Translation of in vitro cannabinoid 1 receptor agonist activity to in vivo pharmacodynamic endpoints

INTRODUCTION

Building an understanding of in vivo efficacy based on the evaluation of in vitro affinity or potency is critical in expediting early decision making in drug discovery and can significantly reduce the need for animal studies. The aim of the present study was to understand the translation of in vitro to in vivo endpoints for the cannabinoid receptor 1 (CB1).

METHODS

Using a selection of CB1 agonists we describe an evaluation of in vitro to in vivo translation comparing in vitro receptor affinity or functional potency, using both cAMP and β-arrestin endpoints, to various in vivo CB1 agonist-associated endpoints.

RESULTS

We demonstrate that in vitro CB1 agonism significantly correlates with the CB1-induced cue in the drug discrimination model in vivo, but not with other purported CB1 agonist-mediated in vivo endpoints, including hypothermia and sedation. Thus, these data challenge common perceptions regarding CB1 agonist-induced tetrad effects in rodents.

DISCUSSION

This work exemplifies how in vitro profiling of receptor affinity or potency can predict in vivo pharmacodynamic effects, using the CB1 as an example system. The translatability of in vitro activity to in vivo efficacy allows for the ability to rapidly contextualize off-target CB1 in vitro findings, allowing clear and rapid definition of the risk posed by such activity without the need for extensive animal studies. This has significant implications in terms of early decision making in drug discovery and reducing the use of animals in research, while also outlining a template for expanding the approach for additional targets.

Incorporation of in vitro techniques for botanicals dietary supplement safety assessment - Towards evaluation of developmental and reproductive toxicity (DART)

As complex mixtures, botanicals present unique challenges when assessing safe use, particularly when endpoint gaps exist that cannot be fully resolved by existing toxicological literature. Here we explore in vitro gene expression as well receptor binding and enzyme activity as alternative assays to inform on developmental and reproductive toxicity (DART) relevant modes of action, since DART data gaps are common for botanicals. Specifically, botanicals suspected to have DART effects, in addition to those with a significant history of use, were tested in these assays. Gene expression changes in a number of different cell types were analysed using the connectivity mapping approach (CMap) to identify modes of action through a functional read across approach. Taken together with ligand affinity data obtained using a set of molecular targets customised towards known DART relevant modes of action, it was possible to inform DART risk using functional analogues, potency comparisons and a margin of internal exposure approach.

Dissociation of GDP dissociation inhibitor and membrane translocation are required for efficient activation of Rac by the Dbl homology-pleckstrin homology region of Tiam

Small G proteins of the Rho/Rac/Cdc42 family are associated with lipid membranes through their prenylated C termini. Alternatively, these proteins form soluble complexes with GDI proteins. To assess how this membrane partitioning influences the activation of Rac by guanine nucleotide exchange factors, GDP-to-GTP exchange reactions were performed in the presence of liposomes using different forms of Rac-GDP. We show that both non-prenylated Rac-GDP and the soluble complex between prenylated Rac-GDP and GDI are poorly activated by the Dbl homology-pleckstrin homology (DH-PH) domain of the exchange factor Tiam1, whereas prenylated Rac-GDP bound to liposomes is activated about 10 times more rapidly. Sedimentation experiments with liposomes reveal that the DH-PH region of Tiam1 forms, with nucleotide-free prenylated Rac, a membrane-bound complex from which GDI is excluded. Taken together, these experiments demonstrate that the dissociation of Rac-GDP from GDI and its translocation to membrane lipids favor DH-PH-catalyzed nucleotide exchange because the steric hindrance caused by GDI is relieved and because the membrane environment favors functional interaction between the DH-PH domain and the small G protein.

Calcium-sensitive particulate guanylyl cyclase as a modulator of cAMP in olfactory receptor neurons

The second messengers cAMP and inositol-1,4,5-triphosphate have been implicated in olfaction in various species. The odorant-induced cGMP response was investigated using cilia preparations and olfactory primary cultures. Odorants cause a delayed and sustained elevation of cGMP. A component of this cGMP response is attributable to the activation of one of two kinetically distinct cilial receptor guanylyl cyclases by calcium and a guanylyl cyclase-activating protein (GCAP). cGMP thus formed serves to augment the cAMP signal in a cGMP-dependent protein kinase (PKG) manner by direct activation of adenylate cyclase. cAMP, in turn, activates cAMP-dependent protein kinase (PKA) to negatively regulate guanylyl cyclase, limiting the cGMP signal. These data demonstrate the existence of a regulatory loop in which cGMP can augment a cAMP signal, and in turn cAMP negatively regulates cGMP production via PKA. Thus, a small, localized, odorant-induced cAMP response may be amplified to modulate downstream transduction enzymes or transcriptional events.

Guanylate-cyclase-inhibitory protein is a frog retinal Ca2+-binding protein related to mammalian guanylate-cyclase-activating proteins

Two guanylate-cyclase-activating proteins (GCAP) encoded by a tail-to-tail gene array have been characterized in the mammalian retina. Using frog retina as a model, we obtained evidence for the presence of a photoreceptor Ca2+-binding protein closely related to GCAP. This protein (206 amino acids) does not stimulate guanylate cyclase (GC) in low [Ca2+], but inhibits GC in high [Ca2+], and is therefore termed guanylate-cyclase-inhibitory protein (GCIP). Sequence analysis indicates that GCIP and GCAP1 and GCAP2 have diverged substantially, but conserved domains present in all vertebrate GCAP are present in GCIP. Moreover, partial characterization of the GCIP gene showed that the positions of two introns in the GCIP gene are identical to positions of corresponding introns of the mammalian GCAP gene array. As to the major differences between GCIP and GCAP, the fourth EF hand Ca2+-binding motif of GCIP is disabled for Ca2+ binding, and GCIP does not stimulate GC. Monoclonal and polyclonal antibodies raised against recombinant GCIP identified high levels of GCIP in the inner segments, somata and synaptic terminals of frog cone photoreceptors. The results suggest that GCIP is a Ca2+-binding protein of the GCAP/recoverin subfamily. Its localization in frog cones closely resembles that of GC in mammalian cones. GCIP inhibits GC at high free [Ca2+], competing with GCAP1 and GCAP2 for GC regulatory sites.

Localization of guanylate cyclase-activating protein 2 in mammalian retinas

Guanylate cyclase-activating proteins (GCAP1 and GCAP2) are thought to mediate the intracellular stimulation of guanylate cyclase (GC) by Ca2+, a key event in recovery of the dark state of rod photoreceptors after exposure to light. GCAP1 has been localized to rod and cone outer segments, the sites of phototransduction, and to photoreceptor synaptic terminals and some cone somata. We used in situ hybridization and immunocytochemistry to localize GCAP2 in human, monkey, and bovine retinas. In human and monkey retinas, the most intense immunolabeling with anti-GCAP2 antibodies was in the cone inner segments, somata, and synaptic terminals and, to a lesser degree, in rod inner segments and inner retinal neurons. In bovine retina, the most intense immunolabeling was in the rod inner segments, with weaker labeling of cone myoids, somata, and synapses. By using a GCAP2-specific antibody in enzymatic assays, we confirmed that GCAP1 but not GCAP2 is the major component that stimulates GC in bovine rod outer segment homogenates. These results suggest that although GCAP1 is involved in the Ca2+-sensitive regulation of GC in rod and cone outer segments, GCAP2 may have non-phototransduction functions in photoreceptors and inner retinal neurons.

Functional reconstitution of photoreceptor guanylate cyclase with native and mutant forms of guanylate cyclase-activating protein 1

In rod and cone photoreceptor cells, activation of particulate guanylate cyclase (retGC1) is mediated by a Ca2+-binding protein termed GCAP1, that detects changes in [Ca2+]free. In this study, we show that N-acylated GCAP1 restored Ca2+ sensitivity of native and recombinant photoreceptor retGC1. ATP increased the affinity of retGC1 for GCAP1 and accelerated catalysis. Using peptides derived from the GCAP1 sequence, we found that at least three regions, encompassing the N-terminus, the EF-1 motif, and the EF-3 motif, were likely involved in the interaction with retGC1. Mutation of 2Gly to Ala (GCAP1-G2A), which abolished myristoylation and a 25 amino acid truncation at the N-terminus (delta25-GCAP1) reduced retGC1-stimulating activity dramatically, while deletion of 10 amino acids (delta10-GCAP1) reduced the specific activity by only approximately 60% and modified the Ca2+ sensitivity. At 10(-6) M [Ca2+]free, in conditions that inactivated native GCAP1, retGC1 showed significant activity in the presence of delta10-GCAP1. Native and all three mutant forms of GCAP1 had similar affinities for Ca2+ as demonstrated by gel filtration and the changes in tryptophan fluorescence. All mutants bound to ROS membranes in a Ca2+-independent manner, except delta25-GCAP1, which was mostly soluble. These findings suggest that the N-terminal region is important in tethering of GCAP1 to the ROS membranes.

Modulation of the GTPase activity of transducin. Kinetic studies of reconstituted systems

We seek to define the influence of retinal cGMP phosphodiesterase (PDE) on the GTPase activity of transducin (T). A novel stopped-flow/fast filtration apparatus [Antonny, B., et al. (1993) Biochemistry 32, 8646-8653] is used to deliver T alpha GTP free of rod outer segment (ROS) membranes to a suspension of phospholipid vesicles bearing holoPDE. As measured by a pH electrode, the decay of cGMP hydrolysis from these samples, which contain no other proteins but T alpha and holoPDE, requires GTP hydrolysis and occurs in 40 s. The addition of T beta gamma to the vesicles does not accelerate this deactivation. When ROS membranes are urea-stripped, reconstituted with transducin + holoPDE, and illuminated, the injection of an amount of GTP that is substoichiometric to holoPDE gives a cGMP hydrolysis pulse that lasts for 30 s. However, the same reconstitution performed with ROS stripped by extensive dilution in isotonic buffer results in a deactivation time of only 8 s, which resembles the 7 s observed with native ROSs. With these isotonically stripped ROSs, when GTP injection comes after a first injection with GTP gamma S, the cGMP hydrolysis pulse is lengthened and lasts for 17 s; with urea-washed ROS, no such lengthening is observed. These results clearly demonstrate that holoPDE by itself cannot enhance the GTPase activity of transducin, even when the two proteins are localized on a membrane surface. Instead, they point to the existence of a membrane-bound, urea-sensitive protein factor that activates the GTPase of T alpha in the transducin-holoPDE complex.

GTP-dependent binding of Gi, G(o) and Gs to the gamma-subunit of the effector of Gt

The gamma-subunit of the cGMP-phosphodiesterase (PDE gamma) of retinal rods forms a tight complex with the activated alpha-subunit of transducin (Gt alpha GTP gamma S). We observe that while PDE gamma is not the physiological effector of other G alpha subtypes, it can still detectably interact with them. This interaction is strong with Gi1 alpha and Gi3 alpha (Kd approximately 10 nM) and weaker with Go alpha and Gs alpha (Kd approximately 1 microM). For all these G alpha subtypes, similar intrinsic fluorescence changes are observed upon PDE gamma binding. Moreover, similar relative decreases in affinity are obtained when the GDP forms of Gi1 alpha, Gi3 alpha or Gt alpha are used in lieu of the GTP forms. This points to a conserved GTP-dependent effector-interaction domain.

Tryptophan W207 in transducin T alpha is the fluorescence sensor of the G protein activation switch and is involved in the effector binding

We have produced a recombinant transducin alpha subunit (rT alpha) in sf9 cells, using a baculovirus system. Deletion of the myristoylation site near the N-terminal increased the solubility and allowed the purification of rT alpha. When reconstituted with excess T beta gamma on retinal membrane, rT alpha displayed functional characteristics of wild-type T alpha vis à vis its coupled receptor, rhodopsin and its effector, cGMP phosphodiesterase (PDE). We further mutated a tryptophan, W207, which is conserved in all G proteins and is suspected to elicit the fluorescence change correlated to their activation upon GDP/GTP exchange or aluminofluoride (AlFx) binding. [W207F]T alpha mutant displayed high affinity receptor binding and underwent a conformational switch upon receptor-catalysed GTP gamma S binding or upon AlFx binding, but this did not elicit any fluorescence change. Thus W207 is the only fluorescence sensor of the switch. Upon the switch the mutant remained unable to activate the PDE. To characterize better its effector-activating interaction we measured the affinity of [W207F]T alpha GDP-AlFx for PDE gamma, the effector subunit that binds most tightly to T alpha. [W207F]T alpha still bound in an activation-dependent way to PDE gamma, but with a 100-fold lower affinity than rT alpha. This suggests that W207 contributes to the G protein effector binding.

GTP hydrolysis by purified alpha-subunit of transducin and its complex with the cyclic GMP phosphodiesterase inhibitor

The single-turn GTP hydrolysis by isolated and soluble transducin has been time-resolved using a rapid flow filtration technique which takes advantage of the GTP-requiring detachment of transducin alpha-subunits (T alpha) from photoactivated rhodopsin (R*). Illuminated rod outer segment (ROS) fragments to which holo-transducin is tightly bound are retained on a syringe filter that is washed continuously with a buffer containing no GTP. When the flow is switched to a buffer with GTP, T alpha GTP is specifically eluted and injected into a cuvette where GTP hydrolysis is monitored via the associated change in the T alpha intrinsic tryptophan fluorescence. Low concentrations of GTP elute the complete pool of T alpha from the filter-retained ROS fragments in less than 1 s. This directly demonstrates that, upon GTP loading, T alpha becomes instantly soluble in physiological buffers (120 mM KC1 and 2 mM MgCl2). When all alone, T alpha hydrolyzes its bound GTP in 21 +/- 1 s (1/e time at 25 degrees C). Replacing chloride by other anions increases the GTPase rate by 2-fold. The K50 for chloride inhibition of GTPase is approximately 2 mM. Slower GTP hydrolysis is observed for cholera-toxin-modified transducin or when GTP alpha S (Sp) replaces GTP in the eluting buffer. No signal is observed when GTP gamma S is used. The GTPase rate is unaffected when T alpha GTP binds to the inhibitory subunit (PDE gamma) of the cGMP phosphodiesterase (PDE), although this binding is fast and of high affinity.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between the retinal cyclic GMP phosphodiesterase inhibitor and transducin. Kinetics and affinity studies

In the retinal cyclic GMP phosphodiesterase (PDE), catalysis by the alpha beta-heterodimer is inhibited in the dark by two identical gamma-subunits and stimulated in the light by the GTP-bearing alpha-subunit of the heterotrimeric G-protein transducin (T beta gamma-T alpha GDP). Two T alpha GTP molecules, dissociated from T beta gamma, bind to and displace the PDE gamma subunits from their inhibitory sites on PDE alpha beta. With GTP gamma S in lieu of GTP, this association becomes persistent. Under physiological conditions, the PDE alpha beta (gamma T alpha)2 active complex stays on the membrane. But in low-salt buffers, it becomes soluble and dissociates into a partially active PDE alpha beta catalytic moiety and two PDE gamma-T alpha GTP gamma S complexes. This indicates that T alpha binds preferentially to PDE gamma. We have studied the interaction of recombinant bovine PDE gamma with purified T alpha in solution or with retinal rod outer segments (ROS) containing both T beta gamma-T alpha GDP and PDE alpha beta gamma 2. When added to dark ROS, recombinant PDE gamma did not bind to inactive PDE alpha beta gamma 2 but extracted T alpha GDP from membrane-bound holo-transducin to form a soluble PDE gamma-T alpha GDP complex. PDE gamma also bound to purified T alpha GDP in solution. The kinetics and affinity of the interaction between PDE gamma and T alpha GDP or T alpha GTP gamma S were determined by monitoring changes in the proteins' tryptophan fluorescence. The Kd's for the binding of recombinant PDE gamma to soluble T alpha GTP gamma S and T alpha GDP are < or = 0.1 and 3 nM, respectively. PDE gamma-T alpha GDP falls apart in 3 s. This slow dissociation means that, in situ, T alpha-PDE gamma cannot physically leave the active PDE alpha beta, since after GTP hydrolysis, an isolated T alpha-PDE gamma complex would dissociate too slowly to allow a fast PDE reinhibition by the liberated PDE gamma. When recombinant PDE gamma was added to PDE that had been persistently activated by T alpha GTP gamma S, reinhibition occurred and T alpha GTP gamma S, complexed to the native PDE gamma, was released, indicating that both had hitherto stayed bound to PDE alpha beta. The mutation W70F does not prevent recombinant PDE gamma from inhibiting PDE alpha beta but diminishes its affinity for T alpha GTP and T alpha GDP 100-fold.(ABSTRACT TRUNCATED AT 400 WORDS)