Seunghyi Kook

Lentiviral Overexpression of GRK6 Alleviates l-Dopa–Induced Dyskinesia in Experimental Parkinson’s Disease

G protein–coupled receptor kinase 6, which promotes desensitization of the dopamine receptor, alleviates dyskinesia without compromising the antiparkinsonian effect of l-dopa. Treatment for Tremors Without Side Effects As neurodegenerative diseases go, Parkinson’s disease is fairly treatable. Oral doses of l-dopa can still the tremors and normalize a patient’s movements—for a time. Eventually, however, most patients develop involuntary aimless gestures call dyskinesias, thought to be a result of oversensitive dopamine responses in the brain, caused by years of taking l-dopa. Now, Bezard and his colleagues have taken aim at a regulator of the dopamine receptor, G protein–coupled receptor kinase 6 (GRK6), to combat these disturbing side effects. The dopamine receptor, like others in its family, will desensitize after use. In this state, the receptor can no longer be activated and is taken up by the cell. The first step in desensitization is the phosphorylation of the receptor by GRK6. After many years of l-dopa, the amount of GRK in the brain starts to decline and the machinery that desensitizes the receptor does not work properly, leading, it is believed, to the uncontrolled movements of dyskinesia. The authors reinstated GRKs with gene therapy in mice that had an induced parkinsonian syndrome and showed that the dyskinesia-like movements of the mice were much reduced and, as expected, desensitization of the dopamine receptor was normalized. Repeating this experiment in macaque monkeys, in which a Parkinson-like disease had been artificially induced by a toxic agent, gave similar results: Increasing GRK6 expression in the brain could markedly improve the dyskinesia-like side effects of long-term l-dopa treatment, likely by correcting the desensitization of dopamine receptors. Notably, correction of GRK6 did not interfere with the therapeutic effects of l-dopa—an important attribute for the eventual application of such a therapy. These authors have identified a signaling pathway that seems to be responsible for the worst side effect of the standard treatment for Parkinson’s disease. Manipulation of one of its members, GRK6, or other components of dopamine receptor sensitization may prove to be an effective treatment for these side effects without hindering the efficacy of one of the most useful drugs in the neurologist’s armamentarium. Parkinson’s disease is caused primarily by degeneration of brain dopaminergic neurons in the substantia nigra and the consequent deficit of dopamine in the striatum. Dopamine replacement therapy with the dopamine precursor l-dopa is the mainstay of current treatment. After several years, however, the patients develop l-dopa–induced dyskinesia, or abnormal involuntary movements, thought to be due to excessive signaling via dopamine receptors. G protein–coupled receptor kinases (GRKs) control desensitization of dopamine receptors. We found that dyskinesia is attenuated by lentivirus-mediated overexpression of GRK6 in the striatum in rodent and primate models of Parkinson’s disease. Conversely, reduction of GRK6 concentration by microRNA delivered with lentiviral vector exacerbated dyskinesia in parkinsonian rats. GRK6 suppressed dyskinesia in monkeys without compromising the antiparkinsonian effects of l-dopa and even prolonged the antiparkinsonian effect of a lower dose of l-dopa. Our finding that increased availability of GRK6 ameliorates dyskinesia and increases duration of the antiparkinsonian action of l-dopa suggests a promising approach for controlling both dyskinesia and motor fluctuations in Parkinson’s disease.

Silent Scaffolds INHIBITION OF c-Jun N-TERMINAL KINASE 3 ACTIVITY IN CELL BY DOMINANT-NEGATIVE ARRESTIN-3 MUTANT

Background: JNK kinases play an important role in cell death and differentiation. Results: Arrestin-3 mutant that binds ASK1, MKK4, and JNK3 normally without promoting JNK3 activation suppresses JNK3 activation in the cell. Conclusion: Modified scaffolding proteins can be used to regulate MAP kinase activity in vivo. Significance: Silent scaffolds are a novel type of molecular tool for manipulation of MAP kinase activity in cells. We established a new in vivo arrestin-3-JNK3 interaction assay based on bioluminescence resonance energy transfer (BRET) between JNK3-luciferase and Venus-arrestins. We tested the ability of WT arrestin-3 and its 3A mutant that readily binds β2-adrenergic receptors as well as two mutants impaired in receptor binding, Δ7 and KNC, to directly bind JNK3 and to promote JNK3 phosphorylation in cells. Both receptor binding-deficient mutants interact with JNK3 significantly better than WT and 3A arrestin-3. WT arrestin-3 and Δ7 mutant robustly promoted JNK3 activation, whereas 3A and KNC mutants did not. Thus, receptor binding, JNK3 interaction, and JNK3 activation are three distinct arrestin functions. We found that the KNC mutant, which tightly binds ASK1, MKK4, and JNK3 without facilitating JNK3 phosphorylation, has a dominant-negative effect, competitively decreasing JNK activation by WT arrestin-3. Thus, KNC is a silent scaffold, a novel type of molecular tool for the suppression of MAPK signaling in living cells.

Role of Receptor-attached Phosphates in Binding of Visual and Non-visual Arrestins to G Protein-coupled Receptors

Background: The relative contribution of phosphates and active GPCR conformation is unknown. Results: Using WT and mutant arrestins and receptors, we show that phosphates are critical for arrestin binding to some GPCRs but not to others. Conclusion: The role of receptor-attached phosphates in arrestin binding varies widely depending on the arrestin-receptor combination. Significance: Distinct molecular mechanisms mediate arrestin recruitment to different GPCRs. Arrestins are a small family of proteins that regulate G protein-coupled receptors (GPCRs). Arrestins specifically bind to phosphorylated active receptors, terminating G protein coupling, targeting receptors to endocytic vesicles, and initiating G protein-independent signaling. The interaction of rhodopsin-attached phosphates with Lys-14 and Lys-15 in β-strand I was shown to disrupt the interaction of α-helix I, β-strand I, and the C-tail of visual arrestin-1, facilitating its transition into an active receptor-binding state. Here we tested the role of conserved lysines in homologous positions of non-visual arrestins by generating K2A mutants in which both lysines were replaced with alanines. K2A mutations in arrestin-1, -2, and -3 significantly reduced their binding to active phosphorhodopsin in vitro. The interaction of arrestins with several GPCRs in intact cells was monitored by a bioluminescence resonance energy transfer (BRET)-based assay. BRET data confirmed the role of Lys-14 and Lys-15 in arrestin-1 binding to non-cognate receptors. However, this was not the case for non-visual arrestins in which the K2A mutations had little effect on net BRETmax values for the M2 muscarinic acetylcholine (M2R), β2-adrenergic (β2AR), or D2 dopamine receptors. Moreover, a phosphorylation-deficient mutant of M2R interacted with wild type non-visual arrestins normally, whereas phosphorylation-deficient β2AR mutants bound arrestins at 20–50% of the level of wild type β2AR. Thus, the contribution of receptor-attached phosphates to arrestin binding varies depending on the receptor-arrestin pair. Although arrestin-1 always depends on receptor phosphorylation, its role in the recruitment of arrestin-2 and -3 is much greater in the case of β2AR than M2R and D2 dopamine receptor.

The 31-kDa Caspase-generated Cleavage Product of p130cas Functions as a Transcriptional Repressor of E2A in Apoptotic Cells

In response to integrin receptor binding to the extracellular matrix, the multidomain docking protein p130cas (Crk-associated substrate) activates various signaling cascades modulating such cellular processes as proliferation, migration, and apoptosis. During apoptosis, caspase-mediated cleavage of p130cas generated a C-terminal 31-kDa fragment (31-kDa) and promoted morphological changes characteristic of apoptosis, including loss of focal adhesions, cell rounding, and nuclear condensation and fragmentation. By contrast, a p130cas D748E mutant, which was unable to produce 31-kDa, attenuated the disassembly of focal adhesions at the bottom of the cell. 31-kDa contains a helix-loop-helix (HLH) domain that shows greater sequence homology with Id proteins than with basic HLH proteins, which enabled heterodimerization with E2A. Once coupled to E2A, 31-kDa was translocated to the cell nucleus, where it inhibited E2A-mediated p21Waf1/Cip1 transcription. Moreover, overexpression of 31-kDa led to cell death that could be inhibited by treatment with the caspase inhibitor ZVAD-fluoromethyl ketone or by ectopic expression of E2A or p21Waf1/Cip1. These data suggest that during etoposide-induced apoptosis, 31-kDa promotes caspase-mediated cell death by inhibiting E2A-mediated activation of p21Waf1/Cip1 transcription.

The Effect of Arrestin Conformation on the Recruitment of c-Raf1, MEK1, and ERK1/2 Activation

Arrestins are multifunctional signaling adaptors originally discovered as proteins that “arrest” G protein activation by G protein-coupled receptors (GPCRs). Recently GPCR complexes with arrestins have been proposed to activate G protein-independent signaling pathways. In particular, arrestin-dependent activation of extracellular signal-regulated kinase 1/2 (ERK1/2) has been demonstrated. Here we have performed in vitro binding assays with pure proteins to demonstrate for the first time that ERK2 directly binds free arrestin-2 and -3, as well as receptor-associated arrestins-1, -2, and -3. In addition, we showed that in COS-7 cells arrestin-2 and -3 association with β2-adrenergic receptor (β2AR) significantly enhanced ERK2 binding, but showed little effect on arrestin interactions with the upstream kinases c-Raf1 and MEK1. Arrestins exist in three conformational states: free, receptor-bound, and microtubule-associated. Using conformationally biased arrestin mutants we found that ERK2 preferentially binds two of these: the “constitutively inactive” arrestin-Δ7 mimicking microtubule-bound state and arrestin-3A, a mimic of the receptor-bound conformation. Both rescue arrestin-mediated ERK1/2/activation in arrestin-2/3 double knockout fibroblasts. We also found that arrestin-2-c-Raf1 interaction is enhanced by receptor binding, whereas arrestin-3-c-Raf1 interaction is not.

Ubiquitin Ligase Parkin Promotes Mdm2–Arrestin Interaction but Inhibits Arrestin Ubiquitination

Numerous mutations in E3 ubiquitin ligase parkin were shown to associate with familial Parkinsons disease. Here we show that parkin binds arrestins, versatile regulators of cell signaling. Arrestin-parkin interaction was demonstrated by coimmunoprecipitation of endogenous proteins from brain tissue and shown to be direct using purified proteins. Parkin binding enhances arrestin interactions with another E3 ubiquitin ligase, Mdm2, apparently by shifting arrestin conformational equilibrium to the basal state preferred by Mdm2. Although Mdm2 was reported to ubiquitinate arrestins, parkin-dependent increase in Mdm2 binding dramatically reduces the ubiquitination of both nonvisual arrestins, basal and stimulated by receptor activation, without affecting receptor internalization. Several disease-associated parkin mutations differentially affect the stimulation of Mdm2 binding. All parkin mutants tested effectively suppress arrestin ubiquitination, suggesting that bound parkin shields arrestin lysines targeted by Mdm2. Parkin binding to arrestins along with its effects on arrestin interaction with Mdm2 and ubiquitination is a novel function of this protein with implications for Parkinsons disease pathology.

Caspase-dependent cleavage of tensin induces disruption of actin cytoskeleton during apoptosis.

Members of both calpain and caspase protease families can degrade several components of focal adhesions, leading to disassembly of these complexes. In this report, we investigated the disappearance of tensin from cell adhesion sites of chicken embryonic fibroblast cells (CEFs) exposed to etoposide and demonstrated that loss of tensin from cell adhesions during etoposide-induced apoptosis may be due to degradation of tensin by caspase-3. Tensin cleavage by caspase-3 at the sequence DYPD(1226)G separates the amino-terminal region containing the actin binding domain and the carboxyl-terminal region containing the SH2 domain. The resultant carboxyl-terminal fragment of tensin is unable to bind phosphoinositide 3-kinase (PI3-kinase) transducing cell survival signaling. We also demonstrated that overexpression of the amino-terminal tensin fragment induced disruption of actin cytoskeleton in chicken embryonic fibroblasts. Therefore, caspase-mediated cleavage of tensin contributes to the disruption of actin organization and interrupts ECM-mediated survival signals through phosphatidylinositol 3-kinase.

Arrestin-3 Binds c-Jun N-terminal Kinase 1 (JNK1) and JNK2 and Facilitates the Activation of These Ubiquitous JNK Isoforms in Cells via Scaffolding

Background: The ability of arrestin-3 to facilitate activation of JNK1 and JNK2 has never been reported. Results: Arrestin-3 binds JNK1α1 and JNK2α2 and promotes their phosphorylation by MKK4 and MKK7 in vitro and in intact cells. Conclusion: Arrestin-3 promotes the activation of ubiquitous JNK1 and JNK2 isoforms. Significance: Arrestin-3 scaffolds MKK4/7-JNK1/2/3 signaling modules and facilitates activation of ubiquitous JNK isoforms. Non-visual arrestins scaffold mitogen-activated protein kinase (MAPK) cascades. The c-Jun N-terminal kinases (JNKs) are members of MAPK family. Arrestin-3 has been shown to enhance the activation of JNK3, which is expressed mainly in neurons, heart, and testes, in contrast to ubiquitous JNK1 and JNK2. Although all JNKs are activated by MKK4 and MKK7, both of which bind arrestin-3, the ability of arrestin-3 to facilitate the activation of JNK1 and JNK2 has never been reported. Using purified proteins we found that arrestin-3 directly binds JNK1α1 and JNK2α2, interacting with the latter comparably to JNK3α2. Phosphorylation of purified JNK1α1 and JNK2α2 by MKK4 or MKK7 is increased by arrestin-3. Endogenous arrestin-3 interacted with endogenous JNK1/2 in different cell types. Arrestin-3 also enhanced phosphorylation of endogenous JNK1/2 in intact cells upon expression of upstream kinases ASK1, MKK4, or MKK7. We observed a biphasic effect of arrestin-3 concentrations on phosphorylation of JNK1α1 and JNK2α2 both in vitro and in vivo. Thus, arrestin-3 acts as a scaffold, facilitating JNK1α1 and JNK2α2 phosphorylation by MKK4 and MKK7 via bringing JNKs and their activators together. The data suggest that arrestin-3 modulates the activity of ubiquitous JNK1 and JNK2 in non-neuronal cells, impacting the signaling pathway that regulates their proliferation and survival.

JNK3 enzyme binding to arrestin-3 differentially affects the recruitment of upstream mitogen-activated protein (MAP) kinase kinases.

Background: An interaction between arrestin-3 and MKK7 has never been elucidated. Results: Arrestin-3 directly binds MKK7 and promotes JNK3α2 phosphorylation by MKK7 in vitro and in intact cells. Conclusion: Arrestin-3 recruits JNK3α2 and both upstream MKKs. Significance: Arrestin-3 promotes full JNK3α2 activation; MKK binding is regulated by JNK3α2. Arrestin-3 was previously shown to bind JNK3α2, MKK4, and ASK1. However, full JNK3α2 activation requires phosphorylation by both MKK4 and MKK7. Using purified proteins we show that arrestin-3 directly interacts with MKK7 and promotes JNK3α2 phosphorylation by both MKK4 and MKK7 in vitro as well as in intact cells. The binding of JNK3α2 promotes an arrestin-3 interaction with MKK4 while reducing its binding to MKK7. Interestingly, the arrestin-3 concentration optimal for scaffolding the MKK7-JNK3α2 module is ∼10-fold higher than for the MKK4-JNK3α2 module. The data provide a mechanistic basis for arrestin-3-dependent activation of JNK3α2. The opposite effects of JNK3α2 on arrestin-3 interactions with MKK4 and MKK7 is the first demonstration that the kinase components in mammalian MAPK cascades regulate each others interactions with a scaffold protein. The results show how signaling outcomes can be affected by the relative expression of scaffolding proteins and components of signaling cascades that they assemble.

Caspase-cleaved arrestin-2 and BID cooperatively facilitate cytochrome C release and cell death

Apoptosis is programmed cell death triggered by activation of death receptors or cellular stress. Activation of caspases is the hallmark of apoptosis. Arrestins are best known for their role in homologous desensitization of G protein-coupled receptors (GPCRs). Arrestins quench G protein activation by binding to activated phosphorylated GPCRs. Recently, arrestins have been shown to regulate multiple signalling pathways in G protein-independent manner via scaffolding signalling proteins. Here we demonstrate that arrestin-2 isoform is cleaved by caspases during apoptosis induced via death receptor activation or by DNA damage at evolutionarily conserved sites in the C-terminus. Caspase-generated arrestin-2-(1-380) fragment translocates to mitochondria increasing cytochrome C release, which is the key checkpoint in cell death. Cells lacking arrestin-2 are significantly more resistant to apoptosis. The expression of wild-type arrestin-2 or its cleavage product arrestin-2-(1-380), but not of its caspase-resistant mutant, restores cell sensitivity to apoptotic stimuli. Arrestin-2-(1-380) action depends on tBID: at physiological concentrations, arrestin-2-(1-380) directly binds tBID and doubles tBID-induced cytochrome C release from isolated mitochondria. Arrestin-2-(1-380) does not facilitate apoptosis in BID knockout cells, whereas its ability to increase caspase-3 activity and facilitate cytochrome C release is rescued when BID expression is restored. Thus, arrestin-2-(1-380) cooperates with another product of caspase activity, tBID, and their concerted action significantly contributes to cell death.