Phosphorylation-dependent ubiquitination of cyclin E by the SCFFbw7 ubiquitin ligase

Cyclin E binds and activates the cyclin-dependent kinase Cdk2 and catalyzes the transition from the G1 phase to the S phase of the cell cycle. The amount of cyclin E protein present in the cell is tightly controlled by ubiquitin-mediated proteolysis. Here we identify the ubiquitin ligase responsible for cyclin E ubiquitination as SCFFbw7 and demonstrate that it is functionally conserved in yeast, flies, and mammals. Fbw7 associates specifically with phosphorylated cyclin E, and SCFFbw7 catalyzes cyclin E ubiquitination in vitro. Depletion of Fbw7 leads to accumulation and stabilization of cyclin E in vivo in human and Drosophila melanogaster cells. Multiple F-box proteins contribute to cyclin E stability in yeast, suggesting an overlap in SCF E3 ligase specificity that allows combinatorial control of cyclin E degradation.     

Beta-arrestin 2: a receptor-regulated MAPK scaffold for the activation of JNK3

beta-Arrestins, originally discovered in the context of heterotrimeric guanine nucleotide binding protein-coupled receptor (GPCR) desensitization, also function in internalization and signaling of these receptors. We identified c-Jun amino-terminal kinase 3 (JNK3) as a binding partner of beta-arrestin 2 using a yeast two-hybrid screen and by coimmunoprecipitation from mouse brain extracts or cotransfected COS-7 cells. The upstream JNK activators apoptosis signal-regulating kinase 1 (ASK1) and mitogen-activated protein kinase (MAPK) kinase 4 were also found in complex with beta-arrestin 2. Cellular transfection of beta-arrestin 2 caused cytosolic retention of JNK3 and enhanced JNK3 phosphorylation stimulated by ASK1. Moreover, stimulation of the angiotensin II type 1A receptor activated JNK3 and triggered the colocalization of beta-arrestin 2 and active JNK3 to intracellular vesicles. Thus, beta-arrestin 2 acts as a scaffold protein, which brings the spatial distribution and activity of this MAPK module under the control of a GPCR.     

Requirement of JNK for stress-induced activation of the cytochrome c-mediated death pathway

The c-Jun NH2-terminal kinase (JNK) is activated when cells are exposed to ultraviolet (UV) radiation. However, the functional consequence of JNK activation in UV-irradiated cells has not been established. It is shown here that JNK is required for UV-induced apoptosis in primary murine embryonic fibroblasts. Fibroblasts with simultaneous targeted disruptions of all the functional Jnk genes were protected against UV-stimulated apoptosis. The absence of JNK caused a defect in the mitochondrial death signaling pathway, including the failure to release cytochrome c. These data indicate that mitochondria are influenced by proapoptotic signal transduction through the JNK pathway.     

Requirement of JNK2 for scavenger receptor A-mediated foam cell formation in atherogenesis

In vitro studies suggest a role for c-Jun N-terminal kinases (JNKs) in proatherogenic cellular processes. We show that atherosclerosis-prone ApoE-/- mice simultaneously lacking JNK2 (ApoE-/- JNK2-/- mice), but not ApoE-/- JNK1-/- mice, developed less atherosclerosis than do ApoE-/- mice. Pharmacological inhibition of JNK activity efficiently reduced plaque formation. Macrophages lacking JNK2 displayed suppressed foam cell formation caused by defective uptake and degradation of modified lipoproteins and showed increased amounts of the modified lipoprotein-binding and -internalizing scavenger receptor A (SR-A), whose phosphorylation was markedly decreased. Macrophage-restricted deletion of JNK2 was sufficient to decrease atherogenesis. Thus, JNK2-dependent phosphorylation of SR-A promotes uptake of lipids in macrophages, thereby regulating foam cell formation, a critical step in atherogenesis.     

Extrusion of cells with inappropriate Dpp signaling from Drosophila wing disc epithelia

Decapentaplegic (Dpp) is a signaling molecule that controls growth and patterning of the developing Drosophila wing. Mutant cells lacking Dpp signal transduction have been shown to activate c-Jun amino-terminal kinase (JNK)-dependent apoptosis and to be lost from the wing disc epithelium. These observations have led to the hypothesis that Dpp promotes cell survival by preventing apoptosis. Here, we show that in the absence of JNK-dependent apoptosis, mutant cells lacking Dpp signal transduction can survive; however, they are still lost from the wing disc epithelium. This loss correlates with extensive cytoskeletal changes followed by basal epithelial extrusion. We propose that Dpp promotes cell survival within disc epithelia by affecting cytoskeletal organization.     

Coupling of stress in the ER to activation of JNK protein kinases by transmembrane protein kinase IRE1

Malfolded proteins in the endoplasmic reticulum (ER) induce cellular stress and activate c-Jun amino-terminal kinases (JNKs or SAPKs). Mammalian homologs of yeast IRE1, which activate chaperone genes in response to ER stress, also activated JNK, and IRE1alpha-/- fibroblasts were impaired in JNK activation by ER stress. The cytoplasmic part of IRE1 bound TRAF2, an adaptor protein that couples plasma membrane receptors to JNK activation. Dominant-negative TRAF2 inhibited activation of JNK by IRE1. Activation of JNK by endogenous signals initiated in the ER proceeds by a pathway similar to that initiated by cell surface receptors in response to extracellular signals.     

Graded regulation of the Kv2.1 potassium channel by variable phosphorylation

Dynamic modulation of ion channels by phosphorylation underlies neuronal plasticity. The Kv2.1 potassium channel is highly phosphorylated in resting mammalian neurons. Activity-dependent Kv2.1 dephosphorylation by calcineurin induces graded hyperpolarizing shifts in voltage-dependent activation, causing suppression of neuronal excitability. Mass spectrometry-SILAC (stable isotope labeling with amino acids in cell culture) identified 16 Kv2.1 phosphorylation sites, of which 7 were dephosphorylated by calcineurin. Mutation of individual calcineurin-regulated sites to alanine produced incremental shifts mimicking dephosphorylation, whereas mutation to aspartate yielded equivalent resistance to calcineurin. Mutations at multiple sites were additive, showing that variable phosphorylation of Kv2.1 at a large number of sites allows graded activity-dependent regulation of channel gating and neuronal firing properties.     

Identification of SCF ubiquitin ligase substrates by global protein stability profiling

Ubiquitin-mediated proteolysis regulates all aspects of cellular function, and defects in this process are associated with human diseases. The limited number of identified ubiquitin ligase-substrate pairs is a major bottleneck in the ubiquitin field. We established and applied genetic technologies that combine global protein stability (GPS) profiling and genetic perturbation of E3 activity to screen for substrates of the Skp1-cullin-F-box (SCF) ubiquitin ligase in mammalian cells. Among the >350 potential substrates identified, we found most known SCF targets and many previously unknown substrates involved in cell cycle, apoptosis, and signaling pathways. Exploring cell cycle-stage stability, we found that several substrates used the SCF and other E3s in different cell cycle stages. Our results demonstrate the potential of these technologies as general platforms for the global discovery of E3-substrate regulatory networks.     

Noncytotoxic lytic granule-mediated CD8+ T cell inhibition of HSV-1 reactivation from neuronal latency

Reactivation of herpes simplex virus type 1 (HSV-1) from neuronal latency is a common and potentially devastating cause of disease worldwide. CD8+ T cells can completely inhibit HSV reactivation in mice, with interferon-gamma affording a portion of this protection. We found that CD8+ T cell lytic granules are also required for the maintenance of neuronal latency both in vivo and in ex vivo ganglia cultures and that their directed release to the junction with neurons in latently infected ganglia did not induce neuronal apoptosis. Here, we describe a nonlethal mechanism of viral inactivation in which the lytic granule component, granzyme B, degrades the HSV-1 immediate early protein, ICP4, which is essential for further viral gene expression.     

健脾补土组方对体外神经元低氧/复氧后PI3K、Akt、Caspase-3表达的影响

目的 观察健脾补土组方对新生SD鼠神经元体外培养低氧/复氧后的凋亡情况以及PI3K、Akt、Caspase-3表达的影响,探讨其减少神经元凋亡的作用机制。方法 原代分离培养新生SD鼠神经元,随机分为正常血清对照组、低氧模型组、神经生长因子组、健脾补土组。将标本低氧诱导24 h,再复氧培养4 h进行造模。用流式细胞术检测神经元凋亡情况,RT-qPCR法检测PI3K、Akt、caspase-3 mRNA的表达,免疫细胞化学法和Western blot法检测PI3K、Akt、caspase-3蛋白的表达。结果 与正常对照组相比,低氧模型组神经元凋亡率显著增加,PI3K、Akt mRNA与蛋白表达显著减少,Caspase-3 mRNA与蛋白表达显著增加;与低氧模型组比较,神经生长因子组、健脾补土组神经元凋亡率均显著减少,PI3K、Akt mRNA及蛋白表达显著增强,Caspase-3 mRNA及蛋白表达显著较少（P<0.01）;与神经生长因子组比较,健脾补土组神经元凋亡率显著减少,PI3K、Akt mRNA及蛋白表达显著增加,Caspase-3 mRNA及蛋白表达显著降低。结论 健脾补土组方能减少脑缺血后神经元的凋亡,其作用机制可能与其通过上调PI3K和Akt的表达水平,最终抑制在细胞凋亡中起关键作用的因子Caspase-3的表达有关。     

Cell type-specific loss of BDNF signaling mimics optogenetic control of cocaine reward

The nucleus accumbens is a key mediator of cocaine reward, but the distinct roles of the two subpopulations of nucleus accumbens projection neurons, those expressing dopamine D1 versus D2 receptors, are poorly understood. We show that deletion of TrkB, the brain-derived neurotrophic factor (BDNF) receptor, selectively from D1+ or D2+ neurons oppositely affects cocaine reward. Because loss of TrkB in D2+ neurons increases their neuronal excitability, we next used optogenetic tools to control selectively the firing rate of D1+ and D2+ nucleus accumbens neurons and studied consequent effects on cocaine reward. Activation of D2+ neurons, mimicking the loss of TrkB, suppresses cocaine reward, with opposite effects induced by activation of D1+ neurons. These results provide insight into the molecular control of D1+ and D2+ neuronal activity as well as the circuit-level contribution of these cell types to cocaine reward.     

Amyloid beta protein enhances the survival of hippocampal neurons in vitro

The beta-amyloid protein is progressively deposited in Alzheimer's disease as vascular amyloid and as the amyloid cores of neuritic plaques. Contrary to its metabolically inert appearance, this peptide may have biological activity. To evaluate this possibility, a peptide ligand homologous to the first 28 residues of the beta-amyloid protein (beta 1-28) was tested in cultures of hippocampal pyramidal neurons for neurotrophic or neurotoxic effects. The beta 1-28 appeared to have neurotrophic activity because it enhanced neuronal survival under the culture conditions examined. This finding may help elucidate the sequence of events leading to plaque formation and neuronal damage in Alzheimer's disease.     

Accumulation of phosphorylated repressor for gibberellin signaling in an F-box mutant

Gibberellin (GA) regulates growth and development in plants. We isolated and characterized a rice GA-insensitive dwarf mutant, gid2. The GID2 gene encodes a putative F-box protein, which interacted with the rice Skp1 homolog in a yeast two-hybrid assay. In gid2, a repressor for GA signaling, SLR1, was highly accumulated in a phosphorylated form and GA increased its concentration, whereas SLR1 was rapidly degraded by GA through ubiquitination in the wild type. We conclude that GID2 is a positive regulator of GA signaling and that regulated degradation of SLR1 is initiated through GA-dependent phosphorylation and finalized by an SCF(GID2)-proteasome pathway.     

2 种原代培养方法对新生大鼠大脑皮质神经元生物学特性的影响

为明确不同培养方法对新生大鼠大脑皮质神经元成熟时间、形态特征、纯度及活力等生物学特性的影响,采用DMEM培养基加Neurobasal无血清培养基法或Neurobasal无血清培养基法原代培养新生24h内SD大鼠大脑皮质神经元,倒置显微镜下观察细胞形态,MTT法检测细胞活力,免疫荧光细胞化学染色法检测神经元纯度及活性.结果显示,2种方法培养的神经元形态差异无统计学意义;但与DMEM培养基加Neurobasal无血清培养基法相比,Neurobasal无血清培养基法培养的神经元成熟更早,数目更多,纯度更高,活力更强(p0.05).结果提示,原代培养的大脑皮质神经元部分生物学特性受培养方法直接影响;Neurobasal无血清培养基法所得神经元纯度与活性较高,这为实验目的导向的神经元原代培养方法选择提供了借鉴.     

NMDA receptor-mediated K+ efflux and neuronal apoptosis

Neuronal death induced by activating N-methyl-D-aspartate (NMDA) receptors has been linked to Ca2+ and Na+ influx through associated channels. Whole-cell recording from cultured mouse cortical neurons revealed a NMDA-evoked outward current, INMDA-K, carried by K+ efflux at membrane potentials positive to -86 millivolts. Cortical neurons exposed to NMDA in medium containing reduced Na+ and Ca2+ (as found in ischemic brain tissue) lost substantial intracellular K+ and underwent apoptosis. Both K+ loss and apoptosis were attenuated by increasing extracellular K+, even when voltage-gated Ca2+ channels were blocked. Thus NMDA receptor-mediated K+ efflux may contribute to neuronal apoptosis after brain ischemia.     

CB1 cannabinoid receptors and on-demand defense against excitotoxicity

Abnormally high spiking activity can damage neurons. Signaling systems to protect neurons from the consequences of abnormal discharge activity have been postulated. We generated conditional mutant mice that lack expression of the cannabinoid receptor type 1 in principal forebrain neurons but not in adjacent inhibitory interneurons. In mutant mice,the excitotoxin kainic acid (KA) induced excessive seizures in vivo. The threshold to KA-induced neuronal excitation in vitro was severely reduced in hippocampal pyramidal neurons of mutants. KA administration rapidly raised hippocampal levels of anandamide and induced protective mechanisms in wild-type principal hippocampal neurons. These protective mechanisms could not be triggered in mutant mice. The endogenous cannabinoid system thus provides on-demand protection against acute excitotoxicity in central nervous system neurons.     

Action-potential modulation during axonal conduction

Once initiated near the soma, an action potential (AP) is thought to propagate autoregeneratively and distribute uniformly over axonal arbors. We challenge this classic view by showing that APs are subject to waveform modulation while they travel down axons. Using fluorescent patch-clamp pipettes, we recorded APs from axon branches of hippocampal CA3 pyramidal neurons ex vivo. The waveforms of axonal APs increased in width in response to the local application of glutamate and an adenosine A(1) receptor antagonist to the axon shafts, but not to other unrelated axon branches. Uncaging of calcium in periaxonal astrocytes caused AP broadening through ionotropic glutamate receptor activation. The broadened APs triggered larger calcium elevations in presynaptic boutons and facilitated synaptic transmission to postsynaptic neurons. This local AP modification may enable axonal computation through the geometry of axon wiring.