📚 Cell Signalling | 细胞信号传导
In multicellular organisms, cells must communicate to coordinate growth, metabolism, reproduction, and responses to the environment. Cell signalling is the molecular mechanism by which cells detect, process, and respond to signals from other cells or from changes in their surroundings. This communication involves a signal molecule (ligand) released by a signalling cell that binds to a specific receptor on a target cell, triggering a cascade of intracellular events known as signal transduction. Ultimately, the cell responds by altering its behaviour, gene expression, or metabolism. Understanding cell signalling is fundamental in A-Level Biology, as it explains how hormones like adrenaline and insulin exert their effects, how nerve impulses are transmitted, and how cells grow and differentiate.
在多细胞生物中，细胞必须进行交流以协调生长、代谢、繁殖和对环境的反应。细胞信号传导是细胞检测、处理并响应来自其他细胞或周围环境变化信号的分子机制。这种交流涉及信号细胞释放的信号分子（配体），该配体与靶细胞上的特异性受体结合，触发一系列称为信号转导的胞内事件。最终，细胞通过改变其行为、基因表达或代谢作出反应。理解细胞信号传导在A-Level生物学中至关重要，因为它解释了肾上腺素和胰岛素等激素如何发挥作用、神经冲动如何传递以及细胞如何生长和分化。
1. Introduction to Cell Signalling | 细胞信号传导简介
Cell signalling can be broken down into three main stages: reception, transduction, and response. During reception, the signal molecule (ligand) binds to a specific receptor, which may be on the plasma membrane or inside the cell. Transduction involves the conversion of the signal into a form that brings about a cellular response – often via a sequence of molecules (signalling cascade). The response is the final cellular action, such as enzyme activation, gene expression, or changes in the cell’s shape or movement. The specificity of signalling is ensured by the complementary shape between the ligand and its receptor, much like enzyme–substrate specificity.
细胞信号传导可分为三个主要阶段：接收、转导和响应。在接收阶段，信号分子（配体）与特异性受体结合，受体可位于质膜上或细胞内部。转导阶段涉及将信号转化为引起细胞响应的形式——通常通过一系列分子（信号级联）。响应是最终的细胞作用，例如酶激活、基因表达，或细胞形状和运动的改变。信号传导的特异性由配体与其受体之间的互补形状所保证，类似于酶与底物的特异性。
2. Types of Signalling | 信号传导类型
Autocrine signalling occurs when a cell releases a signal molecule that binds to receptors on its own surface, leading to a response in the same cell. This is common in immune cells and during development.
自分泌信号传导：细胞释放信号分子并结合自身表面的受体，引起同一细胞的反应。这在免疫细胞和发育过程中常见。
Paracrine signalling involves the release of signal molecules that act on nearby target cells. The signal molecules diffuse locally through the extracellular fluid, so the effect is restricted to the immediate neighbourhood. An example is the action of neurotransmitters in a synapse (though synaptic signalling is often considered a specialised form).
旁分泌信号传导涉及释放作用于邻近靶细胞的信号分子。信号分子通过细胞外液局部扩散，因此效应局限于附近区域。一个例子是突触中神经递质的作用（尽管突触信号常被视为一种特殊形式）。
Endocrine signalling relies on hormones secreted into the bloodstream, allowing signals to act on distant target cells throughout the body. Because hormones are diluted in the blood, they often act at very low concentrations and require high-affinity receptors.
内分泌信号传导依赖分泌到血液中的激素，使信号能作用于全身远距离的靶细胞。由于激素在血液中被稀释，它们通常在极低浓度下起作用，并需要高亲和力受体。
Synaptic signalling is a highly specific form of paracrine signalling where a neurotransmitter is released from a neuron and diffuses across a narrow synaptic cleft to bind receptors on the adjacent postsynaptic cell. This allows rapid, localised communication.
突触信号传导是旁分泌信号的一种高度特异形式，神经递质从神经元释放，穿过狭窄的突触间隙，与相邻突触后细胞上的受体结合。这实现了快速、局部的通信。
Juxtacrine signalling involves direct contact between the signalling cell and the target cell. The signal molecule is a membrane-bound protein or a component of the extracellular matrix, and contact is required for signalling to occur. This type is important in cell differentiation and immune responses.
近分泌信号（接触依赖型信号）要求信号细胞与靶细胞直接接触。信号分子是膜结合蛋白或细胞外基质成分，需要接触才能传递信号。这种类型在细胞分化和免疫应答中很重要。
3. Signal Molecules (Ligands) | 信号分子（配体）
Signal molecules, or ligands, vary widely in structure and chemical properties. Hydrophilic ligands, such as peptide hormones (insulin, glucagon), adrenaline, and most neurotransmitters, cannot cross the hydrophobic core of the plasma membrane. They bind to cell-surface receptors and rely on signal transduction to pass the message into the cell interior.
信号分子或配体在结构和化学性质上差异很大。亲水性配体，如肽类激素（胰岛素、胰高血糖素）、肾上腺素和大多数神经递质，不能穿过质膜的疏水核心。它们与细胞表面受体结合，依赖信号转导将信息传递到细胞内部。
Hydrophobic ligands, such as steroid hormones (oestrogen, testosterone, cortisol) and thyroid hormones, can diffuse through the lipid bilayer. Their receptors are located inside the cell, either in the cytosol or in the nucleus. Upon binding, the receptor–ligand complex often acts directly as a transcription factor to regulate gene expression.
疏水性配体，如类固醇激素（雌激素、睾酮、皮质醇）和甲状腺激素，可穿过脂双层扩散。它们的受体位于细胞内部，在胞质或细胞核内。结合后，受体-配体复合物常直接作为转录因子调控基因表达。
4. Receptors: Cell-Surface vs Intracellular | 受体：细胞表面受体与细胞内受体
Receptors are proteins that recognise and bind specific ligands with high affinity, triggering a cellular response. They are broadly categorised into cell-surface (transmembrane) receptors and intracellular receptors. The table below compares their key features.
受体是能识别并以高亲和力结合特定配体的蛋白质，从而触发细胞响应。它们主要分为细胞表面（跨膜）受体和细胞内受体。下表比较了它们的关键特征。
| Feature | Cell-Surface Receptors | Intracellular Receptors |
|---|---|---|
| Location | Plasma membrane | Cytosol or nucleus |
| Ligand property | Hydrophilic, cannot cross membrane | Hydrophobic, can diffuse across membrane |
| Examples of ligands | Adrenaline, glucagon, insulin, neurotransmitters | Steroid hormones (oestrogen, testosterone), thyroid hormones |
| Signal transduction | Uses second messengers, often activates enzyme cascades | Hormone-receptor complex acts as transcription factor, directly regulates gene expression |
| Speed of response | Rapid (milliseconds to minutes) | Slow (hours to days) |
Cell-surface receptors are embedded in the plasma membrane and bind water-soluble ligands. They relay the external signal across the membrane, often by generating a small intracellular signalling molecule called a second messenger. Intracellular receptors are not membrane-embedded; they bind lipid-soluble ligands and typically alter gene expression directly, leading to slow but sustained responses.
细胞表面受体镶嵌在质膜中，结合水溶性配体。它们将外部信号传递过膜，通常通过产生称为第二信使的小分子胞内信号。细胞内受体不在膜上；它们结合脂溶性配体，通常直接改变基因表达，导致缓慢但持久的响应。
5. G Protein-Coupled Receptors (GPCRs) | G蛋白偶联受体
GPCRs form a large family of cell-surface receptors characterised by seven transmembrane alpha-helices. When a ligand binds on the extracellular side, the receptor undergoes a conformational change that activates a heterotrimeric G protein on the inner surface of the membrane. The G protein consists of α, β, and γ subunits, and in its inactive state, the α subunit is bound to GDP.
G蛋白偶联受体是一个庞大的细胞表面受体家族，特征是具有七次跨膜α-螺旋。当配体在胞外侧结合时，受体发生构象改变，激活膜内表面的异源三聚体G蛋白。G蛋白由α、β和γ亚基组成，在非活性状态下，α亚基与GDP结合。
Upon activation, the receptor causes the α subunit to exchange GDP for GTP. The GTP-bound α subunit then dissociates from the βγ dimer and is free to activate an effector enzyme, such as adenylyl cyclase or phospholipase C. This activation is temporary because the α subunit possesses intrinsic GTPase activity, hydrolysing GTP to GDP and returning to its inactive state, thus terminating the signal.
激活后，受体促使α亚基将GDP置换为GTP。与GTP结合的α亚基随后从βγ二聚体上解离，并可自由激活效应酶，例如腺苷酸环化酶或磷脂酶C。此激活是暂时的，因为α亚基具有内在GTP酶活性，将GTP水解为GDP并恢复到非活性状态，从而终止信号。
6. The cAMP Second Messenger Pathway | cAMP第二信使途径
One of the best-studied GPCR pathways involves the second messenger cyclic AMP (cAMP). The binding of adrenaline to the β-adrenergic receptor (a GPCR) activates the stimulatory G protein (Gαₛ). Gαₛ-GTP then binds to and activates the membrane enzyme adenylyl cyclase, which converts ATP into cAMP and pyrophosphate.
研究最深入的GPCR途径之一涉及第二信使环磷酸腺苷（cAMP）。肾上腺素与β-肾上腺素受体（一种GPCR）结合后，激活刺激性G蛋白（Gαₛ）。Gαₛ-GTP随后结合并激活膜酶腺苷酸环化酶，该酶将ATP转化为cAMP和焦磷酸。
ATP → cAMP + PPᵢ
cAMP then diffuses through the cytoplasm and binds to the regulatory subunits of protein kinase A (PKA), releasing its catalytic subunits. Activated PKA phosphorylates specific serine or threonine residues on target proteins, initiating a cascade. For example, in liver cells, PKA phosphorylates phosphorylase kinase, which in turn activates glycogen phosphorylase, leading to the breakdown of glycogen to glucose-1-phosphate. This ultimately raises blood glucose levels as part of the ‘fight or flight’ response.
cAMP随后在细胞质中扩散，并结合蛋白激酶A（PKA）的调节亚基，释放其催化亚基。活化的PKA磷酸化靶蛋白上的丝氨酸或苏氨酸残基，启动级联反应。例如在肝细胞中，PKA磷酸化磷酸化酶激酶，后者激活糖原磷酸化酶，导致糖原分解为1-磷酸葡萄糖。这最终作为“战斗或逃跑”反应的一部分升高血糖水平。
An important feature of this pathway is signal amplification: one hormone molecule binding a receptor can activate multiple G proteins, each of which activates an adenylyl cyclase that produces many cAMP molecules. Each cAMP activates a PKA that in turn phosphorylates many target molecules, thereby greatly amplifying the original signal.
该途径的一个重要特征是信号放大：一个激素分子结合受体可激活多个G蛋白，每个G蛋白激活一个腺苷酸环化酶，产生众多cAMP分子。每个cAMP激活一个PKA，PKA又磷酸化许多目标分子，从而极大地放大了原始信号。
7. Receptor Tyrosine Kinases (RTKs) | 受体酪氨酸激酶
RTKs are another major class of cell-surface receptors. They have a single transmembrane helix and an intracellular domain with tyrosine kinase activity. Examples include the insulin receptor and receptors for many growth factors. Ligand binding induces receptor dimerisation (pairing of two receptor molecules), which activates the kinase domains, leading to autophosphorylation on tyrosine residues.
受体酪氨酸激酶是另一类主要的细胞表面受体。它们具有单次跨膜螺旋和含有酪氨酸激酶活性的胞内域。例子包括胰岛素受体和许多生长因子的受体。配体结合诱导受体二聚化（两个受体分子配对），激活激酶域，导致酪氨酸残基上的自磷酸化。
The phosphorylated tyrosine residues serve as docking sites for intracellular signalling proteins that contain SH2 or PTB domains. One key pathway involves the recruitment of an adaptor protein that activates Ras, a small G protein. Ras then triggers a phosphorylation cascade of MAP kinases (the MAPK pathway: Raf → MEK → ERK). Activated ERK enters the nucleus and phosphorylates transcription factors, promoting gene expression for cell growth and division.
磷酸化的酪氨酸残基作为含有SH2或PTB结构域的胞内信号蛋白的停靠位点。一条关键途径涉及招募接头蛋白来激活Ras（一种小G蛋白）。Ras随后触发MAP激酶的磷酸化级联（MAPK途径：Raf → MEK → ERK）。活化的ERK进入细胞核，磷酸化转录因子，促进细胞生长和分裂的基因表达。
8. Ion Channel-Linked Receptors | 离子
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