Role of G Protein-Coupled Receptor Kinases 2 and 3 in µ-Opioid Receptor Desensitization and Internalization.

There is ongoing debate about the role of G protein-coupled receptor kinases (GRKs) in agonist-induced desensitization of the µ-opioid receptor (MOPr) in brain neurons. In the present paper, we have used a novel membrane-permeable, small-molecule inhibitor of GRK2 and GRK3, Takeda compound 101 (Cmpd101; 3-[[[4-methyl-5-(4-pyridyl)-4H-1,2,4-triazole-3-yl] methyl] amino]-N-[2-(trifuoromethyl) benzyl] benzamidehydrochloride), to study the involvement of GRK2/3 in acute agonist-induced MOPr desensitization. We observed that Cmpd101 inhibits the desensitization of the G protein-activated inwardly-rectifying potassium current evoked by receptor-saturating concentrations of methionine-enkephalin (Met-Enk), [d-Ala(2), N-MePhe(4), Gly-ol(5)]-enkephalin (DAMGO), endomorphin-2, and morphine in rat and mouse locus coeruleus (LC) neurons. In LC neurons from GRK3 knockout mice, Met-Enk-induced desensitization was unaffected, implying a role for GRK2 in MOPr desensitization. Quantitative analysis of the loss of functional MOPrs following acute agonist exposure revealed that Cmpd101 only partially reversed MOPr desensitization. Inhibition of extracellular signal-regulated kinase 1/2, protein kinase C, c-Jun N-terminal kinase, or GRK5 did not inhibit the Cmpd101-insensitive component of desensitization. In HEK 293 cells, Cmpd101 produced almost complete inhibition of DAMGO-induced MOPr phosphorylation at Ser(375), arrestin translocation, and MOPr internalization. Our data demonstrate a role for GRK2 (and potentially also GRK3) in agonist-induced MOPr desensitization in the LC, but leave open the possibility that another, as yet unidentified, mechanism of desensitization also exists.

µ-Opioid receptor desensitization: homologous or heterologous?

There is considerable controversy over whether µ-opioid receptor (MOPr) desensitization is homologous or heterologous and over the mechanisms underlying such desensitization. In different cell types MOPr desensitization has been reported to involve receptor phosphorylation by various kinases, including G-protein-coupled receptor kinases (GRKs), second messenger and other kinases as well as perturbation of the MOPr effector pathway by GRK sequestration of G protein ßγ subunits or ion channel modulation. Here we report that in brainstem locus coeruleus (LC) neurons prepared from relatively mature rats (5-8 weeks old) rapid MOPr desensitization induced by the high-efficacy opioid peptides methionine enkephalin and DAMGO was homologous and not heterologous to α(2)-adrenoceptors and somatostatin SST(2) receptors. Given that these receptors all couple through G proteins to the same set of G-protein inwardly rectifying (GIRK) channels it is unlikely therefore that in mature neurons MOPr desensitization involves G protein ßγ subunit sequestration or ion channel modulation. In contrast, in slices from immature animals (less than postnatal day 20), MOPr desensitization was observed to be heterologous and could be downstream of the receptor. Heterologous MOPr desensitization was not dependent on protein kinase C or c-Jun N-terminal kinase activity, but the change from heterologous to homologous desensitization with age was correlated with a decrease in the expression levels of GRK2 in the LC and other brain regions. The observation that the mechanisms underlying MOPr desensitization change with neuronal development is important when extrapolating to the mature brain results obtained from experiments on expression systems, cell lines and immature neuronal preparations.

The methylated N-terminal tail of RCC1 is required for stabilisation of its interaction with chromatin by Ran in live cells.

BACKGROUND: Regulator of chromosome condensation 1 (RCC1) is the guanine nucleotide exchange factor for Ran GTPase. Localised generation of Ran-GTP by RCC1 on chromatin is critical for nucleocytoplasmic transport, mitotic spindle assembly and nuclear envelope formation. Both the N-terminal tail of RCC1 and its association with Ran are important for its interaction with chromatin in cells. In vitro, the association of Ran with RCC1 induces a conformational change in the N-terminal tail that promotes its interaction with DNA. RESULTS: We have investigated the mechanism of the dynamic interaction of the alpha isoform of human RCC1 (RCC1alpha) with chromatin in live cells using fluorescence recovery after photobleaching (FRAP) of green fluorescent protein (GFP) fusions. We show that the N-terminal tail stabilises the interaction of RCC1alpha with chromatin and this function can be partially replaced by another lysine-rich nuclear localisation signal. Removal of the tail prevents the interaction of RCC1alpha with chromatin from being stabilised by RanT24N, a mutant that binds stably to RCC1alpha. The interaction of RCC1alpha with chromatin is destabilised by mutation of lysine 4 (K4Q), which abolishes alpha-N-terminal methylation, and this interaction is no longer stabilised by RanT24N. However, alpha-N-terminal methylation of RCC1alpha is not regulated by the binding of RanT24N. Conversely, the association of Ran with precipitated RCC1alpha does not require the N-terminal tail of RCC1alpha or its methylation. The mobility of RCC1alpha on chromatin is increased by mutation of aspartate 182 (D182A), which inhibits guanine-nucleotide exchange activity, but RCC1alphaD182A can still bind nucleotide-free Ran and its interaction with chromatin is stabilised by RanT24N. CONCLUSIONS: These results show that the stabilisation of the dynamic interaction of RCC1alpha with chromatin by Ran in live cells requires the N-terminal tail of RCC1alpha. alpha-N-methylation is not regulated by formation of the binary complex with Ran, but it promotes chromatin binding through the tail. This work supports a model in which the association of RCC1alpha with chromatin is promoted by a conformational change in the alpha-N-terminal methylated tail that is induced allosterically in the binary complex with Ran.

Phosphorylation of Mcl-1 by CDK1-cyclin B1 initiates its Cdc20-dependent destruction during mitotic arrest.

The balance between cell cycle progression and apoptosis is important for both surveillance against genomic defects and responses to drugs that arrest the cell cycle. In this report, we show that the level of the human anti-apoptotic protein Mcl-1 is regulated during the cell cycle and peaks at mitosis. Mcl-1 is phosphorylated at two sites in mitosis, Ser64 and Thr92. Phosphorylation of Thr92 by cyclin-dependent kinase 1 (CDK1)-cyclin B1 initiates degradation of Mcl-1 in cells arrested in mitosis by microtubule poisons. Mcl-1 destruction during mitotic arrest requires proteasome activity and is dependent on Cdc20/Fizzy, which mediates recognition of mitotic substrates by the anaphase-promoting complex/cyclosome (APC/C) E3 ubiquitin ligase. Stabilisation of Mcl-1 during mitotic arrest by mutation of either Thr92 or a D-box destruction motif inhibits the induction of apoptosis by microtubule poisons. Thus, phosphorylation of Mcl-1 by CDK1-cyclin B1 and its APC/C(Cdc20)-mediated destruction initiates apoptosis if a cell fails to resolve mitosis. Regulation of apoptosis, therefore, is linked intrinsically to progression through mitosis and is governed by a temporal mechanism that distinguishes between normal mitosis and prolonged mitotic arrest.

A mitotic role for BRCA1/BARD1 in tumor suppression?

The tumor-suppressor protein BRCA1 is thought to act by preserving genomic integrity. In this issue of Cell, Joukov et al. demonstrate that the BRCA1/BARD1 heterodimer participates in mitotic spindle assembly, a process conducted by the GTPase Ran. Loss of this mitotic function might contribute to tumorigenesis.

High resolution analysis of mammalian nuclear structure throughout the cell cycle: implications for nuclear pore complex assembly during interphase and mitosis.

Nuclear pore complexes (NPCs) are the gateways for both active and passive bidirectional molecular transport between the nucleoplasm and cytoplasm. These mega-dalton assemblies are composed of multiple copies of approximately 30 distinct proteins termed nucleoporins. Higher eukaryotes display an "open" mitosis in which the NPCs, nuclear envelope, and lamina disassemble. During mitosis several nucleoporins are redistributed to kinetochores until they are recruited back to the periphery of chromatin as the NPCs are reassembled. Within this study we have developed and optimized the visualization of mammalian cells and their chromosome profiles throughout the cell-cycle. Close attention has been paid to the preservation of chromatin, membranes, and NPC structure to investigate the ultrastructural locations of specific proteins in both interphase and mitosis.

Cell biology: Ran, mitosis and the cancer connection.

The small GTPase Ran has been shown to regulate HURP, a protein that interacts with several mitotic spindle assembly factors. This discovery sheds new light on the role of Ran in the fidelity of mitosis and in cancer.