Photodynamic Physiology-Photonanomanipulations in Cellular Physiology with Protein Photosensitizers

Chemical and Biological Strategies for Profiling Protein-Protein Interactions in Living Cells

Protein-protein interactions (PPIs) play critical roles in almost all cellular signal transduction events. Characterization of PPIs without interfering with the functions of intact cells is very important for basic biology study and drug developments. However, the ability to profile PPIs especially those weak/transient interactions in their native states remains quite challenging. To this end, many endeavors are being made in developing new methods with high efficiency and strong operability. By coupling with advanced fluorescent microscopy and mass spectroscopy techniques, these strategies not only allow us to visualize the subcellular locations and monitor the functions of protein of interest (POI) in real time, but also enable the profiling and identification of potential unknown interacting partners in high-throughput manner, which greatly facilitates the elucidation of molecular mechanisms underlying numerous pathophysiological processes. In this review, we will summarize the typical methods for PPIs identification in living cells and their principles, advantages and limitations will also be discussed in detail.

Optogenetic Methods in Plant Biology

Optogenetics is a technique employing natural or genetically engineered photoreceptors in transgene organisms to manipulate biological activities with light. Light can be turned on or off, and adjusting its intensity and duration allows optogenetic fine-tuning of cellular processes in a noninvasive and spatiotemporally resolved manner. Since the introduction of Channelrhodopsin-2 and phytochrome-based switches nearly 20 years ago, optogenetic tools have been applied in a variety of model organisms with enormous success, but rarely in plants. For a long time, the dependence of plant growth on light and the absence of retinal, the rhodopsin chromophore, prevented the establishment of plant optogenetics until recent progress overcame these difficulties. We summarize the recent results of work in the field to control plant growth and cellular motion via green light-gated ion channels and present successful applications to light-control gene expression with single or combined photoswitches in plants. Furthermore, we highlight the technical requirements and options for future plant optogenetic research.

Photodynamic Activation of the Cholecystokinin 1 Receptor with Tagged Genetically Encoded Protein Photosensitizers: Optimizing the Tagging Patterns

Cholecystokinin 1 receptor (CCK1R) is activated photodynamically. For this to happen in situ, genetically encoded protein photosensitizers (GEPP) may be tagged to natively expressed CCK1R, but how to best tag GEPP has not been examined. Therefore, GEPP (miniSOG or KillerRed) was tagged to CCK1R and light-driven photodynamic CCK1R activation was monitored by Fura-2 fluorescent calcium imaging, to screen for optimized tagging patterns. Blue light-emitting diode irradiation of CHO-K1 cells expressing miniSOG fused to N- or C-terminus of CCK1R was found to both trigger persistent calcium oscillations-a hallmark of permanent photodynamic CCK1R activation. Photodynamic CCK1R activation was accomplished also with miniSOG fused to N-terminus of CCK1R via linker (GlySerGly)_{4 or 8} , but not linker (GSG)₁₂ or an internal ribosomal entry site insert. KillerRed fused to N- or C-terminus of CCK1R after white light irradiation resulted in similar activation of in-frame CCK1R. Photodynamic CCK1R activation in miniSOG-CCK1R-CHO-K1 cells was blocked by singlet oxygen (¹ O₂ ) quencher uric acid or Trolox C, corroborating the role of ¹ O₂ as the reactive intermediate. It is concluded that photodynamic CCK1R activation can be achieved either with direct GEPP fusion to CCK1R or fusion via a short linker, fusion via long linkers might serve as the internal control.

Transmembrane Domain 3 Is a Transplantable Pharmacophore in the Photodynamic Activation of Cholecystokinin 1 Receptor

Cholecystokinin 1 receptor (CCK1R) is activated in photodynamic action by singlet oxygen, but detailed molecular mechanisms are not elucidated. To identify the pharmacophore(s) in photodynamic CCK1R activation, we examined photodynamic activation of point mutants CCK1R^M121/3.32A, CCK1R^M121/3.32Q, and a chimeric receptor with CCK1R transmembrane domain 3 (TM3) transplanted to muscarinic ACh receptor 3 (M3R) which is unaffected by photodynamic action. These engineered receptors were tagged at the N-terminus with genetically encoded protein photosensitizer miniSOG, and their light-driven photodynamic activation was compared to wild type CCK1R and M3R, as monitored by Fura-2 fluorescent calcium imaging. Photodynamic activations of miniSOG-CCK1R^M121/3.32A and miniSOG-CCK1R^M121/3.32Q were found to be 55% and 73%, respectively, when compared to miniSOG-CCK1R (100%), whereas miniSOG-M3R was not affected (0% activation). Notably, the chimeric receptor miniSOG-M3R-TM3_CCK1R was effectively activated photodynamically (65%). These data suggest that TM3 is an important pharmacophore in photodynamic CCK1R activation, readily transplantable to nonsusceptible M3R for photodynamic activation.

Molecular Tools for Targeted Control of Nerve Cell Electrical Activity. Part I

In modern life sciences, the issue of a specific, exogenously directed manipulation of a cell’s biochemistry is a highly topical one. In the case of electrically excitable cells, the aim of the manipulation is to control the cells’ electrical activity, with the result being either excitation with subsequent generation of an action potential or inhibition and suppression of the excitatory currents. The techniques of electrical activity stimulation are of particular significance in tackling the most challenging basic problem: figuring out how the nervous system of higher multicellular organisms functions. At this juncture, when neuroscience is gradually abandoning the reductionist approach in favor of the direct investigation of complex neuronal systems, minimally invasive methods for brain tissue stimulation are becoming the basic element in the toolbox of those involved in the field. In this review, we describe three approaches that are based on the delivery of exogenous, genetically encoded molecules sensitive to external stimuli into the nervous tissue. These approaches include optogenetics (Part I) as well as chemogenetics and thermogenetics (Part II), which are significantly different not only in the nature of the stimuli and structure of the appropriate effector proteins, but also in the details of experimental applications. The latter circumstance is an indication that these are rather complementary than competing techniques.

Bioengineering a Light-Responsive Encapsulin Nanoreactor: A Potential Tool for In Vitro Photodynamic Therapy

Encapsulins, a prokaryotic class of self-assembling protein nanocompartments, are being re-engineered to serve as "nanoreactors" for the augmentation or creation of key biochemical reactions. However, approaches that allow encapsulin nanoreactors to be functionally activated with spatial and temporal precision are lacking. We report the construction of a light-responsive encapsulin nanoreactor for "on demand" production of reactive oxygen species (ROS). Herein, encapsulins were loaded with the fluorescent flavoprotein mini-singlet oxygen generator (miniSOG), a biological photosensitizer that is activated by blue light to generate ROS, primarily singlet oxygen (¹O₂). We established that the nanocompartments stably encased miniSOG and in response to blue light were able to mediate the photoconversion of molecular oxygen into ROS. Using an in vitro model of lung cancer, we showed that ROS generated by the nanoreactor triggered photosensitized oxidation reactions which exerted a toxic effect on tumor cells, suggesting utility in photodynamic therapy. This encapsulin nanoreactor thus represents a platform for the light-controlled initiation and/or modulation of ROS-driven processes in biomedicine and biotechnology.

Interaction of Genetically Encoded Photosensitizers with Scintillating Nanoparticles for X-ray Activated Photodynamic Therapy

Photodynamic therapy (PDT) applications are limited by the low penetration of UV-visible light into biological tissues. Considering X-rays as an alternative to excite photosensitizers (PS) in a deeper tumor, an intermediate particle able to convert the X-ray energy into visible light (scintillating nanoparticle, ScNP) is necessary. Moreover, accumulation of PS in the target cells is also required. Genetically encoded proteins could be used as a photosensitizer, allowing the exclusive expression of PS inside the tumor cells. Here, the interaction of eGFP, KillerOrange, and KillerRed proteins with LaF₃:Tb³⁺ ScNP was investigated, for the first time, in terms of its physicochemical and energy transfer properties. The protein structure, stability, and function were evaluated upon adverse physiological conditions and X-ray irradiation. Optimal parameters for energy transfer from ScNP to the proteins were investigated, paving the way for the use of genetically encoded photosensitizers for applications in X-ray activated photodynamic therapy.

Light-controlled calcium signalling in prostate cancer and benign prostatic hyperplasia

Background

Identifying ways to reduce the burden of prostate cancer (Pca) or benign prostatic hyperplasia (BPH) is a top research priority. It is a typical entanglement seen in men which is portrayed by trouble in micturition. It stands as a significant problem in our society. Different molecular biomarker has high potential to treat Pca or BPH but also causes serious side effects during treatment.

Main text

The role of calcium signalling in the alteration of different biomarkers of Pca or BPH is important. Therefore, the photoswitch drugs may hold the potential to rebalance the altered calcium signaling cascade and the biomarker levels. Thereby play a significant role in the management of Pca and BPH. Online literature searches such as PubMed, Web of Science, Scopus, and Google Scholar were carried out. The search terms used for this review were photo-pharmacology, photo-switch drug, photodynamic therapy, calcium signalling, etc. Present treatment of Pca or BPH shows absence of selectivity and explicitness which may additionally result in side effects. The new condition of the calcium flagging may offer promising outcomes in restoring the present issues related with prostate malignancy and BPH treatment.

Conclusion

The light-switching calcium channel blockers aim to solve this issue by incorporating photo-switchable calcium channel blockers that may control the signalling pathway related to proliferation and metastasis in prostate cancer without any side effects.

Graphical abstract

Schematic diagram explaining the proposed role of photo-switch therapy in curbing the side effects of active drugs in Pca (prostate cancer) and BPH (benign prostatic hyperplasia). a) Delivery of medication by ordinary strategies and irreversible phototherapy causes side effects during treatment. Utilization of photo-switch drug to control the dynamic and inert condition of the medication can cause the medication impacts as we required in prostate cancer and BPH. b) Support of harmony between the calcium signaling is essential to guarantee ordinary physiology. Increment or abatement in the dimensions of calcium signaling can result in changed physiology. c) Major factors involved in the pathogenesis of BPH; downregulation of vitamin D receptor (VDR) and histone deacetylase (HDAC) can prevent BPH. Similarly, downregulation of α-1 adrenoceptor can reduce muscle contraction, while overexpression of β-3 adrenoceptor in BPH can promote further muscle relaxation in BPH treatment therapy. Inhibition of overexpressed biomarkers in BPH TRPM2-1: transient receptor potential cation channel subfamily M member 1; TRPM2-2: transient receptor potential cation channel subfamily M member 2; Androgens; CXCL5: C-X-C motif chemokine ligand 5; TGFβ-1: transforming growth factor β-1; TXA2; thromboxane-2; NMDA: N-methyl-d-aspartate can be the potential target in BPH therapy.

A Genetically Encoded RNA Photosensitizer for Targeted Cell Regulation

Genetically encoded RNA devices have emerged for various cellular applications in imaging and biosensing, but their functions as precise regulators in living systems are still limited. Inspired by protein photosensitizers, we propose here a genetically encoded RNA aptamer based photosensitizer (GRAP). Upon illumination, the RNA photosensitizer can controllably generate reactive oxygen species for targeted cell regulation. The GRAP system can be selectively activated by endogenous stimuli and light of different wavelengths. Compared with their protein analogues, GRAP is highly programmable and exhibits reduced off-target effects. These results indicate that GRAP enables efficient noninvasive target cell ablation with high temporal and spatial precision. This new RNA regulator system will be widely used for optogenetics, targeted cell ablation, subcellular manipulation, and imaging.

Current Targets and Bioconjugation Strategies in Photodynamic Diagnosis and Therapy of Cancer

Photodynamic diagnosis (PDD) and therapy (PDT) are emerging, non/minimally invasive techniques for cancer diagnosis and treatment. Both techniques require a photosensitizer and light to visualize or destroy cancer cells. However, a limitation of conventional, non-targeted PDT is poor selectivity, causing side effects. The bioconjugation of a photosensitizer to a tumor-targeting molecule, such as an antibody or a ligand peptide, is a way to improve selectivity. The bioconjugation strategy can generate a tumor-targeting photosensitizer conjugate specific for cancer cells, or ideally, for multiple tumor compartments to improve selectivity and efficacy, such as cancer stem cells and tumor neovasculature within the tumor microenvironment. If successful, such targeted photosensitizer conjugates can also be used for specific visualization and detection of cancer cells and/or tumor angiogenesis (an early event in tumorigenesis) with the hope of an early diagnosis of cancer. The purpose of this review is to summarize some current promising target molecules, e.g., tissue factor (also known as CD142), and the currently used bioconjugation strategies in PDT and PDD, with a focus on newly developed protein photosensitizers. These are genetically engineered photosensitizers, with the possibility of generating a fusion protein photosensitizer by recombinant DNA technology for both PDT and PDD without the need of chemical conjugation. We believe that providing an overview of promising targets and bioconjugation strategies will aid in driving research in this field forward towards more effective, less toxic, and non- or minimally invasive treatment and diagnosis options for cancer patients.

Photodynamic Activation of Cholecystokinin 1 Receptor with Different Genetically Encoded Protein Photosensitizers and from Varied Subcellular Sites

Cholecystokinin 1 receptor (CCK1R) is activated by singlet oxygen (¹O₂) generated in photodynamic action with sulphonated aluminum phthalocyanine (SALPC) or genetically encoded protein photosensitizer (GEPP) KillerRed or mini singlet oxygen generator (miniSOG). A large number of GEPP with varied ¹O₂ quantum yields have appeared recently; therefore, in the present work, the efficacy of different GEPP to photodynamically activate CCK1R was examined, as monitored by Fura-2 calcium imaging. KillerRed, miniSOG, miniSOG2, singlet oxygen protein photosensitizer (SOPP), flavin-binding fluorescent protein from Methylobacterium radiotolerans with point mutation C71G (Mr4511^C71G), and flavin-binding fluorescent protein from Dinoroseobacter shibae (DsFbFP) were expressed at the plasma membrane (PM) in AR4-2J cells, which express endogenous CCK1R. Light irradiation (KillerRed: white light 85.3 mW‧cm^-2, 4' and all others: LED 450 nm, 85 mW·cm^-2, 1.5') of GEPP_PM-expressing AR4-2J was found to all trigger persistent calcium oscillations, a hallmark of permanent photodynamic CCK1R activation; DsFbFP was the least effective, due to poor expression. miniSOG was targeted to PM, mitochondria (MT) or lysosomes (LS) in AR4-2J in parallel experiments; LED light irradiation was found to all induce persistent calcium oscillations. In miniSOG_PM-AR4-2J cells, light emitting diode (LED) light irradiation-induced calcium oscillations were readily inhibited by CCK1R antagonist devazepide 2 nM; miniSOG_MT-AR4-2J cells were less susceptible, but miniSOG_LS-AR4-2J cells were not inhibited. In conclusion, different GEPP_PM could all photodynamically activate CCK1R. Intracellular GEPP photodynamic action may prove particularly suited to study intracellular GPCR.

A novel cancer immunotherapy utilizing autologous tumour tissue

BACKGROUND

With the recent interest in personalized medicine for cancer patients and immune therapy, the field of cancer vaccines has been resurrected. Previous autologous, whole cell tumour vaccine trials have not produced convincing results due, in part to poor patient selection and inactivation methos that are harsh on the cells. These methods can alter protein structure and antigenic profiles making vaccine candidates ineffective in stimulating immune response to autochthonous tumour cells.

MATERIALS AND METHODS

We investigated a novel method for inactivating tumour cells that uses UVA/UVB light and riboflavin (vitamin B2) (RF + UV). RF + UV inactivates the tumour cells' ability to replicate, yet preserves tumour cell integrity and antigenicity.

RESULTS

Our results demonstrate that proteins are preserved on the surface of RF + UV-inactivated tumour cells and that they are immunogenic via induction of dendritic cell maturation, increase in IFNγ production and generation of tumour cell-specific IgG. Moreover, when formulated with an adjuvant ('Innocell vaccine') and tested in different murine tumour primary and metastatic disease models, decreased tumour growth, decreased metastatic disease and prolonged survival were observed. In addition, immune cells obtained from tumour tissue following vaccination had decreased exhausted and regulatory T cells, suggesting that activation of intra-tumoural T cells may be playing a role leading to reduced tumour growth.

CONCLUSIONS

These data suggest that the RF + UV inactivation of tumour cells may provide an efficacious method for generating autologous whole tumour cell vaccines for use in cancer patients.

NanoLuc Bioluminescence-Driven Photodynamic Activation of Cholecystokinin 1 Receptor with Genetically-Encoded Protein Photosensitizer MiniSOG

In contrast to reversible activation by agonist, cholecystokinin 1 receptor (CCK1R) is permanently activated by singlet oxygen generated in photodynamic action, with sulphonated aluminium phthalocyanine or genetically encoded mini singlet oxygen generator (miniSOG) as photosensitizer. In these works, a halogen light source was used to power photodynamic action. For possible in vivo application of photodynamic CCK1R physiology, bearing a cumbersome light-delivery device connected to an external light source by experimental animals might interfere with their behavior. Therefore, in the present work, the possibility of bioluminescence-driven miniSOG photodynamic CCK1R activation was examined, as monitored by Fura-2 calcium imaging. In parallel experiments, it was found that, after plasma membrane (PM)-localized expression of miniSOG_PM in AR4-2J cells, light irradiation with blue light-emitting diode (LED) (450 nm, 85 mW·cm^-2, 1.5 min) induced persistent calcium oscillations that were blocked by CCK1R antagonist devazepide 2 nM. NanoLuc was expressed bicistronically with miniSOG_PM via an internal ribosome entry site (IRES) sequence (pminiSOG_PM-IRES-NanoLuc). The resultant miniSOG_PM-IRES-NanoLuc-AR4-2J cells were found to generate strong bioluminescence upon addition of NanoLuc substrate coelenterazine. Strikingly, coelenterazine 5 microM was found to trigger long-lasting calcium oscillations (a hallmark for permanent CCK1R activation) in perifused miniSOG_PM-IRES-NanoLuc-AR4-2J cells. These data indicate that NanoLuc bioluminescence can drive miniSOG_PM photodynamic CCK1R activation, laying the foundation for its future in vivo applications.

Photoinduced damage of AsLOV2 domain is accompanied by increased singlet oxygen production due to flavin dissociation

Flavin mononucleotide (FMN) belongs to the group of very efficient endogenous photosensitizers producing singlet oxygen, ¹O₂, but with limited ability to be targeted. On the other hand, in genetically-encoded photosensitizers, which can be targeted by means of various tags, the efficiency of FMN to produce ¹O₂ is significantly diminished due to its interactions with surrounding amino acid residues. Recently, an increase of ¹O₂ production yield by FMN buried in a protein matrix was achieved by a decrease of quenching of the cofactor excited states by weakening of the protein-FMN interactions while still forming a complex. Here, we suggest an alternative approach which relies on the blue light irradiation-induced dissociation of FMN to solvent. This dissociation unlocks the full capacity of FMN as ¹O₂ producer. Our suggestion is based on the study of an irradiation effect on two variants of the LOV2 domain from Avena sativa; wild type, AsLOV2 wt, and the variant with a replaced cysteine residue, AsLOV2 C450A. We detected irradiation-induced conformational changes as well as oxidation of several amino acids in both AsLOV2 variants. Detailed analysis of these observations indicates that irradiation-induced increase in ¹O₂ production is caused by a release of FMN from the protein. Moreover, an increased FMN dissociation from AsLOV2 wt in comparison with AsLOV2 C450A points to a role of C450 oxidation in repelling the cofactor from the protein.

Iron metabolic pathways in the processes of sponge plasticity

The ability to regulate oxygen consumption evolved in ancestral animals and is intrinsically linked to iron metabolism. The iron pathways have been intensively studied in mammals, whereas data on distant invertebrates are limited. Sea sponges represent the oldest animal phylum and have unique structural plasticity and capacity to reaggregate after complete dissociation. We studied iron metabolic factors and their expression during reaggregation in the White Sea cold-water sponges Halichondria panicea and Halisarca dujardini. De novo transcriptomes were assembled using RNA-Seq data, and evolutionary trends were analyzed with bioinformatic tools. Differential expression during reaggregation was studied for H. dujardini. Enzymes of the heme biosynthesis pathway and transport globins, neuroglobin (NGB) and androglobin (ADGB), were identified in sponges. The globins mutate at higher evolutionary rates than the heme synthesis enzymes. Highly conserved iron-regulatory protein 1 (IRP1) presumably interacts with the iron-responsive elements (IREs) found in mRNAs of ferritin (FTH1) and a putative transferrin receptor NAALAD2. The reaggregation process is accompanied by increased expression of IRP1, the antiapoptotic factor BCL2, the inflammation factor NFκB (p65), FTH1 and NGB, as well as by an increase in mitochondrial density. Our data indicate a complex mechanism of iron regulation in sponge structural plasticity and help to better understand general mechanisms of morphogenetic processes in multicellular species.

Permanent Photodynamic Activation of the Cholecystokinin 2 Receptor

The cholecystokinin 2 receptor (CCK2R) is expressed in the central nervous system and peripheral tissues, playing an important role in higher nervous and gastrointestinal functions, pain sensation, and cancer growth. CCK2R is reversibly activated by cholecystokinin or gastrin, but whether it can be activated permanently is not known. In this work, we found that CCK2R expressed ectopically in CHO-K1 cells was permanently activated in the dark by sulfonated aluminum phthalocyanine (SALPC / AlPcS4, 10-1,000 nM), as monitored by Fura-2 fluorescent calcium imaging. Permanent CCK2R activation was also observed with AlPcS₂, but not PcS₄. CCK2R previously exposed to SALPC (3 and 10 nM) was sensitized by subsequent light irradiation (> 580 nm, 31.5 mW·cm^-2). After the genetically encoded protein photosensitizer mini singlet oxygen generator (miniSOG) was fused to the N-terminus of CCK2R and expressed in CHO-K1 cells, light irradiation (450 nm, 85 mW·cm^-2) activated in-frame CCK2R (miniSOG-CCK2R), permanently triggering persistent calcium oscillations blocked by the CCK2R antagonist YM 022 (30 nM). From these data, it is concluded that SALPC is a long-lasting CCK2R agonist in the dark, and CCK2R is photogenetically activated permanently with miniSOG as photosensitizer. These properties of SALPC and CCK2R could be used to study CCK2R physiology and possibly for pain and cancer therapies.

Permanent Photodynamic Cholecystokinin 1 Receptor Activation: Dimer-to-Monomer Conversion

The G protein-coupled cholecystokinin 1 receptor (CCK1R) is activated permanently by type II photodynamic action (i.e., by singlet oxygen) in the freshly isolated rat pancreatic acini, in contrast to reversible activation by CCK. But how CCK1R is photodynamically activated is not known. Therefore, in the present work, we subjected membrane proteins extracted from isolated rat pancreatic acini to photodynamic action with photosensitiser sulphonated aluminium phthalocyanine (SALPC), and used reducing gel electrophoresis and Western blot to detect possible changes in CCK1R oligomerization status. Photodynamic action (SALPC 1 µM, light 36.7 mW cm^- 2 × 10 min) was found to convert dimeric CCK1R nearly quantitatively to monomers. Such conversion was dependent on both irradiance (8.51-36.7 mW cm^- 2) and irradiation time (1-20 min). Minimum effective irradiance was found to be 11.1 mW cm^- 2 (× 10 min, with SALPC 1 µM), and brief photodynamic action (SALPC 1 µM, 36.7 mW cm^- 2 × 1 min) was effective. Whilst CCK stimulation of purified membrane proteins alone had no effect on CCK1R dimer/monomer balance, sub-threshold photodynamic action (SALPC 100 nM, 36.7 mW cm^- 2 × 10 min) plus CCK revealed a bell-shaped CCK dose response curve for CCK1R monomerization, which was remarkably similar to the dose response curve for CCK-stimulated amylase secretion in isolated rat pancreatic acini. These two lines of evidence together suggest that during photodynamic CCK1R activation, CCK1R is permanently monomerized, thus providing a unique approach for permanent G protein-coupled receptor (GPCR) activation which has not been achieved before.

Singlet oxygen modification abolishes voltage-dependent inactivation of the sea urchin spHCN channel

Photochemically or metabolically generated singlet oxygen (¹O₂) reacts broadly with macromolecules in the cell. Because of its short lifetime and working distance, ¹O₂ holds potential as an effective and precise nanoscale tool for basic research and clinical practice. Here we investigate the modification of the spHCN channel that results from photochemically and chemically generated ¹O₂ The spHCN channel shows strong voltage-dependent inactivation in the absence of cAMP. In the presence of photosensitizers, short laser pulses transform the gating properties of spHCN by abolishing inactivation and increasing the macroscopic current amplitude. Alanine replacement of a histidine residue near the activation gate within the channel's pore abolishes key modification effects. Application of a variety of chemicals including ¹O₂ scavengers and ¹O₂ generators supports the involvement of ¹O₂ and excludes other reactive oxygen species. This study provides new understanding about the photodynamic modification of ion channels by ¹O₂ at the molecular level.

Blue-Light Receptors for Optogenetics

Sensory photoreceptors underpin light-dependent adaptations of organismal physiology, development, and behavior in nature. Adapted for optogenetics, sensory photoreceptors become genetically encoded actuators and reporters to enable the noninvasive, spatiotemporally accurate and reversible control by light of cellular processes. Rooted in a mechanistic understanding of natural photoreceptors, artificial photoreceptors with customized light-gated function have been engineered that greatly expand the scope of optogenetics beyond the original application of light-controlled ion flow. As we survey presently, UV/blue-light-sensitive photoreceptors have particularly allowed optogenetics to transcend its initial neuroscience applications by unlocking numerous additional cellular processes and parameters for optogenetic intervention, including gene expression, DNA recombination, subcellular localization, cytoskeleton dynamics, intracellular protein stability, signal transduction cascades, apoptosis, and enzyme activity. The engineering of novel photoreceptors benefits from powerful and reusable design strategies, most importantly light-dependent protein association and (un)folding reactions. Additionally, modified versions of these same sensory photoreceptors serve as fluorescent proteins and generators of singlet oxygen, thereby further enriching the optogenetic toolkit. The available and upcoming UV/blue-light-sensitive actuators and reporters enable the detailed and quantitative interrogation of cellular signal networks and processes in increasingly more precise and illuminating manners.

Cholecystokinin 1 Receptor - A Unique G Protein-Coupled Receptor Activated by Singlet Oxygen (GPCR-ABSO)

Plasma membrane-delimited generation of singlet oxygen by photodynamic action with photosensitizer sulfonated aluminum phthalocyanine (SALPC) activates cholecystokinin 1 receptor (CCK1R) in pancreatic acini. Whether CCK1R retains such photooxidative singlet oxygen activation properties in other environments is not known. Genetically encoded protein photosensitizers KillerRed or mini singlet oxygen generator (miniSOG) were expressed in pancreatic acinar tumor cell line AR4-2J, CCK1R, KillerRed or miniSOG were expressed in HEK293 or CHO-K1 cells. Cold light irradiation (87 mW⋅cm^-2) was applied to photosensitizer-expressing cells to examine photodynamic activation of CCK1R by Fura-2 fluorescent calcium imaging. When CCK1R was transduced into HEK293 cells which lack endogenous CCK1R, photodynamic action with SALPC was found to activate CCK1R in CCK1R-HEK293 cells. When KillerRed or miniSOG were transduced into AR4-2J which expresses endogenous CCK1R, KillerRed or miniSOG photodynamic action at the plasma membrane also activated CCK1R. When fused KillerRed-CCK1R was transduced into CHO-K1 cells, light irradiation activated the fused CCK1R leading to calcium oscillations. Therefore KillerRed either expressed independently, or fused with CCK1R can both activate CCK1R photodynamically. It is concluded that photodynamic singlet oxygen activation is an intrinsic property of CCK1R, independent of photosensitizer used, or CCK1R-expressing cell types. Photodynamic singlet oxygen CCK1R activation after transduction of genetically encoded photosensitizer in situ may provide a convenient way to verify intrinsic physiological functions of CCK1R in multiple CCK1R-expressing cells and tissues, or to actuate CCK1R function in CCK1R-expressing and non-expressing cell types after transduction with fused KillerRed-CCK1R.