右归丸对大鼠膝骨关节炎的干预作用及其机制*

目的: 探讨右归丸对膝骨关节炎(KOA)模型鼠关节软骨组织骨诱导因子(OGN)、骨黏连蛋白(ON)和纤维蛋白原2(FBN2)的影响。方法: 将大鼠随机分为假手术组,模型组,硫酸氨基葡萄糖组(硫酸氨基葡萄糖),右归丸(高、中、低剂量)组,每组10只。采用改良Hulth法制备大鼠KOA模型,假手术组和模型组给予等体积生理盐水灌胃,右归丸高、中、低剂量组分别灌胃给予右归丸4.8,2.4,1.2 g/kg,硫酸氨基葡萄糖组灌胃给予硫酸氨基葡萄糖 0.17 g/kg,连续给药8周。干预结束24 h后取鼠膝关节软骨,采用HE染色法观察各组软骨的病理改变,并进行Mankin评分;免疫组化法检测各组关节软骨组织中OGN、ON和FBN2的表达;Western blot法检测各组关节软骨组中糖原合成酶激酶-3β(GSK-3β)的表达。结果: 与假手术组比较,模型组大鼠软骨组织Makin 评分显著升高,软骨组织FBN2蛋白表达水平上显著增加,OGN、ON和GSK-3β蛋白表达水平上的显著降低(P＜0.01);模型组关节软骨边缘严重破坏,软骨细胞排列紊乱。与模型组比较,右归丸高剂量干预组大鼠软骨组织Makin 评分和FBN2蛋白表达水平显著降低,GSK-3β蛋白表达水平上显著增加,且右归丸中、高剂量组OGN、ON蛋白表达水平均显著增加(P＜0.05或P＜0.01),软骨结构趋于正常,软骨细胞分布仅偶见不均,关节软骨表面欠光滑。结论: 右归丸能够延缓关节软骨退变,其可能机制是通过提高骨诱导因子和骨粘连蛋白的表达水平来促进关节软骨的骨化和重构。     

SR 59230A对心衰大鼠心脏MicroRNAs表达的影响

目的：观察β3肾上腺素受体（β3-AR）对心衰大鼠心脏MicroRNAs表达的影响及可能的作用机制。方法：大鼠冠脉左前降支结扎造成心衰模型,假手术大鼠只穿线不结扎。造模成功大鼠再随机分为：心衰组（CHF control group）和心衰+SR 59230A组（CHF+SR group）;假手术大鼠也随机分为假手术组（Sham group）和假手术+SR 59230A组（Sham+SR group）。Sham+SR组和CHF+SR组每天两次腹腔注射SR （85 mmol/L,1 ml）,连续注射7周。结果：①miScript miRNA PCR Arrays显示,在体阻断β3-AR后,假手术组与心衰组有18种MicroRNAs共同表达下调;经文献比对,与NF-κB相关的MicroRNAs有6种,分别为miR-125b-5p,miR-143-3p,miR-145-5p,miR-26a-5p,miR-30a-5p和miR-320-5p。②大鼠心脏组织切片观察到NF-κB在心衰大鼠心肌细胞核与细胞质中均有分布,而p53在心肌细胞质分布较多,NF-κB和p53表达明显高于假手术组（P<0.05）。阻断β3-AR后,心衰组心脏NF-κB和p53表达显著减少（P<0.05）,而假手术组NF-κB和p53表达略增加（P<0.05）。③Western blot结果发现心衰大鼠NF-κB p65表达高于假手术组（P<0.05）,给予β3-AR阻断剂后,心衰组心脏NF-κB p65和p53-Phospho-Serine 15表达均下降（P<0.05）,而假手术组心脏阻断β3-AR后,NF-κB、p53和p53-Phospho-Serine 15表达均增加（P<0.05）。结论：阻断β3肾上腺素受体有利于缓解心衰大鼠心脏的损伤;β3-AR可引起MicroRNAs表达变化且与NF-κB信号通路有关。     

姜黄素对过度训练大鼠肾脏细胞凋亡的调控作用及其机制

目的：探讨姜黄素对过度训练所致大鼠氧化应激、肾脏细胞凋亡的作用及其机制。方法：7周龄SPF级雄性Wistar大鼠分为对照组（C组,n=12）、过度训练组（OM组,n=11）、姜黄素+过度训练组（COM组,n=14）。C组不进行任何运动干预,OM组、COM组大鼠进行8周递增负荷游泳训练。训练期间,COM组以200 mg/（kg·d）、5 ml/kg姜黄素进行灌胃,其他组灌胃等体积0.5%浓度羧甲基纤维素纳。末次训练后24 h,光镜观察肾脏组织病理学改变,取血液、肾脏组织检测相关生化指标。结果：8周递增负荷游泳训练后,光镜下C组大鼠肾脏组织结构正常;OM组出现组织病理学改变;COM组较OM组减轻。与C组比较,OM组血清皮质酮（Cor）、肌酐（Cr）和尿素氮（BUN）水平均升高（P<0.01）,睾酮（T）水平降低（P<0.01）;肾脏核因子E2相关因子2（Nrf2）表达水平无显著变化（P>0.05）,血红素氧合酶1（HO-1）表达水平降低（P<0.05）,总抗氧化能力（T-AOC）和超氧化物歧化酶（SOD）活性降低（P<0.01）,丙二醛（MDA）浓度升高（P<0.01）;肾脏细胞凋亡水平升高（P<0.01）,肾脏抗凋亡蛋白B淋巴细胞瘤因子-2（Bcl-2）表达减弱（P<0.01）,促凋亡蛋白Bcl-2相关X蛋白（Bax）表达增强（P<0.01）。与OM组比较,COM组血清Cor水平（P<0.01）降低,T水平升高（P<0.01）,Cr和BUN水平均降低（P<0.05）;肾脏Nrf2和HO-1表达增强（P<0.05）,T-AOC和SOD活性升高（P<0.01）,MDA浓度降低（P<0.05）;肾脏细胞凋亡水平降低（P<0.05）,Bcl-2表达增强（P<0.05）,Bax表达减弱（P<0.01）。组间T/Cor比值变化趋势与T变化相一致,Bcl-2/Bax比值变化趋势与Bcl-2变化相一致。结论：8周递增负荷游泳训练引发大鼠过度训练,氧化应激加剧并加速肾脏细胞凋亡,肾脏组织发生病理改变及功能异常。姜黄素通过上调Nrf2、HO-1蛋白表达,有效缓解过度训练引发的氧化应激,从而增强Bcl-2表达,减弱Bax表达,抑制大鼠肾脏细胞凋亡,保护肾脏组织结构和功能正常。     

Wnt/β-catenin信号通路对重度子痫前期患者胎盘滋养层细胞侵袭和增殖的影响

为了阐明Wnt/β-catenin信号通路在子痫前期发生发展中的作用机制,本研究应用RT-PCR检测了子痫前期和正常妊娠妇女胎盘中的Wnt1、β-catenin和cyclinD1的mRNA水平。通过Western blotting检测了Wnt1、β-catenin、Dickkopf-1 (DKK1)和糖原合成酶激酶3β(GSK-3β)蛋白的表达水平。使用免疫组化定位胎盘中Wnt1、β-catenin和DKK1蛋白的表达。研究显示,与对照组正常胎盘相比,重度子痫前期胎盘中Wnt1、β-catenin和cyclinD1的mRNA表达水平显著降低。Western blotting结果显示,对照组Wnt1、β-catenin和GSK-3β蛋白表达水平显著升高,而DKK1表达水平显著降低。此外,与对照组相比,子痫前期组胎盘中Wnt1和β-catenin的染色强度较弱,而DKK1的染色强度明显增强。说明子痫前期患者胎盘中Wnt/β-catenin信号通路及其下游靶基因被抑制,导致滋养层的侵袭和增殖能力降低,从而促进了子痫前期的发生发展。     

蒲公英多糖对溃疡性结肠炎大鼠IL-6/STAT3信号通路的影响

目的：观察蒲公英多糖对溃疡性结肠炎大鼠模型IL-6/STAT3信号通路的调控作用。方法：清洁级SD大鼠40只,雌雄各半,随机分为4组（n=10）：空白组、模型组、阳性对照组、蒲公英多糖组。采用2,4,6-三硝基苯磺酸（TNBS）诱导结肠炎大鼠模型,阳性对照组采用美沙拉嗪10 mg/kg·d灌胃,蒲公英多糖组采用蒲公英多糖10 mg/kg·d灌胃,治疗4周后处死,观察大鼠结肠粘膜病理改变,检测大鼠血清白介素-6（IL-6）含量、结肠髓过氧化物酶（MPO）、白介素-6受体（sIL-6Rα）、糖蛋白130（gp130）、转录活化因子3（STAT3）、IL-6 mRNA表达。结果：与正常组比较,模型组大鼠血清IL-6含量明显升高（P<0.01）,MPO阳性密度明显增高（P<0.01）,sIL-6Rα、gp130含量明显增高（P<0.01）,肠组织STAT3、IL-6 mRNA相对表达量明显增高（P<0.01）;与模型组比较蒲公英多糖组、美沙拉嗪组大鼠血清IL-6含量明显降低（P<0.01）,MPO阳性密度明显降低（P<0.01）,sIL-6Rα、gp130含量明显降低（P<0.01）,肠组织STAT3、IL-6 mRNA相对表达量与模型组比较明显降低（P<0.05）。结论：蒲公英多糖能够降低溃疡性结肠炎大鼠IL-6水平,下调IL-6/STAT3通路中sIL-6Rα、gp130蛋白表达量,进而下调大鼠肠组织STAT3、IL-6 mRNA的转录水平,缓解结肠组织的炎症状态,保护和修复粘膜组织,起到治疗溃疡性结肠炎的作用。     

电针联合艾灸对胶原诱导性关节炎大鼠关节的保护作用

为了探讨电针联合艾灸对胶原诱导性关节炎大鼠关节的影响,本研究将40只雄性SD大鼠随机分为正常组、模型组、DKK1组和观察组,每组10只。对模型组、DKK1组和观察组大鼠建立胶原诱导性关节炎模型,DKK1组给予Wnt阻断剂DKK1,观察组给予电针联合艾灸,观察各组关节炎指数评分(AI)、放射评分及组织病理表现。采用Western blotting和RT-PCR检测Wnt3a、Wnt5和β-catenin蛋白及mRNA表达。研究显示,观察组和DKK1组造模后21 d、28 d的AI均明显低于模型组(p0.05);观察组和DKK1组造模后28 d的放射评分分别为(2.67±0.87)分和(2.67±0.87)分,明显低于模型组(p0.05);DKK1组和观察组的滑膜肿胀、炎性细胞浸润和骨质破坏情况均轻于模型组;DKK1组和观察组的Wnt3和β-catenin蛋白及m RNA相对表达均明显低于模型组(p0.05);各组Wnt5a蛋白及mRNA相对表达比较差异无统计学意义(p0.05)。研究说明,电针联合艾灸对胶原诱导性关节炎大鼠有较好的关节保护作用,可能与其调节Wnt3a、β-catenin表达有关。     

右美托咪定对大鼠I/R损伤肺组织TLR4表达及保护作用的影响

目的：探讨右美托嘧啶对大鼠再灌注损伤肺组织Toll样受体素4（TLR4）表达的调控,并分析其对肺保护作用机制。方法：采用大鼠在体左侧肺缺血/再灌注（I/R）模型,50只健康雄性成年SD大鼠随机分为5组（n=10）：对照组（Sham组）、缺血/再灌注组（I/R组）、右美托咪定组（Dex组）、阿替美唑组（Atip组）、右美托咪定+阿替美唑组（Dex+Atip组）,实验结束后处死大鼠,留取左肺,检测肺湿干重比（W/D）和总肺水含量（TLW）;光镜下观察肺组织形态结构变化;PCR检测肺组织TLR4 mRNA表达;Western blot检测肺组织TLR4的蛋白表达。结果：与Sham组相比,其余各组W/D和TLW明显升高（P<0.05,P<0.01）,TLR4 mRNA和蛋白表达量上升（P<0.01）,光镜显示肺组织结构出现明显损伤性变化;与I/R组相比,Dex组W/D和TLW下降（P<0.05,P<0.01）,TLR4 mRNA和蛋白表达量降低（P<0.01）,光镜下肺组织损伤减轻;与Dex组比较,Dex+Atip组W/D和TLW明显升高（P<0.05,P<0.01）,TLR4 mRNA和蛋白表达量上升（P<0.01）,光镜肺组织结构损伤严重;I/R组、Atip组、Dex+Atip组两两比较,以上各指标均无统计学差异（P > 0.05）。结论：I/R可引起大鼠肺组织TLR4表达上调和肺组织损伤;右美托咪啶可减轻肺I/R损伤,抑制TLR4表达,这种作用与α2-肾上腺素能受体有关。     

DKK3调控羊驼黑色素细胞中黑色素生成

Dickkopf-3 (DKK3),Wnt/β-catenin信号通路中一个重要的抑制因子,可能参与调控黑色素生成过程。本文研究了DKK3在羊驼黑色素细胞中黑色素生成的作用。在羊驼黑色素细胞中,过表达DKK3显著下调Wnt1,Lef1,Myc和黑色素生成相关基因MITF及其下游基因TYR,TYRP1和TYRP2的表达,在mRNA和蛋白质水平均明显下降(P<0.05);总黑素,褐黑素和真黑素的含量分别下降80.30%、72.17%和64.60% ( P <0.05)。相反,在羊驼黑色素细胞中转染siRNA-DKK3,一种小干扰RNA,可以显著上调Wnt1,Lef1,Myc和黑色素生成相关基因MITF及其下游基因TYR,TYRP1和TYRP2在mRNA和蛋白质水平的表达(P<0.05);总黑素、褐黑素和真黑素的含量分别增加1.65倍、1.25倍和1.21倍(P<0.05)。这些结果表明,DKK3可以通过Wnt/β-catenin信号通路介导MITF下调羊驼黑色素细胞中黑色素的生成。     

ERK1/2通路在低氧上调大鼠PASMCs钙激活性氯离子通道表达的作用

目的：探讨钙激活性氯离子通道（CLCA2）在大鼠低氧性肺动脉平滑肌细胞（PASMCs）中mRNA和蛋白表达的变化及其与ERK1/2信号通路的关系。方法：PASMCs随机分为：常氧组（N组）,低氧组（H组）,DMSO对照组（D组）,U0126干预组（U组）,Staurosporine aglycone干预组（SA组）,采用免疫印迹法检测CLCA2蛋白的表达;选用半定量逆转录-聚合酶链反应（RT-PCR）技术测定CLCA2 mRNA水平的表达。结果：PASMCs中CLCA2 mRNA和蛋白的表达量,H组较N组明显上调（P<0.01）;U组较D组明显上调（P<0.01）;SA组较D组mRNA的表达显著下调（P<0.01）,蛋白的表达轻微下调。结论：低氧可上调CLCA2中mRNA和蛋白在PASMCs的表达;ERK1/2通路激活剂-Staurosporine aglycone能下调CLCA2在PASMCs中mRNA和蛋白的表达量;ERK1/2通路抑制剂-U0126可上调CLCA2在PASMCs中mRNA和蛋白的表达量。     

不同强度运动结合白藜芦醇对老年肥胖大鼠RBP4的影响

目的：探讨不同强度运动结合白藜芦醇对老年肥胖大鼠内脏脂肪组织视黄醇结合蛋白4（RBP4） mRNA蛋白表达及血浆RBP4浓度的影响。方法：选择鼠龄3周的雄性SD大鼠80只,随机分为对照组和实验组：对照组（C）饲喂6.0%脂肪的普通饲料（n=12）;实验组分3个阶段饲喂36%~40%高脂饲料（n=68）。建立老年肥胖大鼠模型,选取24只建模成功的肥胖大鼠随机分为4组（n=6）：肥胖对照组（CO）、白藜芦醇组（RO）、低强度运动+白藜芦醇组（LRO）、中强度运动+白藜芦醇组（MRO）。LRO组和MRO组的运动强度分别为（12 m/min×15 min）和（15 m/min×15 min）,每天运动60 min;补充白藜芦醇各组52.5 mg/kg·d灌胃1次,对照组采用等量的纯净水灌胃,持续干预8周。8周后采血和肾周、睾周、血管及内脏脂肪组织,检测血糖和血浆RBP4浓度、计算胰岛素敏感性（ISI）,检测RBP4 mRNA和蛋白表达。结果：与正常组比较,模型组大鼠RBP4 mRNA和蛋白表达、血浆浓度及血糖指标明显升高（P<0.05,P<0.01）,ISI明显降低（P<0.05）;与模型组比较,RO、LRO组和MRO组大鼠RBP4 mRNA和蛋白表达、血浆浓度及血糖指标明显降低（P<0.05,P<0.01）,ISI明显升高（P<0.05）;RO、LRO组和MRO组之间比较,MRO组大鼠RBP4 mRNA和蛋白表达、血浆浓度及血糖指标明显降低,ISI明显升高,但无显著差异。结论：不同强度运动结合白藜芦醇能降低老年肥胖大鼠内脏脂肪组织RBP4 mRNA和蛋白表达及血浆RBP4浓度,受运动强度影响较小。     

R-spondin1 is a high affinity ligand for LRP6 and induces LRP6 phosphorylation and beta-catenin signaling

R-spondin proteins are newly identified secreted molecules that activate beta-catenin signaling. However, the mechanism of R-spondin action and its relationship with Wnt signaling remain unclear. Here we show that human R-spondin1 (hRspo1) is a high affinity ligand for the Wnt co-receptor LRP6 (K(d) = 1.2 nm). hRspo1 induces glycogen synthase kinase 3-dependent phosphorylation and activation of LRP6. DKK1, an LRP6 antagonist, inhibits hRspo1-induced LRP6 phosphorylation. We further demonstrate that hRspo1 synergizes with Frizzled5 in Xenopus axis induction assays and induces the phosphorylation of Dishevelled, a cytoplasmic component downstream of Frizzled function. Our study reveals interesting similarity and distinction between Wnt and R-spondin signaling.     

In silico studying of the whole protein structure and dynamics of Dickkopf family members showed that N-terminal domain of Dickkopf 2 in contrary to other Dickkopfs facilitates its interaction with low density lipoprotein receptor related protein 5/6

Wnt (Wingless Int) signaling pathway has been known to be dysregulated in several human cancers, especially colorectal cancer (CRC). The Dickkopf (DKK) family which consists of four secreted proteins in vertebrates (DKK 1, 2, 3, 4) is one of the most critical antagonist families for Wnt signaling pathway. They typically antagonize Wnt/β-catenin signaling by binding and inhibiting Wnt co-receptors, LRP5/6 (low density lipoprotein receptor related protein 5/6). However, except for DKK1 (Dickkopf 1), details about structure and function of the members of this family are poorly defined. In this study, main Dickkopf family members were analyzed structurally, using protein structure prediction tools, molecular dynamics (MD), molecular docking and energy analyses. Three dimensional structure of whole DKKs was predicted and their interaction with LRP6 was investigated in detail. The results indicated that in DKK family members, a considerable diversity, in the case of structure, activity and physicochemical properties was seen. This diversity was more profound in DKK3 (Dickkopf3). Interestingly, the interaction mode of DKK2 (Dickkopf2) with its receptor, LRP6, was shown to be substantially different from other Dickkopf family members while N-terminal region of this ligand was also involved in the binding to the LRP6-P3P4. Moreover, the cysteine-rich domain 2 (CRD2) of DKK1 and DKK3 had a higher binding affinity to LRP6 in comparison with the whole protein structures.

Communicated by Ramaswamy H. Sarma     

重组水蛭素的抗血栓形成作用及其机制

目的：研究重组水蛭素抗血栓形成的作用及机制。方法：将60只雄性昆明小鼠随机分为对照组、模型组、阿司匹林组和重组水蛭素低、中、高剂量组（n=10）。除对照组外,其余各组小鼠分别腹腔注射角叉菜胶2.5 mg/kg,诱发小鼠尾部血栓形成。注射角叉菜胶前24 h、0.5 h和注射后24 h,阿司匹林组小鼠分别腹腔注射阿司匹林25 mg/kg,重组水蛭素低、中、高剂量组小鼠分别腹腔注射0.05、0.1、0.2 mg/kg重组水蛭素,对照组和模型组小鼠分别腹腔注射等体积生理盐水。注射角叉菜胶后48 h,观察小鼠黑尾长度并计算黑尾发生率;检测血浆凝血酶原时间（PT）、活化部分凝血活酶时间（APTT）、组织型纤溶酶原激活剂（t-PA）、纤溶酶原激活物抑制因子-1（PAI-1）、6-酮-前列腺素F1α（6-keto-PGF1α）、血栓恶烷B2（TXB2）水平。结果：与对照组比较,模型组小鼠尾部形成血栓;血浆PT明显缩短（P<0.01）,PAI-1、TXB2水平明显升高（P<0.01）,t-PA、6-keto-PGF1α水平明显降低（P<0.01）。与模型组比较,重组水蛭素低、中、高剂量组和阿司匹林组小鼠尾部血栓长度明显缩短（P<0.05或P<0.01）,PT明显延长（P<0.01）,PAI-1、TXB2水平明显降低（P<0.01）,t-PA、6-keto-PGF1α水平明显升高（P<0.01）。与阿司匹林组比较,重组水蛭素低剂量组小鼠尾部血栓长度明显增加（P<0.05）,PT明显缩短（P<0.01）,PAI-1、TXB2水平明显升高（P<0.01）;重组水蛭素低、中剂量组6-keto-PGF1α水平明显降低（P<0.01,P<0.05）;重组水蛭素中剂量组PAI-1、TXB2水平明显升高（P<0.01,P<0.05）。结论：重组水蛭素有明显抗血栓形成作用,其机制可能与影响外源性凝血系统、促进纤溶功能有关。     

The LRP5 high-bone-mass G171V mutation disrupts LRP5 interaction with Mesd

The mechanism by which the high-bone-mass (HBM) mutation (G171V) of the Wnt coreceptor LRP5 regulates canonical Wnt signaling was investigated. The mutation was previously shown to reduce DKK1-mediated antagonism, suggesting that the first YWTD repeat domain where G171 is located may be responsible for DKK-mediated antagonism. However, we found that the third YWTD repeat, but not the first repeat domain, is required for DKK1-mediated antagonism. Instead, we found that the G171V mutation disrupted the interaction of LRP5 with Mesd, a chaperone protein for LRP5/6 that is required for transport of the coreceptors to cell surfaces, resulting in fewer LRP5 molecules on the cell surface. Although the reduction in the number of cell surface LRP5 molecules led to a reduction in Wnt signaling in a paracrine paradigm, the mutation did not appear to affect the activity of coexpressed Wnt in an autocrine paradigm. Together with the observation that osteoblast cells produce autocrine canonical Wnt, Wnt7b, and that osteocytes produce paracrine DKK1, we think that the G171V mutation may cause an increase in Wnt activity in osteoblasts by reducing the number of targets for paracrine DKK1 to antagonize without affecting the activity of autocrine Wnt.     

Wnt proteins induce dishevelled phosphorylation via an LRP5/6- independent mechanism, irrespective of their ability to stabilize beta-catenin

Wnt glycoproteins play essential roles in the development of metazoan organisms. Many Wnt proteins, such as Wnt1, activate the well-conserved canonical Wnt signaling pathway, which results in accumulation of beta-catenin in the cytosol and nucleus. Other Wnts, such as Wnt5a, activate signaling mechanisms which do not involve beta-catenin and are less well characterized. Dishevelled (Dvl) is a key component of Wnt/beta-catenin signaling and becomes phosphorylated upon activation of this pathway. In addition to Wnt1, we show that several Wnt proteins, including Wnt5a, trigger phosphorylation of mammalian Dvl proteins and that this occurs within 20 to 30 min. Unlike the effects of Wnt1, phosphorylation of Dvl in response to Wnt5a is not concomitant with beta-catenin stabilization, indicating that Dvl phosphorylation is not sufficient to activate canonical Wnt/beta-catenin signaling. Moreover, neither Dickkopf1, which inhibits Wnt/beta-catenin signaling by binding the Wnt coreceptors LRP5 and -6, nor dominant-negative LRP5/6 constructs could block Wnt-mediated Dvl phosphorylation. We conclude that Wnt-induced phosphorylation of Dvl is independent of LRP5/6 receptors and that canonical Wnts can elicit both LRP-dependent (to beta-catenin) and LRP-independent (to Dvl) signals. Our data also present Dvl phosphorylation as a general biochemical assay for Wnt protein function, including those Wnts that do not activate the Wnt/beta-catenin pathway.     

DKK1 antagonizes Wnt signaling without promotion of LRP6 internalization and degradation

DKK1 is a secreted protein that antagonizes Wnt signaling and plays essential roles in vertebrate embryogenesis including head induction, skeletal development, and limb patterning. DKK1 is also implicated in osteoporosis, arthritis, and cancer and represents a potential therapeutic target for the treatment of these diseases. DKK1 is a high affinity antagonistic ligand for LRP6, which is a Wnt coreceptor that acts together with the Frizzled serpentine receptor to initiate Wnt signal transduction. Two different models have been proposed to account for the mechanism by which DKK1 antagonizes LRP6 function. One model suggests that DKK1 binding to LRP6 disrupts Wnt-induced Frizzled-LRP6 complex formation, whereas the other model proposes that DKK1 interaction with LRP6 promotes LRP6 internalization and degradation, thereby reducing the cell surface LRP6 level. To clarify the molecular basis of DKK1 action, we examined how DKK1 affects the endogenous LRP6 in several mammalian cell lines including mouse embryonic fibroblasts. Here we show that DKK1 inhibits Wnt signaling but induces neither LRP6 down-regulation from the cell surface nor reduction of total LRP6 protein level and that DKK1 has no effect on the rate of continuous internalization of LRP6 and the half-life (about 4.7 h) of LRP6. We conclude that DKK1 inhibition of LRP6 is independent of LRP6 internalization and degradation.     

Wnt antagonists bind through a short peptide to the first β-propeller domain of LRP5/6

The Wnt pathway inhibitors DKK1 and sclerostin (SOST) are important therapeutic targets in diseases involving bone loss or damage. It has been appreciated that Wnt coreceptors LRP5/6 are also important, as human missense mutations that result in bone overgrowth (bone mineral density, or BMD, mutations) cluster to the E1 propeller domain of LRP5. Here, we report a crystal structure of LRP6 E1 bound to an antibody, revealing that the E1 domain is a peptide recognition module. Remarkably, the consensus E1 binding sequence is a close match to a conserved tripeptide motif present in all Wnt inhibitors that bind LRP5/6. We show that this motif is important for DKK1 and SOST binding to LRP6 and for inhibitory function, providing a detailed structural explanation for the effect of the BMD mutations.