Regulation of arrestin binding by rhodopsin phosphorylation level

Flavors of GPCR signaling bias

GPCRs are inherently flexible molecules existing in an equilibrium of multiple conformations. Binding of GPCR agonists shifts this equilibrium. Certain agonists can increase the fraction of active-like conformations that predispose the receptor to coupling to a particular signal transducer or a select group of transducers. Such agonists are called biased, in contrast to balanced agonists that facilitate signaling via all transducers the receptor couples to. These biased agonists preferentially channel the signaling of a GPCR to particular G proteins, GRKs, or arrestins. Preferential activation of particular G protein or arrestin subtypes can be beneficial, as it would reduce unwanted on-target side effects, widening the therapeutic window. However, biasing GPCRs has two important limitations: a) complete bias is impossible due to inherent flexibility of GPCRs; b) receptor-independent functions of signal transducer proteins cannot be directly affected by GPCR ligands or differential receptor barcoding by GRK phosphorylation. This article is part of the Special Issue on "Ligand Bias".

Arrestins: A Small Family of Multi-Functional Proteins

The first member of the arrestin family, visual arrestin-1, was discovered in the late 1970s. Later, the other three mammalian subtypes were identified and cloned. The first described function was regulation of G protein-coupled receptor (GPCR) signaling: arrestins bind active phosphorylated GPCRs, blocking their coupling to G proteins. It was later discovered that receptor-bound and free arrestins interact with numerous proteins, regulating GPCR trafficking and various signaling pathways, including those that determine cell fate. Arrestins have no enzymatic activity; they function by organizing multi-protein complexes and localizing their interaction partners to particular cellular compartments. Today we understand the molecular mechanism of arrestin interactions with GPCRs better than the mechanisms underlying other functions. However, even limited knowledge enabled the construction of signaling-biased arrestin mutants and extraction of biologically active monofunctional peptides from these multifunctional proteins. Manipulation of cellular signaling with arrestin-based tools has research and likely therapeutic potential: re-engineered proteins and their parts can produce effects that conventional small-molecule drugs cannot.

Rhodopsin, light-sensor of vision

The light sensor of vertebrate scotopic (low-light) vision, rhodopsin, is a G-protein-coupled receptor comprising a polypeptide chain with bound chromophore, 11-cis-retinal, that exhibits remarkable physicochemical properties. This photopigment is extremely stable in the dark, yet its chromophore isomerises upon photon absorption with 70% efficiency, enabling the activation of its G-protein, transducin, with high efficiency. Rhodopsin's photochemical and biochemical activities occur over very different time-scales: the energy of retinaldehyde's excited state is stored in <1 ps in retinal-protein interactions, but it takes milliseconds for the catalytically active state to form, and many tens of minutes for the resting state to be restored. In this review, we describe the properties of rhodopsin and its role in rod phototransduction. We first introduce rhodopsin's gross structural features, its evolution, and the basic mechanisms of its activation. We then discuss light absorption and spectral sensitivity, photoreceptor electrical responses that result from the activity of individual rhodopsin molecules, and recovery of rhodopsin and the visual system from intense bleaching exposures. We then provide a detailed examination of rhodopsin's molecular structure and function, first in its dark state, and then in the active Meta states that govern its interactions with transducin, rhodopsin kinase and arrestin. While it is clear that rhodopsin's molecular properties are exquisitely honed for phototransduction, from starlight to dawn/dusk intensity levels, our understanding of how its molecular interactions determine the properties of scotopic vision remains incomplete. We describe potential future directions of research, and outline several major problems that remain to be solved.

Systematic Assessment of Protein C-Termini Mutated in Human Disorders

All proteins have a carboxyl terminus, and we previously summarized eight mutations in binding and trafficking sequence determinants in the C-terminus that, when disrupted, cause human diseases. These sequence elements for binding and trafficking sites, as well as post-translational modifications (PTMs), are called minimotifs or short linear motifs. We wanted to determine how frequently mutations in minimotifs in the C-terminus cause disease. We searched specifically for PTMs because mutation of a modified amino acid almost always changes the chemistry of the side chain and can be interpreted as loss-of-function. We analyzed data from ClinVar for disease variants, Minimotif Miner and the C-terminome for PTMs, and RefSeq for protein sequences, yielding 20 such potential disease-causing variants. After additional screening, they include six with a previously reported PTM disruption mechanism and nine with new hypotheses for mutated minimotifs in C-termini that may cause disease. These mutations were generally for different genes, with four different PTM types and several different diseases. Our study helps to identify new molecular mechanisms for nine separate variants that cause disease, and this type of analysis could be extended as databases grow and to binding and trafficking motifs. We conclude that mutated motifs in C-termini are an infrequent cause of disease.

The Role of Reversible Phosphorylation of Drosophila Rhodopsin

Vertebrate and fly rhodopsins are prototypical GPCRs that have served for a long time as model systems for understanding GPCR signaling. Although all rhodopsins seem to become phosphorylated at their C-terminal region following activation by light, the role of this phosphorylation is not uniform. Two major functions of rhodopsin phosphorylation have been described: (1) inactivation of the activated rhodopsin either directly or by facilitating binding of arrestins in order to shut down the visual signaling cascade and thus eventually enabling a high-temporal resolution of the visual system. (2) Facilitating endocytosis of activated receptors via arrestin binding that in turn recruits clathrin to the membrane for clathrin-mediated endocytosis. In vertebrate rhodopsins the shutdown of the signaling cascade may be the main function of rhodopsin phosphorylation, as phosphorylation alone already quenches transducin activation and, in addition, strongly enhances arrestin binding. In the Drosophila visual system rhodopsin phosphorylation is not needed for receptor inactivation. Its role here may rather lie in the recruitment of arrestin 1 and subsequent endocytosis of the activated receptor. In this review, we summarize investigations of fly rhodopsin phosphorylation spanning four decades and contextualize them with regard to the most recent insights from vertebrate phosphorylation barcode theory.

Human cone elongation responses can be explained by photoactivated cone opsin and membrane swelling and osmotic response to phosphate produced by RGS9-catalyzed GTPase

Optical coherence tomography has established that human cone photoreceptor outer segments elongate in response to stimuli bleaching large fractions of their visual pigment. Elongation responses are completely described over their 200-fold bleaching range as the sum of two exponentially rising components differing 13-fold in time constants and 4-fold in light sensitivity. Bleaching measurements of individual cones with adaptive optics scanning laser ophthalmoscopy (SLO) suggest that component 2 arises from cone opsin and disk membrane swelling triggered by photoactivation. Application of a model of phototransduction suggests that component 1 corresponds to free phosphate generated by regulator of G-protein signaling 9 (RGS9)-catalyzed hydrolysis of guanosine triphosphate (GTP) in the α-subunit of G protein complexed with phosphodiesterase.

Human cone outer segment (COS) length changes in response to stimuli bleaching up to 99% of L- and M-cone opsins were measured with high resolution, phase-resolved optical coherence tomography (OCT). Responses comprised a fast phase (∼5 ms), during which COSs shrink, and two slower phases (1.5 s), during which COSs elongate. The slower components saturated in amplitude (∼425 nm) and initial rate (∼3 nm ms⁻¹) and are well described over the 200-fold bleaching range as the sum of two exponentially rising functions with time constants of 80 to 90 ms (component 1) and 1,000 to 1,250 ms (component 2). Measurements with adaptive optics reflection densitometry revealed component 2 to be linearly related to cone pigment bleaching, and the hypothesis is proposed that it arises from cone opsin and disk membrane swelling triggered by isomerization and rate-limited by chromophore hydrolysis and its reduction to membrane-localized all-trans retinol. The light sensitivity and kinetics of component 1 suggested that the underlying mechanism is an osmotic response to an amplified soluble by-product of phototransduction. The hypotheses that component 1 corresponds to G-protein subunits dissociating from the membrane, metabolites of cyclic guanosine monophosphate (cGMP) hydrolysis, or by-products of activated guanylate cyclase are rejected, while the hypothesis that it corresponds to phosphate produced by regulator of G-protein signaling 9 (RGS9)-catalyzed hydrolysis of guanosine triphosphate (GTP) in G protein–phosphodiesterase complexes was found to be consistent with the results. These results provide a basis for the assessment with optoretinography of phototransduction in individual cone photoreceptors in health and during disease progression and therapeutic interventions.

Structural basis of GPCR coupling to distinct signal transducers: implications for biased signaling

Three classes of G-protein-coupled receptor (GPCR) partners - G proteins, GPCR kinases, and arrestins - preferentially bind active GPCRs. Our analysis suggests that the structures of GPCRs bound to these interaction partners available today do not reveal a clear conformational basis for signaling bias, which would have enabled the rational design of biased GRCR ligands. In view of this, three possibilities are conceivable: (i) there are no generalizable GPCR conformations conducive to binding a particular type of partner; (ii) subtle differences in the orientation of individual residues and/or their interactions not easily detectable in the receptor-transducer structures determine partner preference; or (iii) the dynamics of GPCR binding to different types of partners rather than the structures of the final complexes might underlie transducer bias.

Structural evidence for visual arrestin priming via complexation of phosphoinositols

Visual arrestin (Arr1) terminates rhodopsin signaling by blocking its interaction with transducin. To do this, Arr1 translocates from the inner to the outer segment of photoreceptors upon light stimulation. Mounting evidence indicates that inositol phosphates (InsPs) affect Arr1 activity, but the Arr1-InsP molecular interaction remains poorly defined. We report the structure of bovine Arr1 in a ligand-free state featuring a near-complete model of the previously unresolved C-tail, which plays a crucial role in regulating Arr1 activity. InsPs bind to the N-domain basic patch thus displacing the C-tail, suggesting that they prime Arr1 for interaction with rhodopsin and help direct Arr1 translocation. These structures exhibit intact polar cores, suggesting that C-tail removal by InsP binding is insufficient to activate Arr1. These results show how Arr1 activity can be controlled by endogenous InsPs in molecular detail.

Agonist-induced phosphorylation of orthologues of the orphan receptor GPR35 functions as an activation sensor

G protein-coupled receptor 35 (GPR35) is poorly characterized but nevertheless has been revealed to have diverse roles in areas including lower gut inflammation and pain. The development of novel reagents and tools will greatly enhance analysis of GPR35 functions in health and disease. Here, we used mass spectrometry, mutagenesis, and [³²P] orthophosphate labeling to identify that all five hydroxy-amino acids in the C-terminal tail of human GPR35a became phosphorylated in response to agonist occupancy of the receptor and that, apart from Ser²⁹⁴, each of these contributed to interactions with arretin-3, which inhibits further G protein-coupled receptor signaling. We found that Ser³⁰³ was key to such interactions; the serine corresponding to human GPR35a residue 303 also played a dominant role in arrestin-3 interactions for both mouse and rat GPR35. We also demonstrated that fully phospho-site–deficient mutants of human GPR35a and mouse GPR35 failed to interact effectively with arrestin-3, and the human phospho-deficient variant was not internalized from the surface of cells in response to agonist treatment. Even in cells stably expressing species orthologues of GPR35, a substantial proportion of the expressed protein(s) was determined to be immature. Finally, phospho-site–specific antisera targeting the region encompassing Ser³⁰³ in human (Ser³⁰¹ in mouse) GPR35a identified only the mature forms of GPR35 and provided effective sensors of the activation status of the receptors both in immunoblotting and immunocytochemical studies. Such antisera may be useful tools to evaluate target engagement in drug discovery and target validation programs.

Dimeric Rhodopsin R135L Mutant-Transducin-like Complex Sheds Light on Retinitis Pigmentosa Misfunctions

Rhodopsin (RHO) is a light-sensitive pigment in the retina and the main prototypical protein of the G-protein-coupled receptor (GCPR) family. After receiving a light stimulus, RHO and its cofactor retinylidene undergo a series of structural changes that initiate an intricate transduction mechanism. Along with RHO, other partner proteins play key roles in the signaling pathway. These include transducin, a GTPase, kinases that phosphorylate RHO, and arrestin (Arr), which ultimately stops the signaling process and promotes RHO regeneration. A large number of RHO genetic mutations may lead to very severe retinal dysfunction and eventually to impaired dark adaptation disease called autosomal dominant retinitis pigmentosa (adRP). In this study, we used molecular dynamics (MD) simulations to evaluate the different behaviors of the dimeric form of wild-type RHO (WT dRHO) and its mutant at position 135 of arginine to leucine (dR135L), both in the free (noncomplexed) and in complex with the transducin-like protein (Gtl). Gtl is a heterotrimeric model composed of a mixture of human and bovine G proteins. Our calculations allow us to explain how the mutation causes structural changes in the RHO dimer and how this can affect the signal that transducin generates when it is bound to RHO. Moreover, the structural modifications induced by the R135L mutation can also account for other misfunctions observed in the up- and downstream signaling pathways. The mechanism of these dysfunctions, together with the transducin activity reduction, provides structure-based explanations of the impairment of some key processes that lead to adRP.

Arrestin-dependent internalization of rhodopsin-like G protein-coupled receptors

The internalization of G protein-coupled receptors (GPCRs) is an important mechanism regulating the signal strength and limiting the opportunity of receptor activation. Based on the importance of GPCRs, the detailed knowledge about the regulation of signal transduction is crucial. Here, current knowledge about the agonist-induced, arrestin-dependent internalization process of rhodopsin-like GPCRs is reviewed. Arrestins are conserved molecules that act as key players within the internalization process of many GPCRs. Based on highly conserved structural characteristics within the rhodopsin-like GPCRs, the identification of arrestin interaction sites in model systems can be compared and used for the investigation of internalization processes of other receptors. The increasing understanding of this essential regulation mechanism of receptors can be used for drug development targeting rhodopsin-like GPCRs. Here, we focus on the neuropeptide Y receptor family, as these receptors transmit various physiological processes such as food intake, energy homeostasis, and regulation of emotional behavior, and are further involved in pathophysiological processes like cancer, obesity and mood disorders. Hence, this receptor family represents an interesting target for the development of novel therapeutics requiring the understanding of the regulatory mechanisms influencing receptor mediated signaling.

Functional modulation of phosphodiesterase-6 by calcium in mouse rod photoreceptors

Phosphodiesterase-6 (PDE6) is a key protein in the G-protein cascade converting photon information to bioelectrical signals in vertebrate photoreceptor cells. Here, we demonstrate that PDE6 is regulated by calcium, contrary to the common view that PDE1 is the unique PDE class whose activity is modulated by intracellular Ca²⁺. To broaden the operating range of photoreceptors, mammalian rod photoresponse recovery is accelerated mainly by two calcium sensor proteins: recoverin, modulating the lifetime of activated rhodopsin, and guanylate cyclase-activating proteins (GCAPs), regulating the cGMP synthesis. We found that decreasing rod intracellular Ca²⁺ concentration accelerates the flash response recovery and increases the basal PDE6 activity (β_dark) maximally by ~ 30% when recording local electroretinography across the rod outer segment layer from GCAPs^-/- recoverin^-/- mice. Our modeling shows that a similar elevation in β_dark can fully explain the observed acceleration of flash response recovery in low Ca²⁺. Additionally, a reduction of the free Ca²⁺ in GCAPs^-/- recoverin^-/- rods shifted the inhibition constants of competitive PDE inhibitor 3-isobutyl-1-methylxanthine (IBMX) against the thermally activated and light-activated forms of PDE6 to opposite directions, indicating a complex interaction between IBMX, PDE6, and calcium. The discovered regulation of PDE6 is a previously unknown mechanism in the Ca²⁺-mediated modulation of rod light sensitivity.

Receptor-Arrestin Interactions: The GPCR Perspective

Arrestins are a small family of four proteins in most vertebrates that bind hundreds of different G protein-coupled receptors (GPCRs). Arrestin binding to a GPCR has at least three functions: precluding further receptor coupling to G proteins, facilitating receptor internalization, and initiating distinct arrestin-mediated signaling. The molecular mechanism of arrestin–GPCR interactions has been extensively studied and discussed from the “arrestin perspective”, focusing on the roles of arrestin elements in receptor binding. Here, we discuss this phenomenon from the “receptor perspective”, focusing on the receptor elements involved in arrestin binding and emphasizing existing gaps in our knowledge that need to be filled. It is vitally important to understand the role of receptor elements in arrestin activation and how the interaction of each of these elements with arrestin contributes to the latter’s transition to the high-affinity binding state. A more precise knowledge of the molecular mechanisms of arrestin activation is needed to enable the construction of arrestin mutants with desired functional characteristics.

GRKs as Modulators of Neurotransmitter Receptors

Many receptors for neurotransmitters, such as dopamine, norepinephrine, acetylcholine, and neuropeptides, belong to the superfamily of G protein-coupled receptors (GPCRs). A general model posits that GPCRs undergo two-step homologous desensitization: the active receptor is phosphorylated by kinases of the G protein-coupled receptor kinase (GRK) family, whereupon arrestin proteins specifically bind active phosphorylated receptors, shutting down G protein-mediated signaling, facilitating receptor internalization, and initiating distinct signaling pathways via arrestin-based scaffolding. Here, we review the mechanisms of GRK-dependent regulation of neurotransmitter receptors, focusing on the diverse modes of GRK-mediated phosphorylation of receptor subtypes. The immediate signaling consequences of GRK-mediated receptor phosphorylation, such as arrestin recruitment, desensitization, and internalization/resensitization, are equally diverse, depending not only on the receptor subtype but also on phosphorylation by GRKs of select receptor residues. We discuss the signaling outcome as well as the biological and behavioral consequences of the GRK-dependent phosphorylation of neurotransmitter receptors where known.

Structure of a GRK5-Calmodulin Complex Reveals Molecular Mechanism of GRK Activation and Substrate Targeting

The phosphorylation of G protein-coupled receptors (GPCRs) by GPCR kinases (GRKs) facilitates arrestin binding and receptor desensitization. Although this process can be regulated by Ca²⁺-binding proteins such as calmodulin (CaM) and recoverin, the molecular mechanisms are poorly understood. Here, we report structural, computational, and biochemical analysis of a CaM complex with GRK5, revealing how CaM shapes GRK5 response to calcium. The CaM N and C domains bind independently to two helical regions at the GRK5 N and C termini to inhibit GPCR phosphorylation, though only the C domain interaction disrupts GRK5 membrane association, thereby facilitating cytoplasmic translocation. The CaM N domain strongly activates GRK5 via ordering of the amphipathic αN-helix of GRK5 and allosteric disruption of kinase-RH domain interaction for phosphorylation of cytoplasmic GRK5 substrates. These results provide a framework for understanding how two functional effects, GRK5 activation and localization, can cooperate under control of CaM for selective substrate targeting by GRK5.

Position of rhodopsin photoisomerization on the disk surface confers variability to the rising phase of the single photon response in vertebrate rod photoreceptors

Retinal rods function as accurate photon counters to provide for vision under very dim light. To do so, rods must generate highly amplified, reproducible responses to single photons, yet outer segment architecture and randomness in the location of rhodopsin photoisomerization on the surface of an internal disk introduce variability to the rising phase of the photon response. Soon after a photoisomerization at a disk rim, depletion of cGMP near the plasma membrane closes ion channels and hyperpolarizes the rod. But with a photoisomerization in the center of a disk, local depletion of cGMP is distant from the channels in the plasma membrane. Thus, channel closure is delayed by the time required for the reduction of cGMP concentration to reach the plasma membrane. Moreover, the local fall in cGMP dissipates over a larger volume before affecting the channels, so response amplitude is reduced. This source of variability increases with disk radius. Using a fully space-resolved biophysical model of rod phototransduction, we quantified the variability attributable to randomness in the location of photoisomerization as a function of disk structure. In mouse rods that have small disks bearing a single incisure, this variability was negligible in the absence of the incisure. Variability was increased slightly by the incisure, but randomness in the shutoff of rhodopsin emerged as the main source of single photon response variability at all but the earliest times. Variability arising from randomness in the transverse location of photoisomerization increased in magnitude and persisted over a longer period in the photon response of large salamander rods. A symmetric arrangement of multiple incisures in the disks of salamander rods greatly reduced this variability during the rising phase, but the incisures had the opposite effect on variability arising from randomness in rhodopsin shutoff at later times.

Biological Role of Arrestin-1 Oligomerization

Members of the arrestin superfamily have great propensity of self-association, but the physiological significance of this phenomenon is unclear. To determine the biological role of visual arrestin-1 oligomerization in rod photoreceptors, we expressed mutant arrestin-1 with severely impaired self-association in mouse rods and analyzed mice of both sexes. We show that the oligomerization-deficient mutant is capable of quenching rhodopsin signaling normally, as judged by electroretinography and single-cell recording. Like wild type, mutant arrestin-1 is largely excluded from the outer segments in the dark, proving that the normal intracellular localization is not due the size exclusion of arrestin-1 oligomers. In contrast to wild type, supraphysiological expression of the mutant causes shortening of the outer segments and photoreceptor death. Thus, oligomerization reduces the cytotoxicity of arrestin-1 monomer, ensuring long-term photoreceptor survival.SIGNIFICANCE STATEMENT Visual arrestin-1 forms dimers and tetramers. The biological role of its oligomerization is unclear. To test the role of arrestin-1 self-association, we expressed oligomerization-deficient mutant in arrestin-1 knock-out mice. The mutant quenches light-induced rhodopsin signaling like wild type, demonstrating that in vivo monomeric arrestin-1 is necessary and sufficient for this function. In rods, arrestin-1 moves from the inner segments and cell bodies in the dark to the outer segments in the light. Nonoligomerizing mutant undergoes the same translocation, demonstrating that the size of the oligomers is not the reason for arrestin-1 exclusion from the outer segments in the dark. High expression of oligomerization-deficient arrestin-1 resulted in rod death. Thus, oligomerization reduces the cytotoxicity of high levels of arrestin-1 monomer.

A non-GPCR-binding partner interacts with a novel surface on β-arrestin1 to mediate GPCR signaling

The multifaceted adaptor protein β-arr1 (β-arrestin1) promotes activation of focal adhesion kinase (FAK) by the chemokine receptor CXCR4, facilitating chemotaxis. This function of β-arr1 requires the assistance of the adaptor protein STAM1 (signal-transducing adaptor molecule 1) because disruption of the interaction between STAM1 and β-arr1 reduces CXCR4-mediated activation of FAK and chemotaxis. To begin to understand the mechanism by which β-arr1 together with STAM1 activates FAK, we used site-directed spin-labeling EPR spectroscopy-based studies coupled with bioluminescence resonance energy transfer-based cellular studies to show that STAM1 is recruited to activated β-arr1 by binding to a novel surface on β-arr1 at the base of the finger loop, at a site that is distinct from the receptor-binding site. Expression of a STAM1-deficient binding β-arr1 mutant that is still able to bind to CXCR4 significantly reduced CXCL12-induced activation of FAK but had no impact on ERK-1/2 activation. We provide evidence of a novel surface at the base of the finger loop that dictates non-GPCR interactions specifying β-arrestin-dependent signaling by a GPCR. This surface might represent a previously unidentified switch region that engages with effector molecules to drive β-arrestin signaling.

Elevated energy requirement of cone photoreceptors

We have used recent measurements of mammalian cone light responses and voltage-gated currents to calculate cone ATP utilization and compare it to that of rods. The largest expenditure of ATP results from ion transport, particularly from removal of Na⁺ entering outer segment light-dependent channels and inner segment hyperpolarization-activated cyclic nucleotide-gated channels, and from ATP-dependent pumping of Ca²⁺ entering voltage-gated channels at the synaptic terminal. Single cones expend nearly twice as much energy as single rods in darkness, largely because they make more synapses with second-order retinal cells and thus must extrude more Ca²⁺ In daylight, cone ATP utilization per cell remains high because cones never remain saturated and must continue to export Na⁺ and synaptic Ca²⁺ even in bright illumination. In mouse and human retina, rods greatly outnumber cones and consume more energy overall even in background light. In primates, however, the high density of cones in the fovea produces a pronounced peak of ATP utilization, which becomes particularly prominent in daylight and may make this part of the retina especially sensitive to changes in energy availability.

Circadian Photoentrainment in Mice and Humans

Light around twilight provides the primary entrainment signal for circadian rhythms. Here we review the mechanisms and responses of the mouse and human circadian systems to light. Both utilize a network of photosensitive retinal ganglion cells (pRGCs) expressing the photopigment melanopsin (OPN4). In both species action spectra and functional expression of OPN4 in vitro show that melanopsin has a λ_max close to 480 nm. Anatomical findings demonstrate that there are multiple pRGC sub-types, with some evidence in mice, but little in humans, regarding their roles in regulating physiology and behavior. Studies in mice, non-human primates and humans, show that rods and cones project to and can modulate the light responses of pRGCs. Such an integration of signals enables the rods to detect dim light, the cones to detect higher light intensities and the integration of intermittent light exposure, whilst melanopsin measures bright light over extended periods of time. Although photoreceptor mechanisms are similar, sensitivity thresholds differ markedly between mice and humans. Mice can entrain to light at approximately 1 lux for a few minutes, whilst humans require light at high irradiance (>100’s lux) and of a long duration (>30 min). The basis for this difference remains unclear. As our retinal light exposure is highly dynamic, and because photoreceptor interactions are complex and difficult to model, attempts to develop evidence-based lighting to enhance human circadian entrainment are very challenging. A way forward will be to define human circadian responses to artificial and natural light in the “real world” where light intensity, duration, spectral quality, time of day, light history and age can each be assessed.

Individual phosphorylation sites at the C-terminus of the apelin receptor play different roles in signal transduction

The apelin and Elabela proteins constitute a spatiotemporal double-ligand system that controls apelin receptor (APJ) signal transduction. Phosphorylation of multiple sites within the C-terminus of APJ is essential for the recruitment of β-arrestins. We sought to determine the precise mechanisms by which apelin and Elabela promote APJ phosphorylation, and to elucidate the influence of β-arrestin phosphorylation on G-protein-coupled receptor (GPCR)/β-arrestin-dependent signaling. We used techniques including mass spectrometry (MS), mutation analysis, and bioluminescence resonance energy transfer (BRET) to evaluate the role of phosphorylation sites in APJ-mediated G-protein-dependent and β-dependent signaling. Phosphorylation of APJ occurred at five serine residues in the C-terminal region (Ser335, Ser339, Ser345, Ser348 and Ser369). We also identified two phosphorylation sites in β-arrestin1 and three in β-arrestin2, including three previously identified residues (Ser412, Ser361, and Thr383) and two new sites, Tyr47 in β-arrestin1 and Tyr48 in β-arrestin2. APJ mutations did not affect the phosphorylation of β-arrestins, but it affects the β-arrestin signaling pathway, specifically Ser335 and Ser339. Mutation of Ser335 decreased the ability of the receptor to interact with β-arrestin1/2 and AP2, indicating that APJ affects the β-arrestin signaling pathway by stimulating Elabela. Mutation of Ser339 abolished the capability of the receptor to interact with GRK2 and β-arrestin1/2 upon stimulation with apelin-36, and disrupted receptor internalization and β-arrestin-dependent ERK1/2 activation. Five peptides act on distinct phosphorylation sites at the APJ C-terminus, differentially regulating APJ signal transduction and causing different biological effects. These findings may facilitate screening for drugs to treat cardiovascular and metabolic diseases.

Phosphorylation barcode-dependent signal bias of the dopamine D1 receptor

Agonist-activated G protein-coupled receptors (GPCRs) must correctly select from hundreds of potential downstream signaling cascades and effectors. To accomplish this, GPCRs first bind to an intermediary signaling protein, such as G protein or arrestin. These intermediaries initiate signaling cascades that promote the activity of different effectors, including several protein kinases. The relative roles of G proteins versus arrestins in initiating and directing signaling is hotly debated, and it remains unclear how the correct final signaling pathway is chosen given the ready availability of protein partners. Here, we begin to deconvolute the process of signal bias from the dopamine D1 receptor (D1R) by exploring factors that promote the activation of ERK1/2 or Src, the kinases that lead to cell growth and proliferation. We found that ERK1/2 activation involves both arrestin and Gαs, while Src activation depends solely on arrestin. Interestingly, we found that the phosphorylation pattern influences both arrestin and Gαs coupling, suggesting an additional way the cells regulate G protein signaling. The phosphorylation sites in the D1R intracellular loop 3 are particularly important for directing the binding of G protein versus arrestin and for selecting between the activation of ERK1/2 and Src. Collectively, these studies correlate functional outcomes with a physical basis for signaling bias and provide fundamental information on how GPCR signaling is directed.

Biased GPCR signaling: Possible mechanisms and inherent limitations

G protein-coupled receptors (GPCRs) are targeted by about a third of clinically used drugs. Many GPCRs couple to more than one type of heterotrimeric G proteins, become phosphorylated by any of several different GRKs, and then bind one or more types of arrestin. Thus, classical therapeutically active drugs simultaneously initiate several branches of signaling, some of which are beneficial, whereas others result in unwanted on-target side effects. The development of novel compounds to selectively channel the signaling into the desired direction has the potential to become a breakthrough in health care. However, there are natural and technological hurdles that must be overcome. The fact that most GPCRs are subject to homologous desensitization, where the active receptor couples to G proteins, is phosphorylated by GRKs, and then binds arrestins, suggest that in most cases the GPCR conformations that facilitate their interactions with these three classes of binding partners significantly overlap. Thus, while partner-specific conformations might exist, they are likely low-probability states. GPCRs are inherently flexible, which suggests that complete bias is highly unlikely to be feasible: in the conformational ensemble induced by any ligand, there would be some conformations facilitating receptor coupling to unwanted partners. Things are further complicated by the fact that virtually every cell expresses numerous G proteins, several GRK subtypes, and two non-visual arrestins with distinct signaling capabilities. Finally, novel screening methods for measuring ligand bias must be devised, as the existing methods are not specific for one particular branch of signaling.

Revealing Conformational Transitions in G-Protein-Coupled Receptor Rhodopsin upon Phosphorylation

G-protein-coupled receptors (GPCRs) have evolved as highly specialized cellular machinery that can dictate biological outcomes in response to diverse stimuli. Specifically, they induce multiple pathway responses upon structural perturbations induced at local protein sites. GPCRs utilize a concurrent strategy involving a central transmembrane topology and biochemical modifications for precise functional implementation. However, the specific role of the latter is not known due to the lack of precise probing techniques that can characterize receptor dynamics upon biochemical modifications. Phosphorylation is known to be one of the critical biochemical modifications in GPCRs that aids in receptor desensitization via arrestin binding. Here, we carry out all-atom molecular dynamics simulations of rhodopsin in a membrane environment to study its conformational dynamics induced upon phosphorylation. Interestingly, our comparative analysis of non-phosphorylated and phosphorylated rhodopsin structure demonstrated enhanced receptor stability upon phosphorylation at the C-terminal region that leads to the opening of the extracellular part of the transmembrane helices. In addition, monitoring the distinct number of phosphorylation states showed that having fewer phosphorylated residues does not bring about appropriate conformational changes in the extracellular region. Since phosphorylation results in receptor desensitization and recycling of the ligand, our findings provide significant insights into the conformational dynamics of the mechanism of ligand exit from the receptor.

Targeting arrestin interactions with its partners for therapeutic purposes

Most vertebrates express four arrestin subtypes: two visual ones in photoreceptor cells and two non-visuals expressed ubiquitously. The latter two interact with hundreds of G protein-coupled receptors, certain receptors of other types, and numerous non-receptor partners. Arrestins have no enzymatic activity and work by interacting with other proteins, often assembling multi-protein signaling complexes. Arrestin binding to every partner affects cell signaling, including pathways regulating cell survival, proliferation, and death. Thus, targeting individual arrestin interactions has therapeutic potential. This requires precise identification of protein-protein interaction sites of both participants and the choice of the side of each interaction which would be most advantageous to target. The interfaces involved in each interaction can be disrupted by small molecule therapeutics, as well as by carefully selected peptides of the other partner that do not participate in the interactions that should not be targeted.

A targeted approach for the synthesis of multi-phosphorylated peptides: a tool for studying the role of phosphorylation patterns in proteins

Protein phosphorylation barcodes, clusters of several phosphorylation sites within a short unfolded region, control many cellular processes. Existing biochemical methods used to study the roles of these barcodes suffer from low selectivity and provide only qualitative data. Chemically synthesized multiphosphopeptide libraries are selective and specific, but their synthesis is extremely difficult using the current peptide synthesis methods. Here we describe a new microwave assisted approach for synthesizing a library of multiphosphopeptides, using the C-terminus of rhodopsin as a proof of concept. Our approach utilizes multiple protocols for synthesizing libraries of multiphosphopeptides instead of the inefficient single protocol methods currently used. Using our approach we demonstrated the synthesis with up to seven phosphorylated amino acids, sometimes next to each other, an accomplishment that was impractical before. Synthesizing the Rhodopsin derived multiphosphopeptide library enabled dissecting the precise phosphorylation barcode required for the recruitment, activation and modulation of the conformation of Arrestin. Since phosphorylation barcodes modulate the activity of hundreds of GPCRs, synthesizing libraries of multiphosphopeptides is the method of choice for studying their molecular mechanisms of action. Our approach provides an invaluable tool for evaluating how protein phosphorylation barcodes regulate their activity.

Multi-scale, numerical modeling of spatio-temporal signaling in cone phototransduction

Mammals have two types of photoreceptors, rods and cones. While rods are exceptionally sensitive and mediate vision at very low illumination levels, cones operate in daylight and are responsible for the bulk of visual perception in most diurnal animals, including humans. Yet the mechanisms of phototransduction in cones is understudied, largely due to unavailability of pure cone outer segment (COS) preparations. Here we present a novel mathematical model of cone phototransduction that explicitly takes into account complex cone geometry and its multiple physical scales, faithfully reproduces features of the cone response, and is orders of magnitude more efficient than the standard 3D diffusion model. This is accomplished through the mathematical techniques of homogenization and concentrated capacity. The homogenized model is then computationally implemented by finite element method. This homogenized model permits one to analyze the effects of COS geometry on visual transduction and lends itself to performing large numbers of numerical trials, as required for parameter analysis and the stochasticity of rod and cone signal transduction. Agreement between the nonhomogenized, (i.e., standard 3D), and homogenized diffusion models is reported along with their simulation times and memory costs. Virtual expression of rod biochemistry on cone morphology is also presented for understanding some of the characteristic differences between rods and cones. These simulations evidence that 3D cone morphology and ion channel localization contribute to biphasic flash response, i.e undershoot. The 3D nonhomogenized and homogenized models are contrasted with more traditional and coarser well-stirred and 1D longitudinal diffusion models. The latter are single-scale and do not explicitly account for the multi-scale geometry of the COS, unlike the 3D homogenized model. We show that simpler models exaggerate the magnitude of the current suppression, yield accelerated time to peak, and do not predict the local concentration of cGMP at the ionic channels.

Distinct G protein-coupled receptor phosphorylation motifs modulate arrestin affinity and activation and global conformation

Cellular functions of arrestins are determined in part by the pattern of phosphorylation on the G protein-coupled receptors (GPCRs) to which arrestins bind. Despite high-resolution structural data of arrestins bound to phosphorylated receptor C-termini, the functional role of each phosphorylation site remains obscure. Here, we employ a library of synthetic phosphopeptide analogues of the GPCR rhodopsin C-terminus and determine the ability of these peptides to bind and activate arrestins using a variety of biochemical and biophysical methods. We further characterize how these peptides modulate the conformation of arrestin-1 by nuclear magnetic resonance (NMR). Our results indicate different functional classes of phosphorylation sites: 'key sites' required for arrestin binding and activation, an 'inhibitory site' that abrogates arrestin binding, and 'modulator sites' that influence the global conformation of arrestin. These functional motifs allow a better understanding of how different GPCR phosphorylation patterns might control how arrestin functions in the cell.

The structural basis of the arrestin binding to GPCRs

G protein-coupled receptors (GPCRs) are the largest family of signaling proteins targeted by more clinically used drugs than any other protein family. GPCR signaling via G proteins is quenched (desensitized) by the phosphorylation of the active receptor by specific GPCR kinases (GRKs) followed by tight binding of arrestins to active phosphorylated receptors. Thus, arrestins engage two types of receptor elements: those that contain GRK-added phosphates and those that change conformation upon activation. GRKs attach phosphates to serines and threonines in the GPCR C-terminus or any one of the cytoplasmic loops. In addition to these phosphates, arrestins engage the cavity that appears between trans-membrane helices upon receptor activation and several other non-phosphorylated elements. The residues that bind GPCRs are localized on the concave side of both arrestin domains. Arrestins undergo a global conformational change upon receptor binding (become activated). Arrestins serve as important hubs of cellular signaling, emanating from activated GPCRs and receptor-independent.

Determination of basal phosphodiesterase activity in mouse rod photoreceptors with cGMP clamp

Light regulates cGMP concentration in the photoreceptor cytoplasm by activating phosphodiesterase (PDE) molecules through a G-protein signalling cascade. Spontaneous PDE activity is present in rod outer segments even in darkness. This basal PDE activity (β_dark) has not been determined in wild type mammalian photoreceptor cells although it plays a key role in setting the sensitivity and recovery kinetics of rod responses. We present a novel method for determination of β_dark using local electroretinography (LERG) from isolated mouse retinas. The method is based on the ability of PDE inhibitors to decrease β_dark, which can be counterbalanced by increasing PDE activity with light. This procedure clamps cytoplasmic cGMP to its dark value. β_dark can be calculated based on the amount of light needed for the "cGMP clamp" and information extracted from the registered rod photoresponses. Here we apply this method to determine β_dark values for the first time in the mammalian rods and obtain the following estimates for different mouse models: 3.9 s^-1 for wild type, 4.5 s^-1 for guanylate cyclase activating proteins (GCAPs) knockout, and 4.4 s^-1 for GCAPs and recoverin double knockout mice. Our results suggest that depletion of GCAPs or recoverin do not affect β_dark.

Cleavage of arrestin-3 by caspases attenuates cell death by precluding arrestin-dependent JNK activation

The two non-visual subtypes, arrestin-2 and arrestin-3, are ubiquitously expressed and bind hundreds of G protein-coupled receptors. In addition, these arrestins also interact with dozens of non-receptor signaling proteins, including c-Src, ERK and JNK, that regulate cell death and survival. Arrestin-3 facilitates the activation of JNK family kinases, which are important players in the regulation of apoptosis. Here we show that arrestin-3 is specifically cleaved at Asp366, Asp405 and Asp406 by caspases during the apoptotic cell death. This results in the generation of one main cleavage product, arrestin-3-(1-366). The formation of this fragment occurs in a dose-dependent manner with the increase of fraction of apoptotic cells upon etoposide treatment. In contrast to a caspase-resistant mutant (D366/405/406E) the arrestin-3-(1-366) fragment reduces the apoptosis of etoposide-treated cells. We found that caspase cleavage did not affect the binding of the arrestin-3 to JNK3, but prevented facilitation of its activation, in contrast to the caspase-resistant mutant, which facilitated JNK activation similar to WT arrestin-3, likely due to decreased binding of the upstream kinases ASK1 and MKK4/7. The data suggest that caspase-generated arrestin-3-(1-366) prevents the signaling in the ASK1-MKK4/7-JNK1/2/3 cascade and protects cells, thereby suppressing apoptosis.

Arrestin-1 engineering facilitates complex stabilization with native rhodopsin

Arrestin-1 desensitizes the activated and phosphorylated photoreceptor rhodopsin by forming transient rhodopsin−arrestin-1 complexes that eventually decay to opsin, retinal and arrestin-1. Via a multi-dimensional screening setup, we identified and combined arrestin-1 mutants that form lasting complexes with light-activated and phosphorylated rhodopsin in harsh conditions, such as high ionic salt concentration. Two quadruple mutants, D303A + T304A + E341A + F375A and R171A + T304A + E341A + F375A share similar heterologous expression and thermo-stability levels with wild type (WT) arrestin-1, but are able to stabilize complexes with rhodopsin with more than seven times higher half-maximal inhibitory concentration (IC₅₀) values for NaCl compared to the WT arrestin-1 protein. These quadruple mutants are also characterized by higher binding affinities to phosphorylated rhodopsin, light-activated rhodopsin and phosphorylated opsin, as compared with WT arrestin-1. Furthermore, the assessed arrestin-1 mutants are still specifically associating with phosphorylated or light-activated receptor states only, while binding to the inactive ground state of the receptor is not significantly altered. Additionally, we propose a novel functionality for R171 in stabilizing the inactive arrestin-1 conformation as well as the rhodopsin–arrestin-1 complex. The achieved stabilization of the active rhodopsin–arrestin-1 complex might be of great interest for future structure determination, antibody development studies as well as drug-screening efforts targeting G protein-coupled receptors (GPCRs).

Light-Induced Thiol Oxidation of Recoverin Affects Rhodopsin Desensitization

The excessive light illumination of mammalian retina is known to induce oxidative stress and photoreceptor cell death linked to progression of age-related macular degeneration. The photochemical damage of photoreceptors is suggested to occur via two apoptotic pathways that involve either excessive rhodopsin activation or constitutive phototransduction, depending on the light intensity. Both pathways are dramatically activated in the absence of rhodopsin desensitization by GRK1. Previously, we have shown that moderate illumination (halogen lamp, 1,500 lx, 1–5 h) of mammalian eyes provokes disulfide dimerization of recoverin, a calcium-dependent regulator of GRK1. Here, we demonstrate under in vivo conditions that both moderate long-term (metal halide lamp, 2,500 lx, 14 h, rat model) and intense short-term (halogen lamp, 30,000 lx for 3 h, rabbit model) illumination of the mammalian retina are accompanied by accumulation of disulfide dimer of recoverin. Furthermore, in the second case we reveal alternatively oxidized derivatives of the protein, apparently including its monomer with sulfinic group. Histological data indicate that thiol oxidation of recoverin precedes apoptosis of photoreceptors. Both disulfide dimer and oxidized monomer (or oxidation mimicking C39D mutant) of recoverin exhibit lowered α-helical content and thermal stability of their apo-forms, as well as increased Ca²⁺ affinity. Meanwhile, the oxidized monomer and C39D mutant of recoverin demonstrate impaired ability to bind photoreceptor membranes and regulate GRK1, whereas disulfide dimer exhibits notably improved membrane binding and GRK1 inhibition in absence of Ca²⁺. The latter effect is expected to slow down rhodopsin desensitization in the light, thereby favoring support of the light-induced oxidative stress, ultimately leading to photoreceptor apoptosis. Overall, the intensity and duration of illumination of the retina affect thiol oxidation of recoverin likely contributing to propagation of the oxidative stress and photoreceptor damage.

Molecular Defects of the Disease-Causing Human Arrestin-1 C147F Mutant

Purpose

The purpose of this study was to identify the molecular defect in the disease-causing human arrestin-1 C147F mutant.

Methods

The binding of wild-type (WT) human arrestin-1 and several mutants with substitutions in position 147 (including C147F, which causes dominant retinitis pigmentosa in humans) to phosphorylated and unphosphorylated light-activated rhodopsin was determined. Thermal stability of WT and mutant human arrestin-1, as well as unfolded protein response in 661W cells, were also evaluated.

Results

WT human arrestin-1 was selective for phosphorylated light-activated rhodopsin. Substitutions of Cys-147 with smaller side chain residues, Ala or Val, did not substantially affect binding selectivity, whereas residues with bulky side chains in the position 147 (Ile, Leu, and disease-causing Phe) greatly increased the binding to unphosphorylated rhodopsin. Functional survival of mutant proteins with bulky substitutions at physiological and elevated temperature was also compromised. C147F mutant induced unfolded protein response in cultured cells.

Conclusions

Bulky Phe substitution of Cys-147 in human arrestin-1 likely causes rod degeneration due to reduced stability of the protein, which induces unfolded protein response in expressing cells.

Structural Basis of Arrestin-Dependent Signal Transduction

Arrestins are a small family of proteins with four isoforms in humans. Remarkably, two arrestins regulate signaling from >800 G protein-coupled receptors (GPCRs) or nonreceptor activators by simultaneously binding an activator and one out of hundreds of other signaling proteins. When arrestins are bound to GPCRs or other activators, the affinity for these signaling partners changes. Thus, it is proposed that an activator alters arrestin's ability to transduce a signal. The comparison of all available arrestin structures identifies several common conformational rearrangements associated with activation. In particular, it identifies elements that are directly involved in binding to GPCRs or other activators, elements that likely engage distinct downstream effectors, and elements that likely link the activator-binding sites with the effector-binding sites.

Arrestins: structural disorder creates rich functionality

Arrestins are soluble relatively small 44–46 kDa proteins that specifically bind hundreds of active phosphorylated GPCRs and dozens of non-receptor partners. There are binding partners that demonstrate preference for each of the known arrestin conformations: free, receptor-bound, and microtubule-bound. Recent evidence suggests that conformational flexibility in every functional state is the defining characteristic of arrestins. Flexibility, or plasticity, of proteins is often described as structural disorder, in contrast to the fixed conformational order observed in high-resolution crystal structures. However, protein-protein interactions often involve highly flexible elements that can assume many distinct conformations upon binding to different partners. Existing evidence suggests that arrestins are no exception to this rule: their flexibility is necessary for functional versatility. The data on arrestins and many other multi-functional proteins indicate that in many cases, “order” might be artificially imposed by highly non-physiological crystallization conditions and/or crystal packing forces. In contrast, conformational flexibility (and its extreme case, intrinsic disorder) is a more natural state of proteins, representing true biological order that underlies their physiologically relevant functions.

Structure and dynamics of GPCR signaling complexes

G-protein-coupled receptors (GPCRs) relay numerous extracellular signals by triggering intracellular signaling through coupling with G proteins and arrestins. Recent breakthroughs in the structural determination of GPCRs and GPCR-transducer complexes represent important steps toward deciphering GPCR signal transduction at a molecular level. A full understanding of the molecular basis of GPCR-mediated signaling requires elucidation of the dynamics of receptors and their transducer complexes as well as their energy landscapes and conformational transition rates. Here, we summarize current insights into the structural plasticity of GPCR-G-protein and GPCR-arrestin complexes that underlies the regulation of the receptor's intracellular signaling profile.

Molecular Mechanisms of GPCR Signaling: A Structural Perspective

G protein-coupled receptors (GPCRs) are cell surface receptors that respond to a wide variety of stimuli, from light, odorants, hormones, and neurotransmitters to proteins and extracellular calcium. GPCRs represent the largest family of signaling proteins targeted by many clinically used drugs. Recent studies shed light on the conformational changes that accompany GPCR activation and the structural state of the receptor necessary for the interactions with the three classes of proteins that preferentially bind active GPCRs, G proteins, G protein-coupled receptor kinases (GRKs), and arrestins. Importantly, structural and biophysical studies also revealed activation-related conformational changes in these three types of signal transducers. Here, we summarize what is already known and point out questions that still need to be answered. Clear understanding of the structural basis of signaling by GPCRs and their interaction partners would pave the way to designing signaling-biased proteins with scientific and therapeutic potential.

Identification of Phosphorylation Codes for Arrestin Recruitment by G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) mediate diverse signaling in part through interaction with arrestins, whose binding promotes receptor internalization and signaling through G protein-independent pathways. High-affinity arrestin binding requires receptor phosphorylation, often at the receptor's C-terminal tail. Here, we report an X-ray free electron laser (XFEL) crystal structure of the rhodopsin-arrestin complex, in which the phosphorylated C terminus of rhodopsin forms an extended intermolecular β sheet with the N-terminal β strands of arrestin. Phosphorylation was detected at rhodopsin C-terminal tail residues T336 and S338. These two phospho-residues, together with E341, form an extensive network of electrostatic interactions with three positively charged pockets in arrestin in a mode that resembles binding of the phosphorylated vasopressin-2 receptor tail to β-arrestin-1. Based on these observations, we derived and validated a set of phosphorylation codes that serve as a common mechanism for phosphorylation-dependent recruitment of arrestins by GPCRs.

Two serines in the distal C-terminus of the human ß1-adrenoceptor determine ß-arrestin2 recruitment

G protein-coupled receptors (GPCRs) undergo phosphorylation at several intracellular residues by G protein-coupled receptor kinases. The resulting phosphorylation pattern triggers arrestin recruitment and receptor desensitization. The exact sites of phosphorylation and their function remained largely unknown for the human β1-adrenoceptor (ADRB1), a key GPCR in adrenergic signal transduction and the target of widely used drugs such as β-blockers. The present study aimed to identify the intracellular phosphorylation sites in the ADRB1 and to delineate their function. The human ADRB1 was expressed in HEK293 cells and its phosphorylation pattern was determined by mass spectrometric analysis before and after stimulation with a receptor agonist. We identified a total of eight phosphorylation sites in the receptor's third intracellular loop and C-terminus. Analyzing the functional relevance of individual sites using phosphosite-deficient receptor mutants we found phosphorylation of the ADRB1 at Ser461/Ser462 in the distal part of the C-terminus to determine β-arrestin2 recruitment and receptor internalization. Our data reveal the phosphorylation pattern of the human ADRB1 and the site that mediates recruitment of β-arrestin2.

Functional map of arrestin binding to phosphorylated opsin, with and without agonist

Arrestins desensitize G protein-coupled receptors (GPCRs) and act as mediators of signalling. Here we investigated the interactions of arrestin-1 with two functionally distinct forms of the dim-light photoreceptor rhodopsin. Using unbiased scanning mutagenesis we probed the individual contribution of each arrestin residue to the interaction with the phosphorylated apo-receptor (Ops-P) and the agonist-bound form (Meta II-P). Disruption of the polar core or displacement of the C-tail strengthened binding to both receptor forms. In contrast, mutations of phosphate-binding residues (phosphosensors) suggest the phosphorylated receptor C-terminus binds arrestin differently for Meta II-P and Ops-P. Likewise, mutations within the inter-domain interface, variations in the receptor-binding loops and the C-edge of arrestin reveal different binding modes. In summary, our results indicate that arrestin-1 binding to Meta II-P and Ops-P is similarly dependent on arrestin activation, although the complexes formed with these two receptor forms are structurally distinct.

Quantitative modeling of the molecular steps underlying shut-off of rhodopsin activity in rod phototransduction

PURPOSE

To examine the predictions of alternative models for the stochastic shut-off of activated rhodopsin (R*) and their implications for the interpretation of experimentally recorded single-photon responses (SPRs) in mammalian rods.

THEORY

We analyze the transitions that an activated R* molecule undergoes as a result of successive phosphorylation steps and arrestin binding. We consider certain simplifying cases for the relative magnitudes of the reaction rate constants and derive the probability distributions for the time to arrestin binding. In addition to the conventional model in which R* catalytic activity declines in a graded manner with successive phosphorylations, we analyze two cases in which the activity is assumed to occur not via multiple small steps upon each phosphorylation but via a single large step. We refer to these latter two cases as the binary R* shut-off and three-state R* shut-off models.

METHODS

We simulate R*'s stochastic reactions numerically for the three models. In the simplifying cases for the ratio of rate constants in the binary and three-state models, we show that the probability distribution of the time to arrestin binding is accurately predicted. To simulate SPRs, we then integrate the differential equations for the downstream reactions using a standard model of the rod outer segment that includes longitudinal diffusion of cGMP and Ca(2+).

RESULTS

Our simulations of SPRs in the conventional model of graded shut-off of R* conform closely to the simulations in a recent study. However, the gain factor required to account for the observed mean SPR amplitude is higher than can be accounted for from biochemical experiments. In addition, a substantial minority of the simulated SPRs exhibit features that have not been reported in published experiments. Our simulations of SPRs using the model of binary R* shut-off appear to conform closely to experimental results for wild type (WT) mouse rods, and the required gain factor conforms to biochemical expectations. However, for the arrestin knockout (Arr(-/-)) phenotype, the predictions deviated from experimental findings and led us to invoke a low-activity state that R* enters before arrestin binding. Our simulations of this three-state R* shut-off model are very similar to those of the binary model in the WT case but are preferred because they appear to accurately predict the mean SPRs for four mutant phenotypes, Arr(+/-), Arr(-/-), GRK1(+/-), and GRK1(-/-), in addition to the WT phenotype. When we additionally treated the formation and shut-off of activated phosphodiesterase (E*) as stochastic, the simulated SPRs appeared even more similar to real SPRs, and there was very little change in the ensemble mean and standard deviation or in the amplitude distribution.

CONCLUSIONS

We conclude that the conventional model of graded reduction in R* activity through successive phosphorylation steps appears to be inconsistent with experimental results. Instead, we find that two variants of a model in which R* activity initially remains high and then declines abruptly after several phosphorylation steps appears capable of providing a better description of experimentally measured SPRs.

Signalling by melanopsin (OPN4) expressing photosensitive retinal ganglion cells

Over the past two decades there have been significant advances in our understanding of both the anatomy and function of the melanopsin system. It has become clear that rather than acting as a simple irradiance detector the melanopsin system is in fact far more complicated. The range of behavioural systems known to be influenced by melanopsin activity is increasing with time, and it is now clear that melanopsin contributes not only to multiple non-image forming systems but also has a role in visual pathways. How melanopsin is capable of driving so many different behaviours is unclear, but recent evidence suggests that the answer may lie in the diversity of melanopsin light responses and the functional specialisation of photosensitive retinal ganglion cell (pRGC) subtypes. In this review, we shall overview the current understanding of the melanopsin system, and evaluate the evidence for the hypothesis that individual pRGC subtypes not only perform specific roles, but are functionally specialised to do so. We conclude that while, currently, the available data somewhat support this hypothesis, we currently lack the necessary detail to fully understand how the functional diversity of pRGC subtypes correlates with different behavioural responses, and ultimately why such complexity is required within the melanopsin system. What we are lacking is a cohesive understanding of how light responses differ between the pRGC subtypes (based not only on anatomical classification but also based on their site of innervation); how these diverse light responses are generated, and most importantly how these responses relate to the physiological functions they underpin.

How rods respond to single photons: Key adaptations of a G-protein cascade that enable vision at the physical limit of perception

Rod photoreceptors are among the most sensitive light detectors in nature. They achieve their remarkable sensitivity across a wide variety of species through a number of essential adaptations: a specialized cellular geometry, a G-protein cascade with an unusually stable receptor molecule, a low-noise transduction mechanism, a nearly perfect effector enzyme, and highly evolved mechanisms of feedback control and receptor deactivation. Practically any change in protein expression, enzyme activity, or feedback control can be shown to impair photon detection, either by decreasing sensitivity or signal-to-noise ratio, or by reducing temporal resolution. Comparison of mammals to amphibians suggests that rod outer-segment morphology and the molecules and mechanism of transduction may have evolved together to optimize light sensitivity in darkness, which culminates in the extraordinary ability of these cells to respond to single photons at the ultimate limit of visual perception.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

C-terminal threonines and serines play distinct roles in the desensitization of rhodopsin, a G protein-coupled receptor

Rod photoreceptors generate measurable responses to single-photon activation of individual molecules of the G protein-coupled receptor (GPCR), rhodopsin. Timely rhodopsin desensitization depends on phosphorylation and arrestin binding, which quenches G protein activation. Rhodopsin phosphorylation has been measured biochemically at C-terminal serine residues, suggesting that these residues are critical for producing fast, low-noise responses. The role of native threonine residues is unclear. We compared single-photon responses from rhodopsin lacking native serine or threonine phosphorylation sites. Contrary to expectation, serine-only rhodopsin generated prolonged step-like single-photon responses that terminated abruptly and randomly, whereas threonine-only rhodopsin generated responses that were only modestly slower than normal. We show that the step-like responses of serine-only rhodopsin reflect slow and stochastic arrestin binding. Thus, threonine sites play a privileged role in promoting timely arrestin binding and rhodopsin desensitization. Similar coordination of phosphorylation and arrestin binding may more generally permit tight control of the duration of GPCR activity.

DOI:http://dx.doi.org/10.7554/eLife.05981.001

‘Rod’ cells in the eye enable us to see in starlight. Inside these cells, a protein called rhodopsin is activated by light, which leads to an electrical signal being produced that travels to the brain. The duration of the electrical signal depends on the time it takes for the rhodopsin to be deactivated. Rhodopsin is a member of a large class of receptor proteins known as G protein-coupled receptors that regulate many processes throughout the body.

Previous studies have shown that rhodopsin is deactivated by the attachment of phosphate groups to the protein. This allows another protein called arrestin to bind to rhodopsin. The phosphates can be attached to particular amino acids—the building blocks of proteins—at one end of rhodopsin. Three of these are a type of amino acid called serine. Previous work has shown that light increases the speed at which phosphate groups are added to these serines, suggesting that they are important for producing rapid electrical signals. The other three amino acids are of a different type—called threonine—but it is less clear what role they play in deactivating rhodopsin.

Here, Azevedo et al. studied mutant forms of rhodopsin that were missing the serines or threonines in mice. The experiments show that loss of the serines only slightly slowed the electrical signals. However, loss of the threonines resulted in much slower electrical signals that ended at random times. This was due to rhodopsin being less able to bind to arrestin.

Azevedo et al. propose a new model for how rhodopsin is deactivated. Once light activates the protein, phosphate groups are rapidly added to the serines, which begins to lower the activity of rhodopsin. However, it is the slower addition of phosphates to the threonines that is essential to promote arrestin binding and fully deactivate the protein. Other proteins belonging to the G protein-coupled receptor family also have these serines and threonines, and thus, may be regulated in a similar way.

DOI:http://dx.doi.org/10.7554/eLife.05981.002