mTOR信号通路对脂肪生成的调控作用

脂肪组织是一种主要的能量储存和内分泌器官。脂肪生成是一系列复杂的细胞分化过程,受到细胞营养水平、激素和代谢物等调节。哺乳动物雷帕霉素靶蛋白(mammalian target of rapamycin, mTOR)复合物包括哺乳动物雷帕霉素靶蛋白复合体1(mammalian target of rapamycin complex 1,mTORC1)和mTORC2两种蛋白质复合体。mTOR复合物含有的脂质激酶样域奠定了mTOR通路调控脂肪生成的基础。对mTORC1和mTORC2的部分组成蛋白质研究也验证了mTOR调控成脂的功能。基于前期的研究,我们综述了miR-199a-3p、miR-103、miR-188、68 kD有丝分裂中的Src相关底物(Src-associated substrate in mitosis of 68 kD,Sam68)、内皮抑素等物质通过mTORC1和mTORC2蛋白质复合体调控脂肪生成的机制。同时,进一步构建了包括胰岛素/IGF通路、PI3K-AKT通路、氨基酸通路、AMPK通路、cAMP通路、cGMP通路、NOTCH通路以及影响上述通路的bta-miR-150、4-O-甲基蔗糖和多种蛋白质在内的mTOR信号通路调控脂肪生成的网络。本文主要综述了mTOR复合物的特性和mTOR通路调控脂肪生成方面的最新研究进展,指出mTORC2具有调控脂质摄取和脂质分解的作用,调控mTORC1功能的作用,但是有关mTORC2的研究相对mTORC1较少,因此,对脂肪生成和脂质代谢的进一步研究需要集中于mTORC2。     

植物特有微丝结合蛋白研究进展

微丝骨架存在于所有真核生物,并且参与了诸多重要的细胞生物学过程和生理活动.微丝骨架在不同的细胞类型和生物学过程中呈现出各自特异的结构和动态变化模式,此过程由许多不同生化特性的微丝结合蛋白(ABPs)家族成员直接调控.不同真核生物的肌动蛋白(actin)都具有较高的同源性和相似的调控过程,但是由于动物和植物在某些特定的组织结构、生理特性等方面存在本质上的不同,进而导致动植物的微丝动态调控机制存在一定差异.ABPs和actin存在共进化的关系,由此许多动物和植物的ABPs也相应地产生了差异.例如,许多哺乳动物和酵母中保守的ABPs在植物基因组中不存在;一系列植物特有的ABPs被相继发现;部分哺乳动物中保守的ABPs成员或一些非ABPs在植物中也演化出了新的生化特性和微丝调控活性.本文概述了目前已发现的植物特有ABPs及其生化特性和生理学功能,归纳了植物中探究新ABPs的研究思路和方法,并对未来植物微丝骨架的研究方向和可能的研究热点进行了展望.     

浅谈物质代谢网络的交汇点——mTOR

当营养、生长因子和能量代谢驱动碳水化合物分解功能,加速蛋白质、脂肪及氨基酸的合成代谢时,细胞便加速生长繁殖。哺乳类动物雷帕霉素靶蛋白(mammalian target of rapamycin,m TOR)可整合营养、生长因子及能量等外界因素对细胞的刺激,调控细胞的增殖分化。m TOR存在两种复合体形式:mTORC1和m TORC2。mTORC1可接受来自生长因子、氨基酸、能量及炎症反应等多种信号,促进细胞增殖,在维持代谢稳态中具有重要作用。当缺失mTORC1时,细胞生长及蛋白质合成受到抑制并诱导自噬的发生。m TORC2则与细胞骨架、脂类分解、胰岛素抵抗、Akt等激酶的激活作用相关。本文中我们将综述近期发现的m TOR上下游信号分子,系统解析m TOR通过调节蛋白质、核酸和脂类代谢来促进机体生长和细胞增殖的分子机制。     

PI3K/Akt/mTOR信号通路及临床相关肿瘤抑制剂

哺乳动物雷帕霉素靶（mTOR）和蛋白激酶B（Akt/PKB）与肿瘤发生的密切关系已被广泛地认可.mTOR是一种丝/苏氨酸激酶,可以通过影响mRNA转录、代谢、自噬等方式调控细胞的生长.它既是PI3K的效应分子,也可以是PI3K的反馈调控因子.mTORC1 和mTORC2是mTOR的两种不同复合物. 对雷帕霉素敏感的mTORC1受到营养、生长因子、能量和应激4种因素的影响.生长因子通过PI3K/Akt信号通路调控mTORC1是最具特征性调节路径.而mTORC2最为人熟知的是作为Akt473磷酸化位点的上游激酶. 同样,Akt/PKB在细胞增殖分化、迁移生长过程中发挥着重要作用. 随着Thr308和Ser473两个位点激活,Akt/PKB也得以全面活化.因此,mTORC2-Akt-mTORC1的信号通路在肿瘤形成和生长中是可以存在的.目前临床肿瘤治疗中,PI3K/Akt/mTOR是重要的靶向治疗信号通路.然而,仅抑制mTORC1活性,不是所有的肿瘤都能得到预期控制.雷帕霉素虽然能抑制mTORC1,但也能反馈性地增加PI3K信号活跃度,从而影响治疗预后.近来发现的第二代抑制剂可以同时抑制mTORC1/2和PI3K活性,这种抑制剂被认为在肿瘤治疗上颇具前景.本综述着重阐述了PI3K/Akt/mTOR信号通路的传导、各因子之间的相互调控以及相关抑制剂的发展.     

植物microRNA跨界调控作用

MicroRNAs (miRNAs)是一类19～24个核苷酸的非编码内源性小分子RNA,功能研究表明，miRNAs参与所有细胞过程的调节，包括生长发育、分化凋亡、炎症、免疫应答、代谢及信号转导等生物学过程。最近研究发现，植物miRNAs作为一类功能广泛的分子调控因子，不仅调控植物自身基因的表达，外源植物miRNAs还通过饮食进入到哺乳动物细胞中，调节某些与人类疾病治疗相关的基因或生物学过程。一些植物miRNAs由于其3′端存在2′-O-甲基化修饰、独特茎环结构、高GC含量以及微囊泡、RNA结合蛋白或外泌体样纳米颗粒等载体对其的保护作用，从而在食物加工与极端胃肠道环境中能保持高度稳定。植物miRNAs主要通过饮食获取，借助跨膜miRNA转运蛋白质、受体介导的内吞作用、肠上皮细胞间隙扩散、肠道微生物群脂质依赖性摄取等不同的转运机制，其被消化系统吸收后进入体循环并递送至器官组织，再调节受体细胞功能。本文综述了植物来源miRNAs的跨界调控研究现状，详细总结了植物miRNAs抵抗降解的可能方法、跨越胃肠道屏障的转运机制以及被摄入后在哺乳动物体内的调控作用，例如抗炎、抗病毒和抗肿瘤等相关发现，揭...     

MicroRNAs对动物皮肤和毛发发育调控作用的研究进展

MicroRNAs是近年来发现的一类由19-25个核苷酸组成的非编码单链小RNA分子,它们通过与靶基因mRNA3’UTR结合抑制靶基因的翻译,在转录后水平调控基因表达.MicroRNAs参与了包括细胞分化、增殖和凋亡及免疫系统应答在内的一系列发育调控和生物学过程.最近研究发现MicroRNAs在多种哺乳动物皮肤中均表达,并参与了哺乳动物皮肤及毛发发育的调控过程,这些都为研究这个新颖的调控因子在干细胞生物学和发育生物学中的功能奠定了基础.本文综述了近年来MicroRNAs对哺乳动物皮肤和毛发发育调控作用的研究状况.     

氨基酸调节mTORC1信号通路的分子机制

正哺乳动物雷帕霉素靶蛋白(mTOR)是存在于哺乳动物体内的一种高度保守的丝氨酸/苏氨酸蛋白激酶,能够调节细胞内多种物质的代谢。它参与组成哺乳动物雷帕霉素靶蛋白复合体1(mTORC1)和哺乳动物雷帕霉素靶蛋白复合体2(mTORC2)2种复合体。在2015年发现的人类mT ORC1结构基础上,Saxton等人揭示了亮氨酸对于mT ORC1通路复杂的调控机制。他     

mTOR信号通路及其与阿尔茨海默病的关系

哺乳动物雷帕霉素靶蛋白mTOR是一个进化上十分保守的蛋白激酶,属于PIKK超家族。在细胞内mTOR存在两种功能不同的复合体mTORC1和mTORC2。mTOR主要通过接受上游信号分子Rheb、TSC1/TSC2的调控来整合细胞内外信号,其下游效应器是4E-BP和p70S6K,通过影响特定mRNA的翻译调节细胞的生长和增殖。在神经系统方面,神经元的发育、突触可塑性的调节、学习和记忆的形成都依赖于适当的mTOR通路的活化。新近的研究显示,神经退行性疾病阿尔茨海默病患者表现mTOR通路的异常,在双转基因鼠中,APP和PS1表达与mTOR/P70S6K下调关联,并影响精神状态评分。mTOR信号通路生理功能和调节机制的研究对了解AD的发病机理和寻找药物靶点具有重要意义。     

小分子量G蛋白Rap2信号途径的生物学功能

Rap2与Rap1同属于Ras超家族小分子量GTP结合蛋白的Rap亚家族,Rap2的氨基酸序列与Rap1具有60%的同源性,推测二者可能具有相似的信号途径和相近的生物学功能,包括细胞的增殖、分化、粘附和细胞骨架重排。然而,Rap2位于效应因子结构域的第39位的苯丙氨酸不同于Rap1及Ras的丝氨酸,这个关键差异表明其可能通过特异的下游信号分子调控独特的生物学功能。最近,随着Rap2特异效应因子的不断发现,Rap2特异的信号通路及功能受到了更多的关注,Rap2具有多样的生物学功能,除调控细胞粘附及细胞骨架动态组装外、Rap2调节中枢神经突触的可塑性以及非洲爪蟾发育中背腹轴特化。此外,也有报道显示Rap2的表达增强与多种肿瘤的形成具有相关性。本文主要针对Rap2的信号途径和生物学功能研究的最新进展进行介绍。     

小非编码RNA在精子发生及男性不育中的功能机制

小非编码RNA是一类特殊的基因调控因子,可在转录、转录后和表观遗传水平上调控真核生物细胞的基因表达.大量研究发现,包括piRNA, miRNA和tsRNA等小非编码RNA在哺乳动物雄性生殖细胞中高丰度表达,为精子发生和雄性生殖必需,其表达异常及相关调控通路基因突变与男性不育密切相关.本文总结近期关于小非编码RNA与精子发生相关的研究进展,将主要概述小非编码RNA在哺乳动物精子发生中的生物学功能和作用机制相关最新研究进展,并探讨其异常调控与男性不育的相关性及其在男性不育临床诊治中的潜在应用.     

mTORC2 regulates neutrophil chemotaxis in a cAMP- and RhoA-dependent fashion

We studied the role of the target of rapamycin complex 2 (mTORC2) during neutrophil chemotaxis, a process that is mediated through the polarization of actin and myosin filament networks. We show that inhibition of mTORC2 activity, achieved via knock down (KD) of Rictor, severely inhibits neutrophil polarization and directed migration induced by chemoattractants, independently of Akt. Rictor KD also abolishes the ability of chemoattractants to induce cAMP production, a process mediated through the activation of the adenylyl cyclase 9 (AC9). Cells with either reduced or higher AC9 levels also exhibit specific and severe tail retraction defects that are mediated through RhoA. We further show that cAMP is excluded from extending pseudopods and remains restricted to the cell body of migrating neutrophils. We propose that the mTORC2-dependent regulation of MyoII occurs through a cAMP/RhoA-signaling axis, independently of actin reorganization during neutrophil chemotaxis.     

Prostaglandin E2 activates and utilizes mTORC2 as a central signaling locus for the regulation of mast cell chemotaxis and mediator release

Prostaglandin (PG) E(2), a potent mediator produced in inflamed tissues, can substantially influence mast cell responses including adhesion to basement membrane proteins, chemotaxis, and chemokine production. However, the signaling pathways by which PGE(2) induces mast cell chemotaxis and chemokine production remains undefined. In this study, we identified the downstream target of phosphatidylinositol 3-kinase, mammalian target of rapamycin (mTOR), as a key regulator of these responses. In mouse bone marrow-derived mast cells, PGE(2) was found to induce activation of mTORC1 (mTOR complexed to raptor) as indicated by increased p70S6K and 4E-BP1 phosphorylation, and activation of mTORC2 (mTOR complexed to rictor), as indicated by increased phosphorylation of AKT at position Ser(473). Selective inhibition of the mTORC1 cascade by rapamycin or by the use of raptor-targeted shRNA failed to decrease PGE(2)-mediated chemotaxis or chemokine generation. However, inhibition of the mTORC2 cascade through the dual mTORC1/mTORC2 inhibitor Torin, or through rictor-targeted shRNA, resulted in a significant attenuation in PGE(2)-mediated chemotaxis, which was associated with a comparable decrease in actin polymerization. Furthermore, mTORC2 down-regulation decreased PGE(2)-induced production of the chemokine monocyte chemoattractant protein-1 (CCL2), which was linked to a significant reduction in ROS production. These findings are consistent with the conclusion that activation of mTORC2, downstream of PI3K, represents a critical signaling locus for chemotaxis and chemokine release from PGE(2)-activated mast cells.     

Intramembrane particle aggregation in erythrocyte ghosts. II. The influence of spectrin aggregation.

Physicochemical properties of mixtures of spectrin and actin extracted from human erythrocyte ghosts have been correlated with ultrastructural changes observed in freeze-fractured erythrocyte membranes. (1) Extracted mixtures of spectrin and actin have a very low solubility (less than 30 mug/ml) near their isoelectric point, pH 4.8. These mixtures are also precipitated by low concentrations of Ca2+, Mg2+, polylysine or basic proteins. (2) All conditions which precipitate extracts of spectrin and actin also induce aggregation of the intramembrane particles in spectrin-depleted erythrocyte ghosts. Precipitation of the residual spectrin molecules into small patches on the cytoplasmic surface of the ghost membrane is thought to be the cause of particle aggregations, implying an association between the spectrin molecules and the intramembrane particles. (3) When fresh ghosts are exposed to conditions which precipitate extracts of spectrin and actin, only limited particle aggregation occurs. Instead, the contraction of the intact spectrin meshwork induced by the precipitation conditions compresses the lipid bilayer of the membrane, causing it to bleb off particle-free, protein-free vesicles. (4) The absence of protein in these lipid vesicles implies that all the proteins of the erythrocyte membrane are immobilized by association with either the spectrin meshwork or the intramembrane particles.     

Actin dynamics at intracellular membranes: the Spir/formin nucleator complex

The assembly of actin monomers into filaments is a highly regulated basic cellular function. The structural organization of a cell, morphological changes or cell motility is dependent on actin filament dynamics. While within the last decade substantial knowledge has been acquired about actin dynamics at the cell membrane, today only little is known about the actin cytoskeleton and its functions at intracellular endosomal and organelle membranes. The Spir actin nucleators are specifically targeted towards endosomal membranes by a FYVE zinc finger membrane localization domain, and provide an important link to study the role of actin dynamics in the regulation of intracellular membrane transport. Spir proteins are the founding members of a novel class of actin nucleation factors, which initiate actin polymerization by binding of actin monomers to one or multiple Wiskott-Aldrich syndrome protein (WASp) homology 2 (WH2) domains. Although Spir proteins can nucleate actin polymerization in vitro by themselves, they form a regulatory complex with the distinct actin nucleators of the formin subgroup (Fmn) of formins. A cooperative mechanism in actin nucleation has been proposed. Ongoing studies on the function and regulation of the Spir proteins in vesicle transport processes will reveal important insights into actin dynamics at intracellular membranes and how this regulates the highly directed and controlled routes of intracellular membrane trafficking.     

Bending over backwards: BAR proteins and the actin cytoskeleton in mammalian receptor-mediated endocytosis

The role of the actin cytoskeleton during receptor-mediated endocytosis (RME) has been well characterized in yeast for many years. Only more recently has the interplay between the actin cytoskeleton and RME been extensively explored in mammalian cells. These studies have revealed the central roles of BAR proteins in RME, and have demonstrated significant roles of BAR proteins in linking the actin cytoskeleton to this cellular process. The actin cytoskeleton generates and transmits mechanical force to promote the extension of receptor-bound endocytic vesicles into the cell. Many adaptor proteins link and regulate the actin cytoskeleton at the sites of endocytosis. This review will cover key effectors, adaptors and signalling molecules that help to facilitate the invagination of the cell membrane during receptor-mediated endocytosis, including recent insights gained on the roles of BAR proteins. The final part of this review will explore associations of alterations to genes encoding BAR proteins with cancer.     

Hyper-mobile water is induced around actin filaments

When introduced into water, some molecules and ions (solutes) enforce the hydrogen-bonded network of neighboring water molecules that are thus restrained from thermal motions and are less mobile than those in the bulk phase (structure-making or positive hydration effect), and other solutes cause the opposite effect (structure-breaking or negative hydration effect). Using a method of microwave dielectric spectroscopy recently developed to measure the rotational mobility (dielectric relaxation frequency) of water hydrating proteins and the volume of hydration shells, the hydration of actin filament (F-actin) has been studied. The results indicate that F-actin exhibits both the structure-making and structure-breaking effects. Thus, apart from the water molecules with lowered rotational mobility that make up a typical hydration shell, there are other water molecules around the F-actin which have a much higher mobility than that of bulk water. No such dual hydration has been observed for myoglobin studied as the representative example of globular proteins which all showed qualitatively similar dielectric spectra. The volume fraction of the mobilized (hyper-mobile) water is roughly equal to that of the restrained water, which is two-thirds of the molecular volume of G-actin in size. The dielectric spectra of aqueous solutions of urea and potassium-halide salts have also been studied. The results suggest that urea and I(-) induce the hyper-mobile states of water, which is consistent with their well-known structure-breaking effect. The molecular surface of actin is rich in negative charges, which along with its filamentous structure provides a structural basis for the induction of a hyper-mobile state of water. A possible implication of the findings of the present study is discussed in relation to the chemomechanical energy transduction through interaction with myosin in the presence of ATP.     

Synaptopodin family of natively unfolded, actin binding proteins: physical properties and potential biological functions

The synaptopodin family of proteins consists of at least 3 members: synaptopodin, the synaptopodin 2 proteins, and the synaptopodin 2-like proteins. Each family member has at least 3 isoforms that are produced by alternative splicing. Synaptopodin family members are basic proteins that are rich in proline and have little regular 2° or 3° structure at physiological temperature, pH and ionic strength. Like other natively unfolded proteins, synaptopodin family members have multiple binding partners including actin and other actin-binding proteins. Several members of the synaptopodin family have been shown to stimulate actin polymerization and to bundle actin filaments either on their own or in collaboration with other proteins. Synaptopodin 2 has been shown to accelerate nucleation of actin filament formation and to induce actin bundling. The actin polymerization activity is inhibited by Ca²⁺-calmodulin. Synaptopodin 2 proteins are localized in Z-bands of striated and heart muscle and dense bodies of smooth muscle cells. Depending on the developmental status and stress, at least one member of the synaptopodin family can occupy nuclei of some cells. Members of the synaptopodin 2 subfamily have been implicated in cancers.     

Mammalian TOR complex 2 controls the actin cytoskeleton and is rapamycin insensitive

The target of rapamycin (TOR) is a highly conserved protein kinase and a central controller of cell growth. In budding yeast, TOR is found in structurally and functionally distinct protein complexes: TORC1 and TORC2. A mammalian counterpart of TORC1 (mTORC1) has been described, but it is not known whether TORC2 is conserved in mammals. Here, we report that a mammalian counterpart of TORC2 (mTORC2) also exists. mTORC2 contains mTOR, mLST8 and mAVO3, but not raptor. Like yeast TORC2, mTORC2 is rapamycin insensitive and seems to function upstream of Rho GTPases to regulate the actin cytoskeleton. mTORC2 is not upstream of the mTORC1 effector S6K. Thus, two distinct TOR complexes constitute a primordial signalling network conserved in eukaryotic evolution to control the fundamental process of cell growth.     

Rapamycin‐mediated mTORC2 inhibition is determined by the relative expression of FK506‐binding proteins

The mechanism by which the drug rapamycin inhibits the mechanistic target of rapamycin (mTOR) is of intense interest because of its likely relevance in cancer biology, aging, and other age‐related diseases. While rapamycin acutely and directly inhibits mTORC1, only chronic administration of rapamycin can inhibit mTORC2 in some, but not all, cell lines or tissues. The mechanism leading to cell specificity of mTORC2 inhibition by rapamycin is not understood and is especially important because many of the negative metabolic side effects of rapamycin, reported in mouse studies and human clinical trials, have been attributed recently to mTORC2 inhibition. Here, we identify the expression level of different FK506‐binding proteins (FKBPs), primarily FKBP12 and FKBP51, as the key determinants for rapamycin‐mediated inhibition of mTORC2. In support, enforced reduction of FKBP12 completely converts a cell line that is sensitive to mTORC2 inhibition to an insensitive cell line, and increased expression can enhance mTORC2 inhibition. Further reduction of FKBP12 in cell lines with already low FKBP12 levels completely blocks mTORC1 inhibition by rapamycin, indicating that relative FKBP12 levels are critical for both mTORC1 and mTORC2 inhibition, but at different levels. In contrast, reduction of FKBP51 renders cells more sensitive to mTORC2 inhibition. Our findings reveal that the expression of FKBP12 and FKBP51 is the rate limiting factor that determines the responsiveness of a cell line or tissue to rapamycin. These findings have implications for treating specific diseases, including neurodegeneration and cancer, as well as targeting aging in general.     

Effect of self-association on the structural organization of partially folded proteins: inactivated actin

The propensity to associate or aggregate is one of the characteristic properties of many nonnative proteins. The aggregation of proteins is responsible for a number of human diseases and is a significant problem in biotechnology. Despite this, little is currently known about the effect of self-association on the structural properties and conformational stability of partially folded protein molecules. G-actin is shown to form equilibrium unfolding intermediate in the vicinity of 1.5 M guanidinium chloride (GdmCl). Refolding from the GdmCl unfolded state is terminated at the stage of formation of the same intermediate state. An analogous form, known as inactivated actin, can be obtained by heat treatment, or at moderate urea concentration, or by the release of Ca(2+). In all cases actin forms specific associates comprising partially folded protein molecules. The structural properties and conformational stability of inactivated actin were studied over a wide range of protein concentrations, and it was established that the process of self-association is rather specific. We have also shown that inactivated actin, being denatured, is characterized by a relatively rigid microenvironment of aromatic residues and exhibits a considerable limitation in the internal mobility of tryptophans. This means that specific self-association can play an important structure-forming role for the partially folded protein molecules.