Investigating the molecular mechanisms through which FTY720-P causes persistent S1P1 receptor internalization

BACKGROUND AND PURPOSE

The molecular mechanism underlying the clinical efficacy of FTY720-P is thought to involve persistent internalization and enhanced degradation of the S1P1 receptor subtype (S1P1R). We have investigated whether receptor binding kinetics and β-arrestin recruitment could play a role in the persistent internalization of the S1P1R by FTY720-P.

EXPERIMENTAL APPROACH

[(3) H]-FTY720-P and [(33) P]-S1P were used to label CHO-S1P1/3Rs for binding studies. Ligand efficacy was assessed through [(35) S]-GTPγS binding and β-arrestin recruitment. Metabolic stability was evaluated using a bioassay measuring intracellular Ca(2+) release. CHO-S1P1/3R numbers were determined, following FTY720-P treatment using flow cytometry.

KEY RESULTS

The kinetic off-rate of [(3) H]-FTY720-P from the S1P1R was sixfold slower than from the S1P3R, and comparable to [(33) P]-S1P dissociation from S1P1/3Rs. S1P and FTY720-P stimulated [(35) S]-GTPγS incorporation to similar degrees, but FTY720-P was over 30-fold less potent at S1P3Rs. FTY720-P stimulated a higher level of β-arrestin recruitment at S1P1Rs, 132% of the total recruited by S1P. In contrast, FTY720-P was a weak partial agonist at S1P3R, stimulating just 29% of the total β-arrestin recruited by S1P. Internalization experiments confirmed that cell surface expression of the S1P1R but not the S1P3R was reduced following a pulse exposure to FTY720-P, which is metabolically stable unlike S1P.

CONCLUSIONS AND IMPLICATIONS

FTY720-P and S1P activation of the S1P1R results in receptor internalization as a consequence of an efficient recruitment of β-arrestin. The combination of slow off-rate, efficacious β-arrestin recruitment and metabolic stability all contribute to FTY720-P's ability to promote prolonged S1P1R internalization and may be critical factors in its efficacy in the clinic.

Pathway specific modulation of S1P1 receptor signalling in rat and human astrocytes

BACKGROUND AND PURPOSE

The sphingosine 1-phosphate receptor subtype 1 (S1P1R) is modulated by phosphorylated FTY720 (pFTY720), which causes S1P1R internalization preventing lymphocyte migration thus limiting autoimmune response. Studies indicate that internalized S1P1Rs continue to signal, maintaining an inhibition of cAMP, thus raising question whether the effects of pFTY720 are due to transient initial agonism, functional antagonism and/or continued signalling. To further investigate this, the current study first determined if continued S1P1R activation is pathway specific.

EXPERIMENTAL APPROACH

Using human and rat astrocyte cultures, the effects of S1P1R activation on cAMP, pERK and Ca(2+) signalling was investigated. In addition, to examine the role of S1P1R redistribution on these events, a novel biologic (MNP301) that prevented pFTY720-mediated S1P1R redistribution was engineered.

KEY RESULTS

The data showed that pFTY720 induced long-lasting S1P1R redistribution and continued cAMP signalling in rat astrocytes. In contrast, pFTY720 induced a transient increase of Ca(2+) in astrocytes and subsequent antagonism of Ca(2+) signalling. Notably, while leaving pFTY720-induced cAMP signalling intact, the novel MNP301 peptide attenuated S1P1R-mediated Ca(2+) and pERK signalling in cultured rat astrocytes.

CONCLUSIONS AND IMPLICATIONS

These findings suggested that pFTY720 causes continued cAMP signalling that is not dependent on S1P1R redistribution and induces functional antagonism of Ca(2+) signalling after transient stimulation. To our knowledge, this is the first report demonstrating that pFTY720 causes continued signalling in one pathway (cAMP) versus functional antagonism of another pathway (Ca(2+)) and which also suggests that redistributed S1P1Rs may have differing signalling properties from those expressed at the surface.

Design of fluorescence resonance energy transfer (FRET)-based cGMP indicators: a systematic approach

The intracellular signalling molecule cGMP regulates a variety of physiological processes, and so the ability to monitor cGMP dynamics in living cells is highly desirable. Here, we report a systematic approach to create FRET (fluorescence resonance energy transfer)-based cGMP indicators from two known types of cGMP-binding domains which are found in cGMP-dependent protein kinase and phosphodiesterase 5, cNMP-BD [cyclic nucleotide monophosphate-binding domain and GAF [cGMP-specific and -stimulated phosphodiesterases, Anabaena adenylate cyclases and Escherichia coli FhlA] respectively. Interestingly, only cGMP-binding domains arranged in tandem configuration as in their parent proteins were cGMP-responsive. However, the GAF-derived sensors were unable to be used to study cGMP dynamics because of slow response kinetics to cGMP. Out of 24 cGMP-responsive constructs derived from cNMP-BDs, three were selected to cover a range of cGMP affinities with an EC50 between 500 nM and 6 microM. These indicators possess excellent specifity for cGMP, fast binding kinetics and twice the dynamic range of existing cGMP sensors. The in vivo performance of these new indicators is demonstrated in living cells and validated by comparison with cGMP dynamics as measured by radioimmunoassays.

Phosphorylated FTY720 promotes astrocyte migration through sphingosine-1-phosphate receptors

Sphingosine-1-phosphate (S1P) receptors are widely expressed in the central nervous system where they are thought to regulate glia cell function. The phosphorylated version of fingolimod/FTY720 (FTY720P) is active on a broad spectrum of S1P receptors and the parent compound is currently in phase III clinical trials for the treatment of multiple sclerosis. Here, we aimed to identify which cell type(s) and S1P receptor(s) of the central nervous system are targeted by FTY720P. Using calcium imaging in mixed cultures from embryonic rat cortex we show that astrocytes are the major cell type responsive to FTY720P in this assay. In enriched astrocyte cultures, we detect expression of S1P1 and S1P3 receptors and demonstrate that FTY720P activates Gi protein-mediated signaling cascades. We also show that FTY720P as well as the S1P1-selective agonist SEW2871 stimulate astrocyte migration. The data indicate that FTY720P exerts its effects on astrocytes predominantly via the activation of S1P1 receptors, whereas S1P signals through both S1P1 and S1P3 receptors. We suggest that this distinct pharmacological profile of FTY720P, compared with S1P, could play a role in the therapeutic effects of FTY720 in multiple sclerosis.

Brain sphingosine-1-phosphate receptors: implication for FTY720 in the treatment of multiple sclerosis

Multiple sclerosis (MS) is an autoimmune, neurological disability with unknown etiology. The current therapies available for MS work by an immunomodulatory action, preventing T-cell- and macrophage-mediated destruction of brain-resident oligodendrocytes and axonal loss. Recently, FTY720 (fingolimod) was shown to significantly reduce relapse rates in MS patients and is currently in Phase III clinical trials. This drug attenuates trafficking of harmful T cells entering the brain by regulating sphingosine-1-phosphate (S1P) receptors. Here, we outline the direct roles that S1P receptors play in the central nervous system (CNS) and discuss additional modalities by which FTY720 may provide direct neuroprotection in MS.

Phosphorylated FTY720 stimulates ERK phosphorylation in astrocytes via S1P receptors

Sphingosine-1-phosphate receptors (S1P1-5) are activated by the endogenous agonist S1P and are expressed in the central nervous system. In astrocytes, activation of S1P receptors leads to phosphorylation of extracellular-signal regulated kinase (ERK), a signaling cascade which plays intimate roles in cell proliferation. Fingolimod (FTY720) is in phase III clinical trials for the treatment of multiple sclerosis and its phosphorylated version (FTY720P) activates S1P receptors. We examined the effects of FTY720P on ERK phosphorylation and determined which S1P receptor subtype(s) mediated this signaling event. FTY720P augmented ERK phosphorylation in cortical cultures prepared from embryonic day 18 rat brains and was blocked by an MEK inhibitor or by pertussis toxin. Co-localisation of phosphorylated ERK occurred in glial fibrillary acidic protein (GFAP) positive astrocytes but not neurons or oligodendrocytes. Furthermore, FTY720P stimulated ERK phosphorylation in highly enriched astrocyte cultures made from postnatal day 2 rat cortices. The effects of FTY720P were mimicked by selective S1P1 receptor agonists and blocked by S1P1 receptor antagonists. Collectively, these results demonstrate that FTY720P mediates ERK phosphorylation in astrocytes via the activation of S1P1 receptors.

Negative feedback in NO/cGMP signalling

Most of the effects of the signalling molecule nitric oxide (NO) are mediated by the stimulation of the NO-sensitive GC (guanylate cyclase) and the subsequent increase in cGMP formation. The enzyme contains a prosthetic haem group, which mediates NO stimulation. In addition to the physiological activator NO, NO-sensitizers like the substance YC-1 sensitize the enzyme towards NO and may therefore have important pharmacological implications. Two isoforms of NO-sensitive GC have been identified to date that share regulatory properties, but differ in the subcellular localization. The more ubiquitously expressed alpha1beta1 heterodimer and the alpha2beta1 isoform are mainly expressed in brain. In intact cells, NO-induced cGMP signalling not only depends on cGMP formation, but is also critically determined by the activity of the enzymes responsible for cGMP degradation, e.g. PDE5 (phosphodiesterase 5). Recently, direct activation of PDE5 by cGMP was demonstrated, limiting the cGMP increase and thus functioning as a negative feedback. As the cGMP-induced PDE5 activation turned out to be sustained, in the range of hours, it is probably responsible for the NO-induced desensitization observed within NO/cGMP signalling.

Desensitization of NO/cGMP signaling in smooth muscle: blood vessels versus airways

The NO/cGMP signaling pathway plays a major role in the cardiovascular system, in which it is involved in the regulation of smooth muscle tone and inhibition of platelet aggregation. Under pathophysiological conditions such as endothelial dysfunction, coronary artery disease, and airway hyperreactivity, smooth muscle containing arteries and bronchi are of great pharmacological interest. In these tissues, NO mediates its effects by stimulating guanylyl cyclase (GC) to form cGMP; the subsequent increase in cGMP is counteracted by the cGMP-specific phosphodiesterase (PDE5), which hydrolyzes cGMP. In platelets, allosteric activation of PDE5 by cGMP paralleled by phosphorylation has been shown to govern the sensitivity of NO/cGMP signaling. Here, we demonstrate that the functional responsiveness to NO correlates with the relative abundance of GC and PDE5 in aortic and bronchial tissue, respectively. We show a sustained desensitization of the NO-induced relaxation of aortic and bronchial rings caused by a short-term exposure to NO. The NO treatment caused heterologous desensitization of atrial natriuretic peptide-induced relaxation, whereas relaxation by the cGMP analog 8-pCPT-cGMP was unperturbed. Impaired relaxation was shown to be paralleled by PDE5 phosphorylation; this indicates enhanced cGMP degradation as a mechanism of desensitization. In summary, our results demonstrate the physiological impact of PDE5 activation on the control of smooth muscle tone and provide an explanation for the apparent impairment of NO-induced vasorelaxation.

NO-sensitive guanylyl cyclase and NO-induced feedback inhibition in cGMP signaling

Most effects of the signaling molecule nitric oxide (NO) are mediated by the stimulation of NO-sensitive guanylyl cyclase (GC) and the subsequent intracellular increase in cGMP. Two isoforms of NO-sensitive GC have been identified to date that share regulatory properties but differ in their subcellular localization; the more ubiquitously expressed alpha1beta1 heterodimer, and the alpha2beta1 isoform mainly expressed in brain. New activators of NO-sensitive GC have been identified which may have beneficial pharmacological effects in cardiovascular diseases. In intact cells, NO-induced cGMP signaling not only depends on cGMP formation but is also critically determined by the activity of the enzyme responsible for cGMP degradation, e.g. phosphodiesterase 5 (PDE5). Sustained activation of PDE5 by cGMP has been identified as the mechanism responsible for the recently observed feedback inhibition within NO/cGMP signaling. Moreover, tuning of PDE5 activity may also represent a regulatory link to mediate cross talk between NO-induced and natriuretic peptide-induced cGMP signaling in general.

Nitric oxide-sensitive guanylyl cyclase: structure and regulation

By the formation of the second messenger cGMP, NO-sensitive guanylyl cyclase (GC) plays a key role within the NO/cGMP signaling cascade which participates in vascular regulation and neurotransmission. The enzyme contains a prosthetic heme group that acts as the acceptor site for NO. High affinity binding of NO to the heme moiety leads to an up to 200-fold activation of the enzyme. Unexpectedly, NO dissociates with a half-life of a few seconds which appears fast enough to account for the deactivation of the enzyme in biological systems. YC-1 and its analogs act as NO sensitizers and led to the discovery of a novel pharmacologically and conceivably physiologically relevant regulatory principle of the enzyme. The two isoforms of the heterodimeric enzyme (alpha1beta1, alpha2beta1) are known that are functionally indistinguishable. The alpha2beta1-isoform mainly occurs in brain whereas the alpha1beta1-enzyme shows a broader distribution and represents the predominantly expressed form of NO-sensitive GC. Until recently, the enzyme has been thought to occur in the cytosol. However, latest evidence suggests that the alpha2-subunit mediates the membrane association of the alpha2beta1-isoform via interaction with a PDZ domain of the post-synaptic scaffold protein PSD-95. Binding to PSD-95 locates this isoform in close proximity to the NO-generating synthases thereby enabling the NO sensor to respond to locally elevated NO concentrations. In sum, the two known isoforms may stand for the neuronal and vascular form of NO-sensitive GC reflecting a possible association to the neuronal and endothelial NO-synthase, respectively.

In vivo reconstitution of the negative feedback in nitric oxide/cGMP signaling: role of phosphodiesterase type 5 phosphorylation

Most effects of the messenger molecule nitric oxide (NO) are mediated by cGMP, which is formed by NO-sensitive guanylyl cyclase (GC) and degraded by phosphodiesterases (PDEs). In platelets, NO elicits a spike-like cGMP response and causes a sustained desensitization. Both characteristics have been attributed to PDE5 activation caused by cGMP binding to its regulatory GAF domain. Activation is paralleled by phosphorylation whose precise function remains unknown. Here, we report reconstitution of all features of the NO-induced cGMP response in human embryonic kidney cells by coexpressing NO-sensitive GC and PDE5. The spike-like cGMP response was blunted when PDE5 phosphorylation was enhanced by additional overexpression of cGMP-dependent protein kinase. Analysis of PDE5 activation in vitro revealed a discrepancy between the cGMP concentrations required for activation (micromolar) and reversal of activation (nanomolar), indicating the conversion of a low-affinity state to a high-affinity state upon binding of cGMP. Phosphorylation even increased the high apparent affinity enabling PDE5 activation to persist at extremely low cGMP concentrations. Our data suggest that the spike-like shape and the desensitization of the cGMP response are potentially inherent to every GC- and PDE5-expressing cell. Phosphorylation of PDE5 seems to act as memory switch for activation leading to long-term desensitization of the signaling pathway.

Inhibition of phosphodiesterase type 5 by the activator of nitric oxide-sensitive guanylyl cyclase BAY 41-2272

BACKGROUND

By the formation of cGMP, nitric oxide (NO)-sensitive guanylyl cyclase (GC) acts as the effector for the signaling molecule NO and mediates the relaxation of vascular smooth muscle and the inhibition of platelet aggregation. The compounds YC-1 and BAY 41-2272 are regarded as NO-independent activators and sensitizers of NO-sensitive GC. In vivo effects, for example, lowering blood pressure and prolonging tail-bleeding times, turn the compounds into promising candidates for the therapy of cardiovascular diseases. However, YC-1 has also been shown to inhibit the major cGMP-degrading enzyme phosphodiesterase type 5 (PDE5). The synergistic properties of YC-1 on cGMP formation and degradation lead to an excessive NO-induced cGMP accumulation in cells, explaining the observed physiological effects. We assessed a potential inhibition of PDE5 by the new GC activator BAY 41-2272.

METHODS AND RESULTS

The effects of BAY 41-2272 on NO-sensitive GC and PDE5 activities were tested in vitro. BAY 41-2272 not only sensitized NO-sensitive GC toward activation by NO but also, with comparable potency, inhibited cGMP degradation by PDE5. In intact platelets, BAY 41-2272 greatly potentiated the NO-induced cGMP response that was caused by a synergistic effect of BAY 41-2272 on cGMP formation and degradation.

CONCLUSIONS

The physiological effects of BAY 41-2272, which are commonly ascribed to the NO-independent activation of NO-sensitive GC, are rather due to the synergism of sensitization of NO-sensitive GC and inhibition of PDE5.

Direct activation of PDE5 by cGMP: long-term effects within NO/cGMP signaling

In platelets, the nitric oxide (NO)-induced cGMP response is indicative of a highly regulated interplay of cGMP formation and cGMP degradation. Recently, we showed that within the NO-induced cGMP response in human platelets, activation and phosphorylation of phosphodiesterase type 5 (PDE5) occurred. Here, we identify cyclic GMP-dependent protein kinase I as the kinase responsible for the NO-induced PDE5 phosphorylation. However, we demonstrate that cGMP can directly activate PDE5 without phosphorylation in platelet cytosol, most likely via binding to the regulatory GAF domains. The reversal of activation was slow, and was not completed after 60 min. Phosphorylation enhanced the cGMP-induced activation, allowing it to occur at lower cGMP concentrations. Also, in intact platelets, a sustained NO-induced activation of PDE5 for as long as 60 min was detected. Finally, the long-term desensitization of the cGMP response induced by a low NO concentration reveals the physiological relevance of the PDE5 activation within NO/cGMP signaling. In sum, we suggest NO-induced activation and phosphorylation of PDE5 as the mechanism for a long-lasting negative feedback loop shaping the cGMP response in human platelets in order to adapt to the amount of NO available.

Inhibition of deactivation of NO-sensitive guanylyl cyclase accounts for the sensitizing effect of YC-1

Many of the physiological effects of the signaling molecule nitric oxide are mediated by the stimulation of the NO-sensitive guanylyl cyclase. Activation of the enzyme is achieved by binding of NO to the prosthetic heme group of the enzyme and the initiation of conformational changes. So far, the rate of NO dissociation of the purified enzyme has only been determined spectrophotometrically, whereas the respective deactivation, i.e. the decline in enzymatic activity, has only been determined in cytosolic fractions and intact cells. Here, we report on the deactivation of purified NO-sensitive guanylyl cyclase determined after addition of the NO scavenger oxyhemoglobin or dilution. The deactivation rate corresponded to a half-life of the NO/guanylyl cyclase complex of approximately 4 s, which is in good agreement with the spectrophotometrically measured NO dissociation rate of the enzyme. The deactivation rate of the enzyme determined in platelets yielded a much shorter half-life indicating either partial damage of the enzyme during the purification procedure or the existence of endogenous deactivation accelerating factors. YC-1, a component causing sensitization of guanylyl cyclase toward NO, inhibited deactivation of guanylyl cyclase, resulting in an extremely prolonged half-life of the NO/guanylyl cyclase complex of more than 10 min. The deactivation of an ATP-utilizing guanylyl cyclase mutant was almost unaffected by YC-1, indicating the existence of a special structure within the catalytic domain required for YC-1 binding or for the transduction of the YC-1 effect. In contrast to the wild type enzyme, YC-1 did not increase NO sensitivity of this mutant, clearly establishing inhibition of deactivation as the underlying mechanism of the NO sensitizer YC-1.

Rapid nitric oxide-induced desensitization of the cGMP response is caused by increased activity of phosphodiesterase type 5 paralleled by phosphorylation of the enzyme

Most of the effects of the signaling molecule nitric oxide (NO) are mediated by cGMP, which is synthesized by soluble guanylyl cyclase and degraded by phosphodiesterases. Here we show that in platelets and aortic tissue, NO led to a biphasic response characterized by a tremendous increase in cGMP (up to 100-fold) in less than 30 s and a rapid decline, reflecting the tightly controlled balance of guanylyl cyclase and phosphodiesterase activities. Inverse to the reported increase in sensitivity caused by NO shortage, concentrating NO attenuated the cGMP response in a concentration-dependent manner. We found that guanylyl cyclase remained fully activated during the entire course of the cGMP response; thus, desensitization was not due to a switched off guanylyl cyclase. However, when intact platelets were incubated with NO and then lysed, enhanced activity of phosphodiesterase type 5 was detected in the cytosol. Furthermore, this increase in cGMP degradation is paralleled by the phosphorylation of phosphodiesterase type 5 at Ser-92. Thus, our data suggest that NO-induced desensitization of the cGMP response is caused by the phosphorylation and subsequent activity increase of phosphodiesterase type 5.