Essential elements of radical pair magnetosensitivity in Drosophila.

Many animals use Earth's magnetic field (also known as the geomagnetic field) for navigation1. The favoured mechanism for magnetosensitivity involves a blue-light-activated electron-transfer reaction between flavin adenine dinucleotide (FAD) and a chain of tryptophan residues within the photoreceptor protein CRYPTOCHROME (CRY). The spin-state of the resultant radical pair, and therefore the concentration of CRY in its active state, is influenced by the geomagnetic field2. However, the canonical CRY-centric radical-pair mechanism does not explain many physiological and behavioural observations2-8. Here, using electrophysiology and behavioural analyses, we assay magnetic-field responses at the single-neuron and organismal levels. We show that the 52 C-terminal amino acid residues of Drosophila melanogaster CRY, lacking the canonical FAD-binding domain and tryptophan chain, are sufficient to facilitate magnetoreception. We also show that increasing intracellular FAD potentiates both blue-light-induced and magnetic-field-dependent effects on the activity mediated by the C terminus. High levels of FAD alone are sufficient to cause blue-light neuronal sensitivity and, notably, the potentiation of this response in the co-presence of a magnetic field. These results reveal the essential components of a primary magnetoreceptor in flies, providing strong evidence that non-canonical (that is, non-CRY-dependent) radical pairs can elicit magnetic-field responses in cells.

Highlighter: An optogenetic system for high-resolution gene expression control in plants.

Optogenetic actuators have revolutionized the resolution at which biological processes can be controlled. In plants, deployment of optogenetics is challenging due to the need for these light-responsive systems to function in the context of horticultural light environments. Furthermore, many available optogenetic actuators are based on plant photoreceptors that might crosstalk with endogenous signaling processes, while others depend on exogenously supplied cofactors. To overcome such challenges, we have developed Highlighter, a synthetic, light-gated gene expression system tailored for in planta function. Highlighter is based on the photoswitchable CcaS-CcaR system from cyanobacteria and is repurposed for plants as a fully genetically encoded system. Analysis of a re-engineered CcaS in Escherichia coli demonstrated green/red photoswitching with phytochromobilin, a chromophore endogenous to plants, but also revealed a blue light response likely derived from a flavin-binding LOV-like domain. We deployed Highlighter in transiently transformed Nicotiana benthamiana for optogenetic control of fluorescent protein expression. Using light to guide differential fluorescent protein expression in nuclei of neighboring cells, we demonstrate unprecedented spatiotemporal control of target gene expression. We implemented the system to demonstrate optogenetic control over plant immunity and pigment production through modulation of the spectral composition of broadband visible (white) light. Highlighter is a step forward for optogenetics in plants and a technology for high-resolution gene induction that will advance fundamental plant biology and provide new opportunities for crop improvement.

Psychiatric diagnoses are common after liver transplantation and are associated with increased health care utilization and patient financial burden.

Psychiatric disorders after liver transplantation (LT) are associated with worse patient and graft outcomes, which may be amplified by inadequate treatment. We aimed to characterize the burden of psychiatric disorders, treatment patterns, and associated financial burden among liver transplantation recipients (LTRs). IQVIA PharMetrics (R) Plus for Academics-a large health plan claims database representative of the commercially insured US population-was used to identify psychiatric diagnoses among adult LTRs and assess treatment. Multivariable logistic regression analysis identified factors associated with post-LT psychiatric diagnoses and receipt of pharmacotherapy. Patient financial liability was estimated using adjudicated medical/pharmacy claims for LTRs with and without psychiatric diagnoses. Post-LT psychiatric diagnoses were identified in 395 (29.5%) of 1338 LTRs, of which 106 (26.8%) were incident cases. Treatment varied, with 67.3% receiving pharmacotherapy, 32.1% psychotherapy, 21.0% combination therapy, and 21.5% no treatment. Among 340 LTRs on psychotropic medications before transplant, 24% did not continue them post-LT. Post-LT psychiatric diagnoses were independently associated with female sex, alcohol-associated liver disease (ALD), prolonged LT hospitalization (>2 wk), and pre-LT psychiatric diagnosis. Incident psychiatric diagnoses were associated with female sex, ALD, and prolonged LT hospitalization. Patients with a post-LT psychiatric diagnosis had higher rates of hospitalization (89.6% vs. 81.5%, p <0.001) and financial liability (median $5.5K vs. $4.6K USD, p =0.006). Having a psychiatric diagnosis post-LT was independently associated with experiencing high financial liability >$5K. Over 1 in 4 LTRs had a psychiatric diagnosis in a large national cohort, yet nearly a quarter received no treatment. LTRs with psychiatric diagnoses experienced increased health care utilization and higher financial liability. Sociodemographic and clinical risk factors could inform high-risk subgroups who may benefit from screening and mitigation strategies.

The financial burden after liver transplantation is significant among commercially insured adults: A large US National Cohort.

Liver transplantation (LT) is lifesaving for patients with cirrhosis; however, the resultant financial burden to patients has not been well characterized. We aimed to provide a nationally representative portrayal of patient financial burden after LT. Adult recipients of LT from 2006 to 2021 were identified using IQVIA PharMetrics® Plus for Academics-a large nationally representative claims database of commercially insured Americans. Patient financial liability (ie, what patients owe) was estimated using the difference between allowed and paid costs for adjudicated medical/pharmacy claims. Descriptive statistics were provided stratified by the financial liability group within 1 year after LT. Multivariable logistic regression modeling identified factors associated with high/extreme liability adjusting for covariates. Potential indirect costs of post-LT care were estimated based on hourly wages lost for care. Among 1412 recipients of LT, financial liability was heterogeneous-~3% had no liability and 21% had extreme liability > $10K for 1-year post-LT care; most (69%) paid between $1 and 10K, with 48% having liability >$5K. Factors associated with >$5K liability included older age, insurance/enrollment type, US region, history of HCC, and simultaneous liver-kidney transplant (for liability >$10K). Medication costs comprised ~30% of outpatient financial liability. Potential indirect costs from wages lost were $2,201-$6,073 per person, depending on an hourly wage. In a large national cohort of commercially insured recipients of LT, financial liability was highly variable across sociodemographic and clinical characteristics; nearly 1 out of 2 recipients of LT owed >$5K for 1 year of post-LT care. Transplant programs should help patients anticipate potential costs and identify vulnerable populations who would benefit from enhanced financial counseling.

Native mass spectrometry reveals the conformational diversity of the UVR8 photoreceptor.

UVR8 is a plant photoreceptor protein that regulates photomorphogenic and protective responses to UV light. The inactive, homodimeric state absorbs UV-B light, resulting in dissociation into monomers, which are considered to be the active state and comprise a ß-propeller core domain and intrinsically disordered N- and C-terminal tails. The C terminus is required for functional binding to signaling partner COP1. To date, however, structural studies have only been conducted with the core domain where the terminal tails have been truncated. Here, we report structural investigations of full-length UVR8 using native ion mobility mass spectrometry adapted for photoactivation. We show that, while truncated UVR8 photoconverts from a single conformation of dimers to a single monomer conformation, the full-length protein exists in numerous conformational families. The full-length dimer adopts both a compact state and an extended state where the C terminus is primed for activation. In the monomer the extended C terminus destabilizes the core domain to produce highly extended yet stable conformations, which we propose are the fully active states that bind COP1. Our results reveal the conformational diversity of full-length UVR8. We also demonstrate the potential power of native mass spectrometry to probe functionally important structural dynamics of photoreceptor proteins throughout nature.

Covalent Attachment of Active Enzymes to Upconversion Phosphors Allows Ratiometric Detection of Substrates.

Upconverting phosphors (UCPs) convert multiple low energy photons into higher energy emission via the process of photon upconversion and offer an attractive alternative to organic fluorophores for use as luminescent probes. Here, UCPs were capped with functionalized silica in order to provide a surface to covalently conjugate proteins with surface-accessible cysteines. Variants of green fluorescent protein (GFP) and the flavoenzyme pentaerythritol tetranitrate reductase (PETNR) were then attached via maleimide-thiol coupling in order to allow energy transfer from the UCP to the GFP or flavin cofactor of PETNR, respectively. PETNR retains its activity when coupled to the UCPs, which allows reversible detection of enzyme substrates via ratiometric sensing of the enzyme redox state.

Observation of the Δg mechanism resulting from the ultrafast spin dynamics that follow the photolysis of coenzyme B12.

Throughout nature, both free radicals and transient radical reaction intermediates are vital to many biological functions. Coenzyme B12 is a case in point. This organometallic cofactor generates a radical pair upon activation in its dependent enzymes by substrate binding and following photolysis. The resulting cob(ii)alamin/5'-deoxyadenosyl radical pair has unusual magnetic properties that present a challenge to detailed investigation at ambient temperatures. Here, we use femtosecond transient absorption spectroscopy adapted for magnetic field exposure to reveal that the spin dynamics of the B12 radical pair are sufficiently fast for magnetic field effects to be observed on the ultrafast reaction kinetics. Moreover, the large difference in g-values between the radicals of the pair means that effects of the Δg mechanism are observed for the first time for a radical pair system exposed to magnetic fields below 1 T. Spin dynamic simulations allow a value of the cob(ii)alamin radical g-value (2.105) at ambient temperature to be extracted and, because the spin dynamic time scale is faster than the diffusional rotation of the cob(ii)alamin radical, the observed value corresponds to the anisotropic g|| value for this radical.

Two photon spectroscopy and microscopy of the fluorescent flavoprotein, iLOV.

LOV-domains are ubiquitous photosensory proteins that are commonly re-engineered to serve as powerful and versatile fluorescent proteins and optogenetic tools. The photoactive, flavin chromophore, however, is excited using short wavelengths of light in the blue and UV regions, which have limited penetration into biological samples and can cause photodamage. Here, we have used non-linear spectroscopy and microscopy of the fluorescent protein, iLOV, to reveal that functional variants of LOV can be activated to great effect by two non-resonant photons of lower energy, near infrared light, not only in solution but also in biological samples. The two photon cross section of iLOV has a significantly blue-shifted S0 → S1 transition compared with the one photon absorption spectrum, suggesting preferential population of excited vibronic states. It is highly likely, therefore, that the two photon absorption wavelength of engineered, LOV-based tools is tuneable. We also demonstrate for the first time two photon imaging using iLOV in human epithelial kidney cells. Consequently, two photon absorption by engineered, flavin-based bio-molecular tools can enable non-invasive activation with high depth resolution and the potential for not only improved image clarity but also enhanced spatiotemporal control for optogenetic applications.

Magnetic Fields Modulate Blue-Light-Dependent Regulation of Neuronal Firing by Cryptochrome.

Many animals are able to sense the Earth's geomagnetic field to enable behaviors such as migration. It is proposed that the magnitude and direction of the geomagnetic field modulates the activity of cryptochrome (CRY) by influencing photochemical radical pair intermediates within the protein. However, this proposal will remain theoretical until a CRY-dependent effect on a receptor neuron is shown to be modified by an external magnetic field (MF). It is established that blue-light (BL) photoactivation of CRY is sufficient to depolarize and activate Drosophila neurons. Here, we show that this CRY-dependent effect is significantly potentiated in the presence of an applied MF (100 mT). We use electrophysiological recordings from larval identified motoneurons, in which CRY is ectopically expressed, to show that BL-dependent depolarization of membrane potential and increased input resistance are markedly potentiated by an MF. Analysis of membrane excitability shows that these effects of MF exposure evoke increased action potential firing. Almost nothing is known about the mechanism by which a magnetically induced change in CRY activity might produce a behavioral response. We further report that specific structural changes to the protein alter the impact of the MF in ways that are strikingly similar to those from recent behavioral studies into the magnetic sense of Drosophila These observations provide the first direct experimental evidence to support the hypothesis that MF modulation of CRY activity is capable of influencing neuron activity to allow animal magnetoreception. SIGNIFICANCE STATEMENT: The biophysical mechanism of animal magnetoreception is still unclear. The photoreceptor protein cryptochrome has risen to prominence as a candidate magnetoreceptor molecule based on multiple reports derived from behavioral studies. However, the role of cryptochrome as a magnetoreceptor remains controversial primarily because of a lack of direct experimental evidence linking magnetic field (MF) exposure to a change in neuronal activity. Here, we show that exposure to an MF (100 mT) is sufficient to potentiate the ability of light-activated cryptochrome to increase neuronal action potential firing. Our results provide critical missing evidence to show that the activity of cryptochrome is sensitive to an external MF that is capable of modifying animal behavior.

The photochemistry and photobiology of vitamin B12.

Biologically active derivatives of vitamin B12 are organometallic cobalt-corrinoid complexes crucial for the healthy function of humans, animals and microbes. Their role as cofactors to numerous, thermally-driven metabolic enzymes is well described, and varies depending on the nature of the upper axial ligand. This ligand also significantly influences the photophysics and photochemistry of B12. In this Perspective I will discuss the various aspects of B12 photochemistry, from its dynamic spin chemistry to the considerable potential it has for biotechnology applications. Although for many years this photochemistry was thought to have no biological function, in recent years it has become apparent that B12 photochemistry at very least has a role in light-dependent bacterial transcriptional regulation. I will provide an overview of what has been reported about B12 photobiology to date, with particular emphasis on the mechanism of the transcriptional repressor, CarH, the subject of my Young Investigator Award Plenary Lecture at the European Society for Photobiology Congress 2015 in Aveiro, Portugal.

Probing reversible chemistry in coenzyme B12 -dependent ethanolamine ammonia lyase with kinetic isotope effects.

Coenzyme B12 -dependent enzymes such as ethanolamine ammonia lyase have remarkable catalytic power and some unique properties that enable detailed analysis of the reaction chemistry and associated dynamics. By selectively deuterating the substrate (ethanolamine) and/or the ß-carbon of the 5'-deoxyadenosyl moiety of the intrinsic coenzyme B12 , it was possible to experimentally probe both the forward and reverse hydrogen atom transfers between the 5'-deoxyadenosyl radical and substrate during single-turnover stopped-flow measurements. These data are interpreted within the context of a kinetic model where the 5'-deoxyadenosyl radical intermediate may be quasi-stable and rearrangement of the substrate radical is essentially irreversible. Global fitting of these data allows estimation of the intrinsic rate constants associated with CoC homolysis and initial H-abstraction steps. In contrast to previous stopped-flow studies, the apparent kinetic isotope effects are found to be relatively small.

Tuning of B12 photochemistry in the CarH photoreceptor to avoid radical photoproducts.

Time-resolved infrared spectroscopy reveals the flow of electron density through coenzyme B12 in the light-activated, bacterial transcriptional regulator, CarH. The protein stabilises a series of charge transfer states that result in a photoresponse that avoids reactive, and potentially damaging, radical photoproducts.

Ultrafast infrared spectral fingerprints of vitamin B12 and related cobalamins.

Vitamin B(12) (cyanocobalamin, CNCbl) and its derivatives are structurally complex and functionally diverse biomolecules. The excited state and radical pair reaction dynamics that follow their photoexcitation have been previously studied in detail using UV-visible techniques. Similar time-resolved infrared (TRIR) data are limited, however. Herein we present TRIR difference spectra in the 1300-1700 cm(-1) region between 2 ps and 2 ns for adenosylcobalamin (AdoCbl), methylcobalamin (MeCbl), CNCbl, and hydroxocobalamin (OHCbl). The spectral profiles of all four cobalamins are complex, with broad similarities that suggest the vibrational excited states are related, but with a number of identifiable variations. The majority of the signals from AdoCbl and MeCbl decay with kinetics similar to those reported in the literature from UV-visible studies. However, there are regions of rapid (<10 ps) vibrational relaxation (peak shifts to higher frequencies from 1551, 1442, and 1337 cm(-1)) that are more pronounced in AdoCbl than in MeCbl. The AdoCbl data also exhibit more substantial changes in the amide I region and a number of more gradual peak shifts elsewhere (e.g., from 1549 to 1563 cm(-1)), which are not apparent in the MeCbl data. We attribute these differences to interactions between the bulky adenosyl and the corrin ring after photoexcitation and during radical pair recombination, respectively. Although spectrally similar to the initial excited state, the long-lived metal-to-ligand charge transfer state of MeCbl is clearly resolved in the kinetic analysis. The excited states of CNCbl and OHCbl relax to the ground state within 40 ps with few significant peak shifts, suggesting little or no homolysis of the bond between the Co and the upper axial ligand. Difference spectra from density functional theory calculations (where spectra from simplified cobalamins with an upper axial methyl were subtracted from those without) show qualitative agreement with the experimental data. They imply the excited state intermediates in the TRIR difference spectra resemble the dissociated states vibrationally (the cobalamin with the upper axial ligand missing) relative to the ground state with a methyl in this position. They also indicate that most of the TRIR signals arise from vibrations involving some degree of motion in the corrin ring. Such coupling of motions throughout the ring makes specific peak assignments neither trivial nor always meaningful, suggesting our data should be regarded as IR spectral fingerprints.

Investigating radical pair reaction dynamics of B12 coenzymes 2: Time-resolved electron paramagnetic resonance spectroscopy.

The chemistry of B12 coenzymes is highly sensitive to the nature of their upper axial ligand and can be further tuned by their environment. Methylcobalamin, for example, generates RPs photochemically but undergoes non-radical biochemistry when bound to its dependent enzymes. Owing to the transient nature of the reaction intermediates, it remains a challenge to investigate how their environment controls reactivity. Here, we describe how to use time-resolved electron paramagnetic spectroscopy to directly monitor the generation and evolution of transient radicals that result from the photolysis of a B12 coenzyme. This method produces evolving, spin-polarized spectra that are rich in mechanistic detail.

Investigating radical pair reaction dynamics of B12 coenzymes 1: Transient absorption spectroscopy and magnetic field effects.

B12 coenzymes are vital to healthy biological function across nature. They undergo radical chemistry in a variety of contexts, where spin-correlated radical pairs can be generated both thermally and photochemically. Owing to the unusual magnetic properties of B12 radical pairs, however, most of the reaction and spin dynamics occur on a timescale (picoseconds-nanoseconds) that cannot be resolved by most measurement techniques. Here, we describe a method that combines femtosecond transient absorption spectroscopy with magnetic field exposure, which enables the direct scrutiny of such rapid processes. This approach should provide a means by which to investigate the apparently profound effect protein environments have on the generation and reactivity of B12 radical pairs.

Interplay between chromophore binding and domain assembly by the B12-dependent photoreceptor protein, CarH.

Organisms across the natural world respond to their environment through the action of photoreceptor proteins. The vitamin B12-dependent photoreceptor, CarH, is a bacterial transcriptional regulator that controls the biosynthesis of carotenoids to protect against photo-oxidative stress. The binding of B12 to CarH monomers in the dark results in the formation of a homo-tetramer that complexes with DNA; B12 photochemistry results in tetramer dissociation, releasing DNA for transcription. Although the details of the response of CarH to light are beginning to emerge, the biophysical mechanism of B12-binding in the dark and how this drives domain assembly is poorly understood. Here - using a combination of molecular dynamics simulations, native ion mobility mass spectrometry and time-resolved spectroscopy - we reveal a complex picture that varies depending on the availability of B12. When B12 is in excess, its binding drives structural changes in CarH monomers that result in the formation of head-to-tail dimers. The structural changes that accompany these steps mean that they are rate-limiting. The dimers then rapidly combine to form tetramers. Strikingly, when B12 is scarcer, as is likely in nature, tetramers with native-like structures can form without a B12 complement to each monomer, with only one apparently required per head-to-tail dimer. We thus show how a bulky chromophore such as B12 shapes protein/protein interactions and in turn function, and how a protein can adapt to a sub-optimal availability of resources. This nuanced picture should help guide the engineering of B12-dependent photoreceptors as light-activated tools for biomedical applications.