刺猬

刺猬信号通路（英语：Hedgehog signaling pathway）是重要的信号传导通路。刺猬信号通路是动物发育的关键调控之一，在所有的两侧对称动物中都有表达。刺猬信号通路得名于在其多肽配体——Drosophila果蝇中发现的一种名为Hh的细胞间信号分子。Hh是 Drosophila体节极性基因的表达物，是果蝇体形发育形成的基础。该分子在后期胚胎开成及变态期及起重要作用。。共有五种刺猬因子：音猬因子（Sonic hedgehog，SHH）、沙漠刺猬因子（desert hedgehog，DHH）、印度刺猬因子（Indian hedgehog，IHH）、Echidna Hedgehog，EHH和Tiggywinkle Hedgehog，TwHH）。仅在鱼类中发现有EHH和TwHH，哺乳类和其他动物中没有这两种刺猬因子。哺乳动物有三种刺猬信号路径同系物，DHH、IHH及SHH,其中Sonic研究得最深入。该路径在脊椎动物胚胎发育中同样的重要。缺乏该路径组件的基因敲除小鼠，其大大脑、骨骼、肌肉系统、胃肠道及肺都未能正正常发育。近期研究表明刺猬信号路径在调节对成人组织起到维护及再生作用的成体干细胞中的作用。同样该路径也与某些癌症的发生的关系。几家制药公司正在积极地开发针对刺猬信号传导以对抗癌症的药物。在20世纪70年代，发育生物学的基本问题是一个相对简单的受精卵是如何发展成一个复杂的身体分节（英语：Segmentation (biology)）的动物的。70年代末，克里斯汀·纽斯林-沃尔哈德和艾瑞克·威斯乔斯分离了控制果蝇前后体轴发育分节的突变基因，他们的“饱和诱变”技术导致了一系列参与体轴形态发生基因的发现。1995年，他们因在对果蝇胚胎发育（英语：Drosophila embryogenesis）基因突变的研究工作与爱德华·路易斯共同获得诺贝尔奖。黑腹果蝇的刺猬 (hh) 基因被发现为其中一种可以造成个体体节前后差异的重要基因。果蝇的刺猬基因在1992年分别被 Jym Mohler, Philip Beachy, 以及 Thomas B. Kornberg的实验室所复制出来。 若其中一些刺猬基因发生突变，将会造成相较于野生型来说，较为短小且矮胖的不正常形状胚胎。刺猬基因的功能为影响体节极性，其突变会会影响幼体的表皮棘刺和成体的附支，如腿和触角。Rather than the normal pattern of denticles, hedgehog mutant larvae tend to have "solid lawns" of denticles (Figure 1). 刺猬基因突变的果蝇幼虫具有短小且多毛的特征，形似刺猬，故此得名。昆虫细胞表达全长含有锌指的转录因子Ci蛋白（英语：Cubitus interruptus）, 而此蛋白会在细胞质中和类似kinesin蛋白Costal-2 (Cos2) 形成复合体并结合到微管上（图2）。 SCF复合物对155 kDa全长Ci蛋白的依赖于蛋白酶体的剪切产生75 kDa片段(CiR)。CiR在细胞内累计并扩散到细胞核，在那里它作为刺猬因子靶基因的共抑制物。 The steps leading to Ci protein proteolysis include phosphorylation of Ci protein by several protein kinases; PKA, GSK3β and CK1 (Figure 2). The Drosophila protein Slimb is part of an SCF complex that targets proteins for ubiquitylation. Slimb binds to phosphorylated Ci protein.In the absence of Hh (图 3), a cell-surface transmembrane protein called Patched (PTCH) acts to prevent high expression and activity of a 7 membrane spanning receptor called Smoothened (SMO). Patched has sequence similarity to known membrane transport proteins. When extracellular Hh is present (Figure 3), it binds to and inhibits Patched, allowing Smoothened to accumulate and inhibit the proteolytic cleavage of the Ci protein. This process most likely involves the direct interaction of Smoothened and Costal-2 and may involve sequestration of the Ci protein-containing complex to a microdomain where the steps leading to Ci protein proteolysis are disrupted. The mechanism by which Hh binding to Patched leads to increased levels of Smoothened is not clear (Step 1 in Figure 3). Following binding of Hh to Patched, Smoothened levels increase greatly over the level maintained in cells when Patched is not bound to Hh. It has been suggested that phosphorylation of Smoothened plays a role in Hh-dependent regulation of Smoothened levels.In cells with Hh-activated Patched (Figure 3), the intact Ci protein accumulates in the cell cytoplasm and levels of CiR decrease, allowing transcription of some genes such as decapentaplegic (dpp, a member of the BMP growth factor family). For other Hh-regulated genes, expression requires not only the loss of CiR but also the positive action of uncleaved Ci to act as a transcriptional activator. Costal-2 is normally important for holding Ci protein in the cytoplasm, but interaction of Smoothened with Costal-2 allows some intact Ci protein to go to the nucleus. The Drosophila protein Fused (Fu in Figure 3) is a protein kinase that binds to Costal-2. Fused can inhibit Suppressor of Fused (SUFU), which in turn interacts with Ci to regulate gene transcription in some cell types.Hedgehog has roles in larval body segment development and in formation of adult appendages. During the formation of body segments in the developing Drosophila embryo, stripes of cells that synthesize the transcription factor Engrailed can also express the cell-to-cell signaling protein Hedgehog (green in Figure 4). Hedgehog is not free to move very far from the cells that make it and so it only activates a thin stripe of cells adjacent to the Engrailed-expressing cells. When acting in this local fashion, hedgehog works as a paracrine factor. Only cells to one side of the Engrailed-expressing cells are competent to respond to Hedgehog following interaction of Hh with the receptor protein Patched (blue in Figure 4).Cells with Hh-activated Patched receptor synthesize the Wingless protein (red in Figure 4). If a Drosophila embryo is altered so as to produce Hh in all cells, all of the competent cells respond and form a broader band of Wingless-expressing cells in each segment. The wingless gene has an upstream transcription regulatory region that binds the Ci transcription factor in a Hh-dependent fashion resulting in an increase in wingless transcription (interaction 2 in Figure 3) in a stripe of cells adjacent to the stripe of Hh-producing cells.Wingless protein acts as an extracellular signal and patterns the adjacent rows of cells by activating its cell surface receptor Frizzled. Wingless acts on Engrailed-expressing cells to stabilize the stripes of Engrailed expression. Wingless is a member of the Wnt family of cell-to-cell signaling proteins. The reciprocal signaling by Hedgehog and Wingless stabilizes the boundary between parasegments (Figure 4, top). The effects of Wingless and Hedgehog on other stripes of cells in each segment establishes a positional code that accounts for the distinct anatomical features along the anterior-posterior axis of the segmentsThe Wingless protein is called "wingless" because of the phenotype of some wingless fly mutants. Wingless and Hedgehog functioned together during metamorphosis to coordinate wing formation. Hedgehog is expressed in the posterior part of developing Drosophila limbs. Hedgehog also participates in the coordination of eye, brain, gonad, gut and tracheal development. Hedgehog has been implicated in reduced eye development in the amphipod Gammarus minus. Specifically, downregulation of hedgehog results in reduced eyes.胚胎发育时缺少刺猬因子（因基因突变或孕妇误食致畸剂）都会导致严重的发育异常。