell viability assay), the complex of TP10 + cPt produced a more potent effect on the cancer cell lines (HeLa, OS143B) in comparison to that observed after separate treatment with TP10 or cPt. At the same time, the action of the complex and its components was rather small on non-cancer cell lines. On the other hand, a complex of another CPP with cPt, i.e., PTD4 + cPt, was without a significant effect on the cancer cell line (OS143B). The images of the fluorescent microscopy showed TAMRA-TP10 or TAMRA-TP10 + cPt in the interior of the HeLa cells. In the case of TAMRA-PTD4 or TAMRA-PTD4 + cPt, only the first compound was found inside the cancer cell line. In contrast, none of the tested compounds gained access to the interior of the non-cancer cells (HEK293, HEL299). Long-term incubation with the TP10 + cPt (estimated by inverted phase contrast microscopy) lead to an enhanced action of the complex on cell viability (decrease in the number of cells and change in their morphology) as compared with that produced by each single agent. With regard to the tested CPPs, only TP10 improved the anticancer activity of cisplatin if both compounds were used in the form of a complex. Additionally, the complex was relatively safe for non-cancer cells. What is more, TP10 also produced an anticancer effect on HeLa and OS143B cell lines.

Combining chemical and genetic approaches for development of responsive FRET-based sensor systems for protein kinases

Chemical and genetic approaches were combined for the development of responsive FRET-based sensor systems for protein kinases, using PIM2 as the model kinase. Fusions of PIM2 and a red fluorescent protein, TagRFP were expressed in mammalian cells and small-molecule ARC-Lum photoluminescent probes possessing different phosphorescent and fluorescent properties were constructed. Based on a variety of Förster-type resonant energy transfer (FRET) mechanisms (including intermolecular or intramolecular energy transfer and transfer between singlet-singlet or triplet-singlet electronic states of interacting luminophores) of the probe and that of the fluorescently tagged PIM2, FRET-based sensor systems were constructed. The developed assays can be applied for analysis of PIM2 in biological samples and screening and characterization of PIM2 inhibitors in cell lysates. In screening studies sub-micromolar affinity of a d-arginine-rich peptide, nona(d-arginine) amide [(d-Arg)9-NH2], towards PIM2 was discovered that points to possible specific effect of this widely used transport peptide to cellular protein phosphorylation balance.

FRET-based screening assay using small-molecule photoluminescent probes in lysate of cells overexpressing RFP-fused protein kinases

An assay was developed for the characterization of protein kinase inhibitors in lysates of mammalian cells based on the measurement of FRET between overexpressed red fluorescent protein (TagRFP)-fused protein kinases (PKs) and luminophore-labeled small-molecule inhibitors (ARC-Photo probes). Two types of the assay, one using TagRFP as the photoluminescence donor together with ARC-Photo probes containing a red fluorophore dye as acceptor, and the other using TagRFP as the acceptor fluorophore in combination with a terbium cryptate-based long-lifetime photoluminescence donor, were used for FRET-based measurements in lysates of the cells overexpressing TagRFP-fused PKs. The second variant of the assay enabled the performance of the measurements under time-resolved conditions that led to substantially higher values of the signal/background ratio and further improved the reliability of the assay.

Cytoplasmic delivery and selective, multicomponent labeling with oligoarginine-linked protein tags

Strategies that leverage bio-orthogonal interactions between small molecule ligands and genetically encoded amino acid sequences can be used to attach high-performance fluorophores to proteins in living cells. However, a major limitation of chemical protein labeling is that cells' plasma membranes are impermeable to many useful probes and biolabels. Here, we show that conjugation to nonaarginine, a cell penetrating peptide (CPP), enables passive cytoplasmic delivery of otherwise membrane-impermeant, small molecule protein labels. Heterodimers consisting of a luminescent Tb(3+) complex, Lumi4, linked to benzyl guanine, benzyl cytosine, and trimethoprim were conjugated to the peptide CysArg9 with a reducible disulfide linker. When added to culture medium, the peptide conjugates rapidly (<30 min) enter the cytoplasm and diffuse freely throughout cells. The benzyl guanine, benzyl cytosine, and trimethoprim derivatives bind selectively to fusion proteins tagged with SNAP-Tag, CLIP-Tag, and Escherichia coli dihydrofolate reductase (eDHFR), respectively. Furthermore, eDHFR and SNAP-Tag fusions can be labeled with Lumi4 analogues in the same cell, and this labeling can be detected using two-color, time-gated Förster resonance energy transfer (FRET) microscopy. Finally, we present quantitative data showing that cytoplasmic uptake of nonaarginine-conjugated probes occurs in multiple cell types (MDCK, HeLa, NIH 3T3), most cells in a culture (>75%) are loaded with probe, and the cellular probe concentration can be controlled by varying incubation conditions. CPP-mediated delivery of Lumi4-linked protein labels will greatly increase the utility of lanthanide-based FRET microscopy. Moreover, our results strongly suggest that this approach can be adapted to deliver a wide variety of protein-targeted fluorophores or other functional probes that were previously unavailable for intracellular imaging studies.

Effect of lanthanide complex structure on cell viability and association

A systematic study of the effect of hydrophobicity and charge on the cell viability and cell association of lanthanide metal complexes is presented. The terbium luminescent probes feature a macrocyclic polyaminocarboxylate ligand (DOTA) in which the hydrophobicity of the antenna and that of the carboxyamide pendant arms are independently varied. Three sensitizing antennas were investigated in terms of their function in vitro: 2-methoxyisophthalamide (IAM(OMe)), 2-hydroxyisophthalamide (IAM), and 6-methylphenanthridine (Phen). Of these complexes, Tb-DOTA-IAM exhibited the highest quantum yield, although the higher cell viability and more facile synthesis of the structurally related Tb-DOTA-IAM(OMe) platform renders it more attractive. Further modification of this latter core structure with carboxyamide arms featuring hydrophobic benzyl, hexyl, and trifluoro groups as well as hydrophilic amino acid based moieties generated a family of complexes that exhibit high cell viability (ED50 > 300 μM) regardless of the lipophilicity or the overall complex charge. Only the hexyl-substituted complex reduced cell viability to 60% in the presence of 100 μM complex. Additionally, cellular association was investigated by ICP-MS and fluorescence microscopy. Surprisingly, the hydrophobic moieties did not increase cell association in comparison to the hydrophilic amino acid derivatives. It is thus postulated that the hydrophilic nature of the 2-methoxyisophthalamide antenna (IAM(OMe)) disfavors the cellular association of these complexes. As such, responsive luminescent probes based on this scaffold would be appropriate for the detection of extracellular species.

Lanthanide-based imaging of protein-protein interactions in live cells

In order to deduce the molecular mechanisms of biological function, it is necessary to monitor changes in the subcellular location, activation, and interaction of proteins within living cells in real time. Förster resonance energy-transfer (FRET)-based biosensors that incorporate genetically encoded, fluorescent proteins permit high spatial resolution imaging of protein-protein interactions or protein conformational dynamics. However, a nonspecific fluorescence background often obscures small FRET signal changes, and intensity-based biosensor measurements require careful interpretation and several control experiments. These problems can be overcome by using lanthanide [Tb(III) or Eu(III)] complexes as donors and green fluorescent protein (GFP) or other conventional fluorophores as acceptors. Essential features of this approach are the long-lifetime (approximately milliseconds) luminescence of Tb(III) complexes and time-gated luminescence microscopy. This allows pulsed excitation, followed by a brief delay, which eliminates nonspecific fluorescence before the detection of Tb(III)-to-GFP emission. The challenges of intracellular delivery, selective protein labeling, and time-gated imaging of lanthanide luminescence are presented, and recent efforts to investigate the cellular uptake of lanthanide probes are reviewed. Data are presented showing that conjugation to arginine-rich, cell-penetrating peptides (CPPs) can be used as a general strategy for the cellular delivery of membrane-impermeable lanthanide complexes. A heterodimer of a luminescent Tb(III) complex, Lumi4, linked to trimethoprim and conjugated to nonaarginine via a reducible disulfide linker rapidly (∼10 min) translocates into the cytoplasm of Maden Darby canine kidney cells from the culture medium. With this reagent, the intracellular interaction between GFP fused to FK506 binding protein 12 (GFP-FKBP12) and the rapamycin binding domain of mTOR fused to Escherichia coli dihydrofolate reductase (FRB-eDHFR) were imaged at high signal-to-noise ratio with fast (1-3 s) image acquisition using a time-gated luminescence microscope. The data reviewed and presented here show that lanthanide biosensors enable fast, sensitive, and technically simple imaging of protein-protein interactions in live cells.

How to build a time-gated luminescence microscope

The sensitivity of filter-based fluorescence microscopy techniques is limited by autofluorescence background. Time-gated detection is a practical way to suppress autofluorescence, enabling higher contrast and improved sensitivity. In the past few years, three groups of authors have demonstrated independent approaches to build robust versions of time-gated luminescence microscopes. Three detailed, step-by-step protocols are provided here for modifying standard fluorescent microscopes to permit imaging time-gated luminescence.

Utility of tris(4-bromopyridyl) europium complexes as versatile intermediates in the divergent synthesis of emissive chiral probes

The tribromopyridyl complex is a versatile late intermediate in the divergent synthesis of emissive triazole and aryl–alkynyl substituted complexes.

Chemical tags: inspiration for advanced imaging techniques

This review summarizes recent applications of chemical tags in conjunction with advanced bio-imaging techniques including single-molecule fluorescence, spatiotemporally resolved ensemble microscopy techniques, and imaging modalities beyond fluorescence. We aim to illustrate the unique advantages of chemical tags in facilitating contemporary microscopy to address biological problems that are difficult or near impossible to approach otherwise. We hope our review will inspire more innovative applications enabled by the mingling of these two growing fields.

Polarization and symmetry of electronic transitions in long fluorescence lifetime triangulenium dyes

To fully exploit the capabilities of fluorescence probes in modern experiments, where advanced instrumentation is used to probe complex environments, other photophysical properties than emission color and emission intensity are monitored. Each dye property can be addressed individually as well as collectively to provide in-depth information unavailable from the standard intensity measurements. Dyes with long emission lifetimes and strongly polarized transitions enable the monitoring of lifetime changes as well as emission polarization (anisotropy). Thus experiments can be designed to follow slow dynamics. The UV and visible electronic transitions of a series of red-emitting dyes based on the triangulenium motif are investigated. We resolve overlapping features in the spectra and assign the orientation of the transition moments to the molecular axes. The result is the complete Jablonski diagram for the UV and visible spectral region. The symmetries of the studied dyes are shown to have a large influence on the optical response, and they are clearly separated into two groups of symmetry by their photophysical properties. The C(2v) symmetric dyes, azadioxatriangulenium (ADOTA(+)) and diazaoxatriangulenium (DAOTA(+)), have high emission anisotropies, fluorescence lifetimes around 20 ns, and fluorescence quantum yields of ∼50%. The trioxatriangulenium (TOTA(+)) and triazatriangulenium (TATA(+)) dyes-nominally of D(3h) symmetry-have fluorescence lifetimes around 10 ns lifetimes and fluorescence quantum yields of 10-15%. However, the D(3h) symmetry is shown to be lowered to a point group, where the axes transform uniquely such that the degeneracy of the E' states is lifted.