Chapter 5.2 Cell Signalling

5.2.1 cell communication and cell recognition A Cell communication Three ways commmunication a Secret chemical signal b Contact-depend signal( 接触性依赖的通讯） c Gap junction signal

signal transduction systems Steroid hormones hormones ion channels Seven-spanning G protein-linked receptors adenylate cyclase G proteins cAMP antagonists Agonists kinases phosphatases Tyrosine Phosphorylation

Cell Signals Multiple signals regulate cell survival. Additional signals regulate cell behavior. An absence of signals results in cell death

Endocrine signaling – signaling molecules act on target cells distant from their site of synthesis by cells of endocrine organs Paracrine signaling – signaling molecules released by a cell only affect target cells in close proximity Autocrine signaling – cells respond to substances that they themselves release The Signal

Secret chemical signal Endocrine 内分泌 旁分泌 Secret chemical signal

B cell recognition 细胞通过其表面受体与胞内信号物质选择性结合，导致胞内一系列生理生化变化，最终表现为细胞整体的生物学效应

Events Stimuli Responses Impinge from the outside and interact with the cell membrane Neurotransmitters Responses Depend on the stimuli Signal trasduction cascade for activation of gene/s. Genetic programs

Signaling is about communication between different groups of cells and tissues…how one group of cells informs another group of cells what to do. Signal transduction refers to how the presence of an extracellular signal can produce a change in the intracellular state of the cell without the initial signal crossing the membrane.

Communication by extracellular signals usually involves six steps: 1) synthesis and 2) release of signaling molecules by the signaling cell 3) transport of the signal to the target cell 4) detection of the signal by a specific receptor protein 5) and change in cellular metabolism or gene expression triggered by the receptor-signaling molecule complex 6) removal of the signal, often terminating the cellular response

C Cell singnal Cell signal 亲脂性分子（菑类激素；甲状腺素）（与胞内受体结合） 亲水性信号分子)(与细胞表面受体结合） 气体信号分子 NO

Cells constantly communicate with one another, through direct contact and by chemical messengers, such as hormones. This communication is cell signaling.

Nitric Oxide In response to a signal from nerve cells, endothelial cells that line blood vessels make and release nitric oxide. Nitric oxide enters muscle cells in the vessel wall causing them to relax and dilate. Blood flow increases and more oxygen can reach organs such as the heart. The 1998 Nobel prize was awarded to two U.S. scientists for discovering this mechanism.

Nitric oxide binds directly to an intracellular enzyme causing a rapid response.

Signal Transduction

Signal molecules move between cells Growth factors promote survival and stimulate cell proliferation. Acetylcholine is a neurotransmitter. Thyroid hormone stimulates metabolism. Nitric oxide relaxes muscle cells in vessels. Adrenaline increases heart contraction. Testosterone induces secondary male sexual characteristics.

Types of signals Extracellular Receptors which have N terminal face outwards and C terminal inside the cell. When bound to a signal molecule, changes its conformation Signal molecules are specific to their receptors

Types of signals contd. Intracellular

Intercellular Mostly triggered by the extracellular signal Types Endocrine – Travel through blood Paracrine – In the visinity Autocrine – Same cell type Juxtacrine – Along cell membranes Intercellular Mostly triggered by the extracellular signal Which converts the extracellular into an intracellular signal Eg. - G protein, GTPase, cAMP, Ca++, Kinases, phosphatases and many more Also called as second messengers

Hormones

Fast and slow responses

Post induction events Amplification Intracellular signal transduction life span of the receptor – ligand complex Intracellular signal transduction Ca++ activation NO (Nitric Oxide)

D Cell receptor Receptor:一种能够识别和选择结合某种配体（信号）的大分子，通过信号转导（signal transduction),将胞外信号转换为胞内物理或者化学信号，以启动一系列过程，最终表现为生物学效应。 Cell receptor:intracellular receptor cell surface receptor

Types of receptors Transmembrane receptors That span the thickness of the plasma membrane Intracellular domain and extracellular domain Signal to concentration Signal trasduction/through small molecules (Ca++) – Ion Channels Cell potential

Types of receptors Nuclear receptors Soluble proteins localised within the cytoplasm or the nuceloplasm Ligand activated trascription activators Hormone regulation Steroid receptors (located within cytosol) Apocomplex formation in the absence of the steroid (also contains chaperone proteins) RXR and Orphan receptors

In general, signaling by cell-surface receptors initiates a cascade of biochemical events that result changes in the metabolism of the cell.

How are signals transmitted into the cell? Hydrophobic signal molecules such as testosterone or gaseous signal molecules such as nitric oxide can cross the membrane. Most signal molecules bind to a cell-surface receptor protein where the information is converted from one form to another in a process called signal transduction.

Three classes of cell-surface receptors The Receptor Ion-channel-linked receptors open an ion channel in response to the signal molecule. G-protein-linked receptors activate an intracellular G-protein that in turn activates intracellular enzymes. Enzyme-linked receptors directly activate a membrane bound enzyme.

Receptors Signal molecules that do not enter the cell bind to cell-surface receptors. Signal molecules that enter the cell bind to intracellular receptors.

The Response Depending on type of cell and stimulus, the response might involve: 1. Change in gene expression. 2. Alteration of activity of metabolic enzymes. 3. Reconfiguration of cytoskeleton. 4. Change in ion permeability. 5. Secretion of a hormone or protein. 6. Activation of DNA synthesis. 7. Death of cell.

The same signal molecule can induce different responses in different cell types

第二信使与分子开关 Second messenger:第一信使与受体结合后最早产生的的信号（ cAMP,Cgmp,IP3,DG,) 分子开关蛋白：switch protein:蛋白激酶使之P而开启，蛋白磷酸酯酶去P而关闭 GTP结合蛋白：结合GTP活化，结合GDP失活

The Second Messenger Examples of Signal Transduction Systems Ca2+/calmodulin adenylate cyclase/cAMP NO PLC/PKC PI/IP3/DAG membrane receptors G proteins steroid hormones

5.2.2通过细胞内受体介导的信号传递 亲脂小分子与细胞内受体结合 细胞内受体的本质是激素激活的基因调控蛋白，构成细胞内受体超家族

5.2.3通过细胞表面受体介导的信号跨膜传递 亲水性的信号与细胞表面受体结合 细胞表面受体 A离子通道欧联的受体(ion-channel-linked receptor) B 酶偶联的受体（enzyme-linked receptor) C G蛋白欧联的受体(G protein -linked receptor)

Cell-surface receptor categories Can be separated into four classes: 1) G protein-linked receptors 2) Ion-channel receptors 3) Receptors lacking intrinsic catalytic activity but directly associated with cytosolic protein tyrosine kinases 4) Receptors with intrinsic enzymatic activity (RTKs)

A 离子通道偶联的受体(ion-channel-linked receptor

B 酶偶联的受体（enzyme-linked receptor）

血小板衍生生长因子 成纤维生长因子

SUMMARY Receptor tyrosine kinases (RTKs), which bind to peptide/protein hormones, may exist as dimers or dimerize during binding to ligands. Ligand binding leads to activation of the kinase activity of the receptor and autophosphorylation of tyrosine residues in its cytosolic domain (see Figure 20-31). The activated receptor also can phosphorylate other protein substrates. Ras is an intracellular GTPase switch protein that acts downstream from most RTKs. Like Gsa , Ras cycles between an inactive GDP-bound form and active GTP-bound form. Ras cycling requires the assistance of two proteins, GEF and GAP, (see Figure 20-22), whereas Gsa cycling does not. Unlike GPCRs, which interact directly with an associated G protein, RTKs are linked indirectly to Ras via two proteins, GRB2 and Sos (see Figure 20-23).

The SH2 domain in GRB2, an adapter protein, binds to specific phosphotyrosines in activated RTKs. The two SH3 domains in GRB2 then bind Sos, a guaninenucleotide exchange factor, thereby bringing Sos close to membrane-bound Ras · GDP and activating its exchange function. Binding of Sos to inactive Ras causes a large conformational change that permits release of GDP and binding of GTP. Normally, Ras activation and the subsequent cellular response is induced by ligand binding to an RTK. However, in cells that contain a constitutively active Ras, the cellular response occurs in the absence of ligand binding.

C G蛋白欧联的受体(G protein -linked receptor) C-1 CAMP signal pathway C-2 磷脂酰肌醇信号通路(Double messenger pathway)

G proteins C-1 CAMP signal pathway 1994 Nobel prize. Discovery of G-protein coupled receptors and their role in signal transduction. Gilman, Alfred G. and Rodbell, Martin. http://www.nobel.se/medicine/laureates/1994/index.html 43,409 articles in Medline that mention G proteins. 1. Basic facts about G proteins Each G-protein consists of 3 sub-units: Ga, Gb, and Gg. These sub-units can be together or separated. Each G-protein has a binding site that can be occupied by either GDP or GTP. A G-protein is said to be inactive when it is bound to GDP. A G-protein is said to be active when it is bound to GTP.

Small GTP-binding proteins include (roles indicated): initiation & elongation factors (protein synthesis). Ras (growth factor signal cascades). Rab (vesicle targeting and fusion). ARF (forming vesicle coatomer coats). Ran (transport of proteins into & out of the nucleus). Rho (regulation of actin cytoskeleton) All GTP-binding proteins differ in conformation depending on whether GDP or GTP is present at their nucleotide binding site. Generally, GTP binding induces the active state.

cAMP Adenylate Cyclase (Adenylyl Cyclase) catalyzes: ATP  cAMP + PPi The reaction is driven forward by the cleavage of PPi, catalyzed by Pyrophosphatase: PPi  2 Pi

Cyclic AMP Cyclic-AMP is suited to be a transient signal. Synthesis & degradation of cAMP are both spontaneous, but enzymes are required to synthesize these reactions. Enzymes that synthesize and degrade cAMP are regulated.

cyclicAMP (cAMP) pathway Up regulation: When a ligand binds to a receptor in a cell membrane the effect is that A. the receptor becomes activated B. the nucleotide binding site on the G-protein is altered, C. GTP replaces GDP, D. GDP is released, E. and Ga-GTP gets disassociated from GbGg. F. Ga being active triggers the binding of Ga-GTP to a membrane bound adenylate cyclase molecule, activating it for production of cyclicAMP (cAMP).

Cyclic AMP (cAMP) pathway Down regulation – when the ligand dissociates from the receptor the effects are A. GTP is hydrolyzed by a GTPase activity on Ga B. Ga-GTP becomes Ga-GDP, C. and disassociates from adenylate cyclase molecule, making the later inactive. D. Ga then reassociates with GbGg E. CyclicAMP (in the cytoplasm) is then inactivated by the enzyme phosphodiesterase, which hydrolyzes it to AMP.

G protein activation/inactivation cycle

2. Hormone binding to a 7-helix receptor (GPCR) causes a conformational change in the receptor that is transmitted to the G protein. The nucleotide-binding site on Ga becomes more accessible to the cytosol, where [GTP] > [GDP]. Ga releases GDP & binds GTP (GDP-GTP exchange).

Turn off of the signal: 1. Ga hydrolyzes GTP to GDP + Pi. (GTPase). The presence of GDP on Ga causes it to rebind to the inhibitory bg complex. Adenylate Cyclase is no longer activated. 2. Phosphodiesterase catalyzes hydrolysis of cAMP  AMP.

Turn off of the signal (cont.): 3. Hormone receptor desensitization occurs. This process varies with the hormone. Some receptors are phosphorylated via G-protein-coupled receptor kinases. The phosphorylated receptor may then bind to a protein arrestin that blocks receptor-G-protein activation and promotes removal of the receptor from the membrane by clathrin-mediated endocytosis. 4. Protein Phosphatase catalyzes removal by hydrolysis of phosphates that were attached to proteins via Protein Kinase A.

Signal amplification is an important feature of signal cascades: One hormone molecule can lead to formation of many cAMP molecules. Each catalytic subunit of Protein Kinase A catalyzes phosphorylation of many proteins during the life-time of the cAMP. View an animation of a G-protein signal cascade.

C-2 磷脂酰肌醇信号通路(Double messenger pathway)

5.2.4 细胞表面整联蛋白介导的信号传递 整联蛋白是细胞表面的跨膜蛋白包括纤连蛋白，胶原，和蛋白聚糖 粘着斑功能：机械结构功能；信号传递功能 细胞表面到细胞核的信号通路 细胞表面到细胞质核糖体的信号通路

5.2.5 细胞信号传递的基本特征与蛋白激酶的网络整合信息 A 细胞信号传递的基本特征 多途径多层次，具备收敛和发散特点 既有专一性又具备作用机制的相似性 信号可以有控制的适度放大 具备进行适应的特点 逐渐降低表面受体的数目 快速钝化受体

Signal transduction across the plasma membrane can cause a cascade of events that amplify the signal and distribute it to influence several cell processes in parallel.

B 蛋白激酶的网络整合信息

Why do cells communicate? During development, cells differentiate to adopt specialized roles. Cells need to know whether to live, die, or divide. Neurotransmission. Regulation of metabolism. Contraction-expansion. Secondary sexual characteristics.

Cell Communication Why do cells communicate? How are signals transmitted between cells? How are signals transmitted across cell membranes into the cell interior? How are signals transmitted within a cell? How do signals affect cell function? What is the relationship between cell signaling and cancer?

Cell signaling and cancer RAS is an intracellular GTP binding protein that is activated by enzyme linked receptors Activated RAS proteins create a cascade of events that can change protein activity and gene expression RAS proteins control cell proliferation, cell survival, and cell differentiation 30% of human cancers have mutations in RAS genes which are then called oncogenes

Review 1. Compare the structures, locations and functions of preproinsulin and insulin. 2. Outline the events from translation of the preproinsulin gene to exocytosis of insulin. 3. Describe the mechanisms by which glucose promotes insulin secretion. 4. Identify the components of insulin secretion that are defective in type II diabetes mellitus.