Light-activated GTPase in vertebrate photoreceptors

The dynamic landscape of chromatin accessibility and active regulatory elements in the mediobasal hypothalamus influences the seasonal activation of the reproductive axis in the male quail under long light exposure

BACKGROUND

In cold and temperate zones, seasonal reproduction plays a crucial role in the survival and reproductive success of species. The photoperiod influences reproductive processes in seasonal breeders through the hypothalamic-pituitary-gonadal (HPG) axis, in which the mediobasal hypothalamus (MBH) serves as the central region responsible for transmitting light information to the endocrine system. However, the cis-regulatory elements and the transcriptional activation mechanisms related to seasonal activation of the reproductive axis in MBH remain largely unclear. In this study, an artificial photoperiod program was used to induce the HPG axis activation in male quails, and we compared changes in chromatin accessibility changes during the seasonal activation of the HPG axis.

RESULTS

Alterations in chromatin accessibility occurred in the mediobasal hypothalamus (MBH) and stabilized at LD7 during the activation of the HPG axis. Most open chromatin regions (OCRs) are enriched mainly in introns and distal intergenic regions. The differentially accessible regions (DARs) showed enrichment of binding motifs of the RFX, NKX, and MEF family of transcription factors that gained-loss accessibility under long-day conditions, while the binding motifs of the nuclear receptor (NR) superfamily and BZIP family gained-open accessibility. Retinoic acid signaling and GTPase-mediated signal transduction are involved in adaptation to long days and maintenance of the HPG axis activation. According to our footprint analysis, three clock-output genes (TEF, DBP, and HLF) and the THRA were the first responders to long days in LD3. THRB, NR3C2, AR, and NR3C1 are the key players associated with the initiation and maintenance of the activation of the HPG axis, which appeared at LD7 and tended to be stable under long-day conditions. By integrating chromatin and the transcriptome, three genes (DIO2, SLC16A2, and PDE6H) involved in thyroid hormone signaling showed differential chromatin accessibility and expression levels during the seasonal activation of the HPG axis. TRPA1, a target of THRB identified by DAP-seq, was sensitive to photoactivation and exhibited differential expression levels between short- and long-day conditions.

CONCLUSION

Our data suggest that trans effects were the main factors affecting gene expression during the seasonal activation of the HPG axis. This study could lead to further research on the seasonal reproductive behavior of birds, particularly the role of MBH in controlling seasonal reproductive behavior.

Druggable Lipid GPCRs: Past, Present, and Prospects

G protein-coupled receptors (GPCRs) have seven transmembrane spanning domains and comprise the largest superfamily with ~800 receptors in humans. GPCRs are attractive targets for drug discovery because they transduce intracellular signaling in response to endogenous ligands via heterotrimeric G proteins or arrestins, resulting in a wide variety of physiological and pathophysiological responses. The endogenous ligands for GPCRs are highly chemically diverse and include ions, biogenic amines, nucleotides, peptides, and lipids. In this review, we follow the KonMari method to better understand druggable lipid GPCRs. First, we have a comprehensive tidying up of lipid GPCRs including receptors for prostanoids, leukotrienes, specialized pro-resolving mediators (SPMs), lysophospholipids, sphingosine 1-phosphate (S1P), cannabinoids, platelet-activating factor (PAF), free fatty acids (FFAs), and sterols. This tidying up consolidates 46 lipid GPCRs and declutters several perplexing lipid GPCRs. Then, we further tidy up the lipid GPCR-directed drugs from the literature and databases, which identified 24 clinical drugs targeting 16 unique lipid GPCRs available in the market and 44 drugs under evaluation in more than 100 clinical trials as of 2019. Finally, we introduce drug designs for GPCRs that spark joy, such as positive or negative allosteric modulators (PAM or NAM), biased agonism, functional antagonism like fingolimod, and monoclonal antibodies (MAbs). These strategic drug designs may increase the efficacy and specificity of drugs and reduce side effects. Technological advances will help to discover more endogenous lipid ligands from the vast number of remaining orphan GPCRs and will also lead to the development novel lipid GPCR drugs to treat various diseases.

The discovery of signal transduction by G proteins: a personal account and an overview of the initial findings and contributions that led to our present understanding

The realization that there existed a G-protein coupled signal transduction mechanism developed gradually and was initially the result of an ill fated quest for uncovering the mechanism of action of insulin, followed by a refocused research in many laboratories, including mine, on how GTP acted to increase hormonal stimulation of adenylyl cyclase. Independent research into how light-activated rhodopsin triggers a response in photoreceptor cells of the retina and the attendant biochemical studies joined midway and, without the left hand knowing well what the right hand was doing, preceded classical G protein research in identifying the molecular players responsible for signal transduction by G proteins.

Rethinking the role of phosducin: light-regulated binding of phosducin to 14-3-3 in rod inner segments

Phosducin (Pd), a small protein found abundantly in photoreceptors, is widely assumed to regulate light sensitivity in the rod outer segment through interaction with the heterotrimeric G protein transducin. But, based on histochemistry and Western blot analysis, Pd is found almost entirely in the inner segment in both light and dark, most abundantly near the rod synapse. We report a second small protein, 14-3-3, in the rod with a similar distribution. By immunoprecipitation, phospho-Pd is found to interact with 14-3-3 in material from dark-adapted retina, and this interaction is markedly diminished by light, which dephosphorylates Pd. Conversely, unphosphorylated Pd binds to inner segment G protein(s) in the light. From these results and reported functions of 14-3-3, we have constructed a hypothesis for the regulation of light sensitivity at the level of rod synapse. By dissociating the Pd/14-3-3 complex, light enables both proteins to function in this role.

Subunit stoichiometry of retinal rod cGMP phosphodiesterase

The cyclic GMP phosphodiesterase of the retinal rod is composed of three distinct types of polypeptides: alpha (90 kDa), beta (86 kDa), and gamma (10 kDa). The gamma subunit has been shown to inhibit phosphodiesterase activity associated with alpha and beta. To investigate the subunit stoichiometry of the retinal phosphodiesterase, we have developed a panel of monoclonal and peptide antibodies that recognize individual phosphodiesterase subunits. By quantitative and immunochemical analysis of the purified subunits, we have shown that each phosphodiesterase molecule contains one copy each of alpha and beta subunit and two copies of gamma subunit. Moreover, gamma can be chemically cross-linked to both alpha and beta, but not to itself, suggesting that alpha and beta may each bind one gamma. The phosphodiesterase is fully activated when both copies of gamma were removed by proteolysis with trypsin. Upon recombination of the purified gamma subunit with the trypsin-activated phosphodiesterase containing alpha beta, the alpha beta gamma 2 stoichiometry is once again restored, with concomitant total inhibition of activity. Our results suggest that at least two activated transducin molecules are required to fully activate one molecule of phosphodiesterase in retinal rods.

Contribution of the guanosinetriphosphatase activity of G-protein to termination of light-activated guanosine cyclic 3',5'-phosphate hydrolysis in retinal rod outer segments

Light activation of GTP binding to G-protein and its eventual hydrolysis are hypothesized to lead to activation and inactivation of cGMP phosphodiesterase (PDE) in vertebrate rod disk membranes (RDM). However, the reported GTPase rate of 3 per minute is too slow to account for the observed rapid inactivation of PDE. Our investigations on GTPase activity showed that RDM isolated in the dark have considerable dark GTPase activity, which is enhanced by light. In dark and light, the enzyme exhibits biphasic substrate dependence with two Km's for GTP of 2-3 and 40-80 microM at 22 degrees C and less than 1 and 10-25 microM at 37 degrees C. The Km's were not influenced by light. On the basis of G-protein content of the RDM, the Vmax's for the two activities at 37 degrees C in light are 4-5 and 20-30 GTPs hydrolyzed per minute per G-protein. RDM washed free of soluble and peripheral proteins do not have measurable GTPase activity in the dark or light. Purified G-protein alone also did not turn over GTP, apparently because bleached rhodopsin is required for it to bind GTP. Reconstitution of washed membranes with purified G-protein restores both the low- and high-Km GTPase activities. Inactivation of G-protein as measured by PDE turnoff and dissociation signal recovery is found to be faster at higher than lower [GTP], consistent with the observation that the higher GTPase activity associated with the higher Km alos resides in the G-protein.(ABSTRACT TRUNCATED AT 250 WORDS)

GTP binding proteins: a key role in cellular communication

One of the major steps in the understanding of the hormonal and sensory transduction mechanisms in eukaryotic cells has been the discovery of a family of GTP binding proteins which couple receptors to specific cellular effectors. The absolute requirement of GTP for hormonal stimulation of adenylate cyclase was the initial observation which led to the purification of the protein involved: Gs. Gs couples stimulatory receptors to adenylate cyclase. It is a heterotrimer composed of an alpha chain (45 or 52 kDa), a beta chain (35-36 kDa) and a gamma chain (8 kDa). Several other G proteins of known functions have been purified: Gi, which couples inhibitory receptors to adenylate cyclase, and transducin which couples photoexcited rhodopsin to cyclic GMP phosphodiesterase. Some G proteins of uncertain function have also been purified: Go, a G protein mainly localized in nervous tissues and Gp, a G protein isolated from placenta and platelets. All these G proteins have a common design. Like Gs they all consist of 3 chains: alpha, beta and gamma. The beta chains are nearly identical, whereas the gamma chains are more variable. The alpha chains are different, but share common domains (especially at the level of the GTP binding site). These domains of homologies are also similar to those of other GTP binding proteins, such as the product of the ras gene (p21) and the initiation or elongation factors. alpha Chains are also ADP ribosylated by bacterial toxins. Gs and transducin are targets for cholera toxin, whereas Gi, Go and transducin are targets for pertussis toxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Signal transduction by guanine nucleotide binding proteins

High affinity binding of guanine nucleotides and the ability to hydrolyze bound GTP to GDP are characteristics of an extended family of intracellular proteins. Subsets of this family include cytosolic initiation and elongation factors involved in protein synthesis, and cytoskeletal proteins such as tubulin (Hughes, S.M. (1983) FEBS Lett. 164, 1-8). A distinct subset of guanine nucleotide binding proteins is membrane-associated; members of this subset include the ras gene products (Ellis, R.W. et al. (1981) Nature 292, 506-511) and the heterotrimeric G-proteins (also termed N-proteins) (Gilman, A.G. (1984) Cell 36, 577-579). Substantial evidence indicates that G-proteins act as signal transducers by coupling receptors (R) to effectors (E). A similar function has been suggested but not proven for the ras gene products. Known G-proteins include Gs and Gi, the G-proteins associated with stimulation and inhibition, respectively, of adenylate cyclase; transducin (TD), the G-protein coupling rhodopsin to cGMP phosphodiesterase in rod photoreceptors (Bitensky, M.W. et al. (1981) Curr. Top. Membr. Transp. 15, 237-271; Stryer, L. (1986) Annu. Rev. Neurosci. 9, 87-119), and Go, a G-protein of unknown function that is highly abundant in brain (Sternweis, P.C. and Robishaw, J.D. (1984) J. Biol. Chem. 259, 13806-13813; Neer, E.J. et al. (1984) J. Biol. Chem. 259, 14222-14229). G-proteins also participate in other signal transduction pathways, notably that involving phosphoinositide breakdown. In this review, I highlight recent progress in our understanding of the structure, function, and diversity of G-proteins.

cGMP- and phosphodiesterase-dependent light-scattering changes in rod disk membrane vesicles: relationship to disk vesicle-disk vesicle aggregation

Visible light activates a large guanosine cyclic 3',5'-phosphate (cGMP)- and phosphodiesterase (PDE)-dependent infrared light-scattering change in suspensions of photoreceptor disk membranes. Reconstitution experiments show that this signal requires bleached rhodopsin, G protein (three polypeptide subunits of Mr 39 000, 37 000, and 6000 which comprise the GTPase), phosphodiesterase, cGMP, and GTP. The lowest light intensity which elicits the light-scattering signal bleaches 0.002% rhodopsin. cGMP and GTP hydrolysis occurs more slowly than the initial phase of the scattering signal, and the kinetics of nucleotide hydrolysis do not correlate with any phase of the signal. Hydrolysis-resistant analogues of cGMP and GTP support the initial decreasing phase of the signal. Thus, the signal apparently depends upon nucleotide binding rather than hydrolysis. Microscopic observations made under the same conditions as light-scattering experiments show that vesicle-vesicle aggregation and disaggregation occur. The data suggest that light and nucleotide activations of the cyclic nucleotide cascade enzymes are responsible for the vesicle aggregation process and nucleotide hydrolysis for vesicle disaggregation. The vesicle aggregation-disaggregation phenomenon appears likely to be the physical basis of the cGMP- and PDE-dependent changes in infrared transmission.

A light-stimulated increase of cyclic GMP in squid photoreceptors

Photoreceptor outer segments isolated from squid retina are known to contain a light-activated GTP-binding protein. Here it is shown that these photoreceptors contain around 0.01 mol cyclic GMP per mol rhodopsin. Adding GTP in the dark stimulates the production of 0.0003-0.001 mol cyclic GMP/mol rhodopsin per min. GTP and light cause a 2-fold faster increase in cyclic GMP. These results show that either (1) squid rhodopsin activates a guanylate cyclase, or (2) there is a constant guanylate cyclase activity and photoexcited rhodopsin inhibits a cyclic GMP phosphodiesterase.

Sulfhydryl group modification of photoreceptor G-protein prevents its light-induced binding to rhodopsin

The effect of sulfhydryl modification on the light-induced interaction between rhodopsin and the peripheral GTP-binding protein of the photoreceptor membrane (G-protein) has been investigated by time-resolved near-infrared light-scattering and polyacrylamide gel electrophoresis. It has been found that the modification of rhodopsin with the alkylating agent N-ethylmaleimide (NEM) does not affect its light-induced interaction with the G-protein. Modification of G-protein with NEM or other sulfhydryl agents prevents any light-induced binding to rhodopsin. Dark-association of G to the membrane as well as the light-induced complex with rhodopsin (once formed) is insensitive to NEM.

Light enhances the turnover of phosphatidylinositol in rat retinas

Light stimulation of isolated rat retinas is shown to enhance the turnover of phosphatidylinositol (PI) as demonstrated by a light-dependent increase in [3H]inositol incorporation and concurrent hydrolysis of existing PI. Studies with rat retinas incubated with [3H]inositol and then microdissected at the level of the outer plexiform layer into photoreceptor cell and inner retina layers indicated that the light-enhanced incorporation of [3H]inositol was associated with the photoreceptor cell layer. The rate of PI hydrolysis in retinas prelabeled in vivo with [3H]inositol was higher in light than in dark incubations and was higher in the photoreceptor cell layer than within the inner retina. Within the photoreceptor cell layer. PI turnover involved 2%/min of the total PI content in dark and 6-8%/min in light. In contrast to what has been reported for stimulus-enhanced turnover of PI in some tissues, this light-enhanced turnover of PI in the retina was not associated with detectable reductions in PI content. Parallel studies of sodium (22Na) uptake demonstrated that the photoreceptor cells remained functional during these incubations as they retained the capacity to restrict the entry of 22Na in light but not in dark.

Size changes of phosphodiesterase in bovine rod outer segments on illumination

Light activates a 3',5'-cyclic GMP phosphodiesterase (PDE) in bovine retinal rod outer segments. The light is absorbed by rhodopsin situated in the disk membranes. PDE is a three-subunit peripheral protein on the disks and appears to be activated via a guanine nucleotide binding protein (G) in the presence of activated rhodopsin and GTP. Does the activation occur by collision coupling of G and PDE? We have studied the protein-protein interactions of PDE in situ in disk membranes by radiation inactivation. Irradiation of a protein with high-energy electrons leads to loss of activity in proportion to radiation dose and the molecular weight of the protein. We see no change in the size of PDE upon activation by light and 100 microM guanosine 5'-(beta, gamma-imidotriphosphate) (Gpp[NH]p) compared with PDE in dark with 260 microM GTP. Application of statistics to our data shows that a 27 000 change in molecular weight would be significant at the 95% level but that smaller changes would go undetected. The apparent molecular weight is 176 000 +/- 27 000 (mean +/- 95% confidence limit), in agreement with the size determined by polyacrylamide gel electrophoresis. Thus there appears to be either (i) no permanent change in PDE size on activation or (ii) a small change, undetectable by the technique, or (iii) an exchange of subunits such that no net change in molecular weight is seen.

The chemistry of vision

The visual response is initiated by light reception and transduction into chemical and electrical energy in the outer-segment membranes of rod and cone cells. Recent research on the molecular events controlled by light has clarified the roles of some of the rod outer-segment biomolecules. These developments and the current unresolved questions are described.

Reciprocal effects of an inhibitory factor on catalytic activity and noncatalytic cGMP binding sites of rod phosphodiesterase

In illuminated rod outer segment membranes, GTP and guanosine 5'-[beta, gamma-imido]triphosphate (p[NH]ppG) have reciprocal effects on cGMP phosphodiesterase (PDEase; 3':5'-cyclic-nucleotide 5'-nucleotidohydrolase, EC 3.1.4.17) activity and cGMP binding to noncatalytic sites on that enzyme. Two micromolar p[NH]ppG increased PDEase activity more than 2-fold while inhibiting cGMP binding more than 40%. Reduction of noncatalytic cGMP binding, which followed addition of p[NH]ppG, was not a result of PDEase activation. Both effects of p[NH]ppG were completely dependent on the presence of bleached rhodopsin. A heat-stable factor has been found to inhibit PDEase activity and also to stimulate cGMP binding to noncatalytic cGMP binding sites. Addition of p[NH]ppG reversed the effects of this factor on both PDEase activity and cGMP binding. During purification of this material, the activity peaks for both PDEase inhibition and activation of noncatalytic cGMP binding comigrated on both Blue Sepharose CL-6B column chromatography and sucrose density gradients centrifugation, suggesting that the same factor could be responsible for both inhibition of PDEase activity and enhancement of noncatalytic cGMP binding. Limited tryptic proteolysis of PDEase, which markedly reduced cGMP binding to the noncatalytic sites, and experiments using highly purified cAMP (free of cGMP) as substrate for PDEase showed that the binding of cGMP to noncatalytic sites was not required for the heat-stable inhibitory factor to inhibit PDEase activity. We discuss possible relationships between the regulation of PDEase and the binding of cGMP to noncatalytic sites.

Purification and characteristics of photoreceptor light-activated guanosinetriphosphatase

We describe a reconstitution of light-activated vertebrate photoreceptor GTPase and a purification of the GTP-binding protein (G protein), which is a component of the GTPase and modulates the light-activated phosphodiesterase (PDE) enzyme system. Rod outer segments (ROS) of bull frogs were treated with ethylenediaminetetraacetic acid (EDTA), and the GTPase and PDE fractions were solubilized (EDTA supernatant). When the EDTA supernatant and EDTA-treated membrane fraction (EDTA-washed membranes) were recombined, light-dependent GTPase activity appeared. In the reconstituted system, the Km for GTP as substrate was 0.5 microM; the optimum pH was 7.5-8.0. The isoelectric point of GTPase in EDTA supernatant was 4.8. G protein was purified 400-fold from ROS, and the molecular weight of G protein was determined to be 40 000 by polyacrylamide gel electrophoresis. The amount of G protein in ROS was calculated as at least 1 molecule per 400 rhodopsin molecules. By recombining (in the presence or absence of GTP) purified G protein, PDE, H fraction (an additional component of GTPase), and illuminated or unilluminated EDTA-washed membranes (as a source of rhodopsin), we showed that illuminated rhodopsin, G protein, PDE, and GTP are the minimum requirements for light-dependent PDE activity. We discuss the significance of these findings in the regulation of the light-activated GTPase and PDE activities, especially with regard to the mechanism of activation.

Isolation of an inhibitory protein for the cyclic guanosine 3','5'-monophosphate phosphodiesterase of bovine rod outer segments

Cyclic guanosine 3',5'-monophosphate phosphodiesterase in crude extracts from bovine rod outer segments can be activated by the addition of bleached rod outer segment membranes and GTP. In the absence of rhodopsin-containing membranes, the phosphodiesterase specific activity decreases with increasing concentration. A trypsin-sensitive inhibitor believed to be responsible for this phenomenon can be separated from the phosphodiesterase by DEAE-cellulose chromatography of the crude extract. Phosphodiesterase eluted from the DEAE-cellulose column shows considerably less concentration-dependence than in the crude extract. This partially purified phosphodiesterase was used as the substrate to assay for inhibitor. A GTPase which is active only in the presence of bleached rod outer segment membranes coelutes with the phosphodiesterase and is distinct from the phosphodiesterase inhibitor we have isolated.

Molecular mechanisms of photoreception. IV. Ca2+-inhibited GTPase of rod outer segments of the frog retina

Ca2+-dependent GTPase activity is found to be present in the rod outer segments of frog retina. GTPase localization in rod outer segments is shown by fractionating the rod outer segment preparation in the sucrose density gradient. The enzyme is readily washed out of cells with isotonic NaCl solution. The Km is 0.6 mM for GTP. The activity is inhibited by 78 +/- 12% with the increase in Ca2+ concentration from 10(-9) to 10(-7) M. GTP hydrolysis is inhibited by the same concentrations of Ca2+ which block the sodium conductivity of the rod outer segment cytoplasmic membrane.

Light and GTP effects on the turbidity of frog visual membrane suspensions

The time course of turbidity changes of frog visual membranes, dependent on osmotic shocks, on light and on nucleotide substrates or effectors of enzyme activities, were measured as absorption changes in a rapid mixing stopped-flow spectrophotometer. As a result of studies on different preparations, it is concluded that light can cause both rapid (within 50 msec) and slow (within 90 sec) changes in the turbidity of visual membranes, not associated with permeability changes, and that they are affected by GTP or its analog guanyl-5'-yl imidodiphosphate; however, the light and GTP effects are lost when a water soluble fraction containing the light-sensitive enzyme cGMP-phosphodiesterase, is removed from the rod outer segments membranes. It is suggested that the fast light and GTP-sensitive response is related to the activation of cGMP-phosphodiesterase.

Additional component required for activity and reconstitution of light-activated vertebrate photoreceptor GTPase

A light-activated GTPase that functions as a component of the rhodopsin-linked, light-activated phosphodiesterase (PDEase) system in vertebrate photoreceptors has been reported. In our efforts to purify photoreceptor GTPase we encountered another component (which we call "helper" or "H" component) whose presence is required for expression of light-activated GTPase activity. We report here the characterization of this heat-labile, macromolecular factor and that the presence of helper is absolutely required for light- and rhodopsin-dependent activation of photoreceptor GTPase. Of equal importance, we find that the "G" component (which requires the presence of H for expression of GTPase activity) can bind GTP and can support light- and GTP-dependent PDEase activation in the absence of H component. These data support a model in which GTP binding to G component is a necessary condition for PDEase activation. Hydrolysis of GTP at the G activator locus (an H-dependent activity) is a regulatory event which reverses PDEase activation. The complexity of this regulatory mechanism provides opportunities for signal modulation and amplification.

Phosphodiesterase and GTPase in rod outer segments. Kinetics in vitro

The hydrolysis of cyclic guanosine monophosphate (cyclic GMP) and of guanosine triphosphate (GTP) by the broken rods of the frog retina after a flash of light have been studied in vitro with a constant perfusion method. The activation has an onset apparently instantaneous as observed with the existing possible time resolution of 3 s. The activation is followed by a partial inactivation that does not bring the activity back to the pre-flash level. GTP or the non-hydrolysable guanyl-5'-ylimidodiphosphate (GMP-PNP) is required for the normal light-activation of the phosphodiesterase and in its absence both the speed of activation and the sensitivity are greatly reduced. The activation speed, the sensitivity (threshold at approx. 0.00004% bleaching), and the kinetic constants do not exclude a direct role in the process of excitation for the phosphodiesterase and suggest a subsidiary but as yet undefined role for the GTPase.

Predictive value of the analogy between hormone-sensitive adenylate cyclase and light-sensitive photoreceptor cyclic GMP phosphodiesterase: a specific role for a light-sensitive GTPase as a component in the activation sequence

We report experiments which involve a light sensitive GTPase in the light dependent activation of retinal rod 3'5'-cyclic guanosine monophosphate (cGMP) phosphodiesterase (PDE). The data suggest that the light activated GTPase is intermediate between rhodopsin and PDE in the light-dependent activation sequence. We list the many striking similarities between hormone sensitive adenylate cyclase and light activated PDE in order to emphasize that the findings presented herein may have predictive value for ongoing studies of the hormone sensitive adenylate cyclase specifically regarding the role of the hormone activated GTPase in the activation sequence.