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CHAPTER 7

CELL COMMUNICATION

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Transportation: (See your text book)

70% energy of cell will be used for transportation. There are two types of transproteins:

Carrier protein (carrier, permease, transporter) Channel protein

Three types of transportations:

Free diffusion (non-polarized molecules)Passive transportation Facilitated diffusion (polarized molecules)

Co-transportation Na+-K+ ATPase (Pump)Automatic transportation Proton pump Ca+ pump ABC transporter

Endocytosis and exocytosis

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Roles of cell communication

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I. Basic concepts

Some concepts that are easy to be confused:

Now, there are many terms about cell communication used in cell biology, especially to the cell biology of tumor. But, some of them are easy to be confused. I define them as the follows: Cell signaling: Cells release some signal out to some cells else. Cell communication: The signal from a cell is transmitted to another cell by some transmitter, and causes a specific reaction. Cell recognition: A cell interacts with another cell by the signal molecules located on cell surface, and causes specific response of another cell. Signal transduction: The signals from out side of cell (Light, electricity, and molecules) are received by the receptors located on cell surface, cause the change of intracellular signal level, and start a serial responses of cell.

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Signal molecules:

Signals can be chemical signals or physical signals. Chemical signal is much more important than physical signal in cell biology. Chemical signals include oligopeptides, proteins, gas molecules (NO 、 CO), amino acids, nucleotides, lipids, cholesterol, and others. The characters of chemical signals are ① specificity; ② efficiency. One or more molecules can cause a strong response because the signal transduction system can enlarge the signal stimulation; ③ they can be inactivated after the signal transmission. This is a protection mechanism that organs obtained during the evolutionary history. By the routes of generation and role, the chemical signal molecules can be sorted as 4 types: hormones, neuron transmitters, local mediated factors, and gas molecules. By the solubility, the signal molecules can be sorted as two types: lipid soluble and water soluble molecules. Lipid soluble signal, such as some hormone, can be transported into cell directly passing through the bilayers membrane. Water soluble signal molecule, such as neuron transmitters, can not pass through the bilayers membrane. They have to bind to their receptors and exchange the signal type, transfer the information to the intracellular signal (cAMP) or activate the kinase for receptor to result in cell responses. So, we call these signal molecules as primary messenger, and intracellular signal molecules (cAMP, cGMP, IP3 and DG) as secondary messenger.

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Ca2+ can be named as third messenger because its signal transmission is depended on secondary messenger.

Secondary messenger can enlarge the signal function.

Receptors: Receptor can selectively bind to its ligand (Signal molecules). Receptor is glycoprotein usually composed of two function domains at least (ligand binding part and activating part). The features of the interaction between ligand and receptor are: ① specificity; ② saturated limit; ③ high affinity. By the receptor’s location, we can sort receptors as intracellular receptor and cell surface receptor. Intracellular receptor receives lipid soluble signal, and cell surface receptor receives water soluble signal. The response of cell to a signal depends on both receptor and cell type. Same signal can cause different responses on different cells. For examples, Ach can cause the contraction of skeleton muscle, inhibit the contraction rate of heart muscle, and cause the secretion of saliva. Different signal can cause same responses also. For examples, both adrenalin and pancreatic glucagon can enhance the level of blood sugar. If some signal stimulates cell consistently, cell can make its receptor obtuse by the ways as following: ① modify and inactivate receptor. ② move the receptor into inside of cell (receptor sequestration). ③ By endocytosis, digest receptor with lysosome (receptor down-regulation).

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Cell surface receptor and intracellular receptor

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Protein Kinase

Protein kinase is a type of phosphate transferase that can transfer the Pi from ATP to specific amino acid residue to phosphate and activate protein. The functions of protein kinase during the signal transduction include: 1. regulate protein activity by phosphorylation. Some proteins will be activated by phosphorylation, and some will be inactivated by this modification. 2. enlarge the signal responses by the phosphorylation.

Types of protein kinase

KinasesKinases Receivers for PiReceivers for Pi

Ser/Thr kinaseSer/Thr kinase Hydroxyl of Ser/ThrHydroxyl of Ser/Thr

Tyr kinaseTyr kinase Phenol hydroxyl of TyrPhenol hydroxyl of Tyr

His/Lys/Arg kinase His/Lys/Arg kinase Imidazole ring, guanidine, Imidazole ring, guanidine, εε-amin-amin

Cys kinaseCys kinase SulfydrylSulfydryl

Asp/Glu kinaseAsp/Glu kinase Acyl Acyl

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Communication types among cells

There are three main types for the communication.

Communication by gap junction: By gap junction, cells can communicate each other directly based on connexons that are tubes with a hydrophilic tunnel at 1.5nm diameter inside. Connexons allow micromolecules, such as, Ca2+, cAMP passed through, that enhances the same type and bordered cells to response to the signals from other cells, such as, electric excitation.

1111Gap junction

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The communication by plasma membrane bound molecules:

We can call this communication as cell recognition that means the interaction between receptors and their ligands. We can sort the recognitions as the following types:

① The recognition between same type cells from same species of animals. For examples, cell can recognize the bordered cells during the development of embryo. The reactions of blood transmission and skin transplantation are the recognition between same type cells from different resource. ② The recognition between different cells from same species. For examples, sperm and ovum, T cell and B cell. ③ The recognition between different cells from different species. For example, pathogens and host cells. ④The recognition between same type cells from different species. This recognition can be managed under experimental condition.

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Chemical communication:

Chemical communication is the indirect communication of cells. This communication means that cells secret (signaling) the signal molecules, such as, hormone, and excite the target cells to regulate the functions of the target cells. 1. Endocrine: Hormone can regulate the function of target cells distributed anywhere inside of body efficiently, systemically, specifically, and consistently. 2.  Paracrine: The signal molecules secreted by cells can spread to the neighbored cells to regulate them. For example, cytokines and gas molecules (NO). 3.  Synapse signaling. 4.  Autocrine: The signaling cell and target cell are same type, or same one (signaling and targeting itself). Autocrine exists in cancer cells usually. For example, Colic cancer cells can secrete gastrin to mediate the expression of oncogenes (c-myc, c-fos, ras, p21, and others) for the enhancement of tumor growth.

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Chemical communication

1515The types of chemical communication

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II. The signal transductions mediated by plasma membrane bound receptors

Hydrophilic signal molecules (neuron transmitters, hormones, growth factors, and others) can not enter cell directly, they must combine to the specific receptors bound on membrane surface to cause cell responses. The membrane surface bound receptors can be sorted as three types as the follows: ① Ion-channel-linked receptor; ② G-protein-linked receptor; ③ Enzyme-linked receptor. Ion-channel-linked receptor distributes on excitable cells. G-protein-linked receptor and enzyme-linked receptor are located on most of cell types that can work as kinase cascade to phosphate proteins with a serial phosphorylation reactions to transfer and enlarge the signal step by step.

1717Three types of membrane surface bound receptors

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Ion-channel-linked receptor:

Ion-channel-linked receptor is a ligand-gated channel receptor distributed on excitable cells, such as, nerve and muscle cells using neuron transmitters as signal molecules. Neuron transmitter can bind to the receptor and change the structure of the receptor, that leads the ion channel shut down or opened. The permeability of the membrane to ion will be changed at this time. Meanwhile, the chemical signal will be exchanged as electric signal (depolarization), and the electric signal (electric excitation) will be transferred to postsynapse cell …… For example, acetylcholine receptor exists as three structures, they can be opened for 1/1,000,000 second when acetylcholine molecule bind to them. Ion-channel-linked receptors can be sorted as positive ion channels (receptors for acetylcholine, glutamic acid, and 5-TH) and negative channels (receptors for glycine and γ － aminobutyric acid).

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Ion-channel-linked receptor

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A model structure of the receptor of acetylcholine

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Ion-channel-linked receptor at the junction of nerve and muscle

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G-protein-linked receptor:

Trimeric GTP-binding regulatory protein is called as G protein located on plasma side of membrane. G protein is composed of subunit α, β, and γ. G protein is a switch during the signal transduction that can shut down when subunit α binds to GDP, opened up when subunit α binds to GTP. Subunit α is of GTPase activity that can hydrolyze GTP.

G-protein-linked receptor is seven-times-transmembrane protein. The extracellular part of the receptor recognizes and bind signal molecule, and intracellular part of the receptor links to G protein. The extracellular signal bind to the receptor can cause the second messenger formed inside cell by the linked G protein.

The receptors for neuron transmitters, peptide hormones are the G-protein-linked receptor. The receptors for the physical and chemical excitations of taste sense and visual sense are G-protein-linked receptors too.

The signal ways mediated by G-protein-linked receptor include cAMP signal way and phosphatidylinositol signal way.

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The molecule switch of G protein

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G-protein-linked receptor is seven-times-

transmembrane protein

2525Three structures of the acetylcholine receptor

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cAMP signal way:

Extracellular signal binds to the extracellular part of the G-protein-linked receptor. By the mediation of G protein, the intracellular part of the receptor will form the second messenger, cAMP signal that is intracellular signal.

Components of cAMP signal:

①.    Activating hormone receptor (Rs) / inhibiting hormone receptor (Ri).②.    Activating regulatory protein (Gs) / inhibiting regulatory protein (Gi).③.    Adenylyl cyclase: A glycoprotein (150KD) that passes through the plasma membrane 12 times. With Mg2+ or Mn2+, adenylyl cyclase catalyze ATP into cAMP.

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Adenylyl cyclase

Mg2+ or Mn2+

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④ .    Protein Kinase A (PKA): PKA is composed of two catalytic subunits and two regulatory subunits. cAMP binds to regulatory subunits and releases out the activated catalytic subunits that can phosphate the serine and threonine residues of some proteins in cells to change the activity of them.

⑤.    cAMP phosphodiesterase: It can degenerate cAMP to form 5’-AMP, that stops signal.

2929Protein Kinase A

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Degeneration of cAMP

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The model of Gs regulation:

When Gs is inactivated, α subunit is combined with GDP, and adenylyl cyclase has no activity. When the ligand, such as hormone, combines to Rs (extracellular part of receptor), the structure of receptor (Rs) will be changed and the Gs binding site will be explored and the ligand-receptor complex will bind to Gs. The structure of the Gs α subunit will be changed. This α subunit rejects GDP and combine GTP to activate it. Gs will releases out its α, β, and γ subunits, and explore the adenylyl cyclase binding site on α subunit. The explored and GTP combined α subunit will combine adenylyl cyclase and activate it. The activated adenylyl cyclase can cause ATP changed to cAMP. With the GTP hydrolysis, α subunit will return to original structure and be separated from adenylyl cyclase. The activation of adenylyl cyclase will be stopped. α subunit combine β and γ subunits. Gs regulation return back to original. The cholera enterotoxin can catalyze ADP bind to the α subunit of Gs, that cause α subunit to lose its GTPase activity resulting in GTP consistent combination to the α subunit. The α subunit will keep activated consistently, and the adenylyl cyclase will be activated forever. These pathological changes will lead the Na+ and water floated out of cells. The patient will take a severe diarrhea and dehydration.

3232The model of Gs regulation

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The cAMP signal way can be summarized as the follows:

Hormone G protein linked receptor G protein adenylyl cyclase cAMP cAMP dependent PKA Regulatory protein for genetranscription

The different cell responses to cAMP signal way with different speed. For examples, The degeneration of glycogen to glycose-1-phosphate can be started within 1 second in muscle cells. But, it needs several hours in some secreting cells because activated PKA will enter nucleus to phosphorate CRE (cAMP response element) bound protein and regulate the expression of relative gene. CRE is the regulation region of DNA sequence to a gene.

3434cAMP signal and glycogen degeneration

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cAMP signal and gene expression

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The model of Gi regulation:

Gi can inhibit adenylyl cyclase by the following routes: combine to the α ①subunit of adenylyl cyclase and inhibit the activity of enzyme. combine to the α ②subunit of free Gs by binding to the complex of β, γ subunits complex. As result, the activation of adenylyl cyclase by α subunit of Gs will be inhibited.

The pertussis toxin can inhibit the binding of the α subunit of Gi to GTP, and block the inhibition of adenylyl cyclase by Ri receptor. So, the development and pathological syndrome of chin cough are associated with the inhibition of Gi regulation way. But, the detailed mechanism about this inhibition keeps unknown so far.

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The model of Gi regulation

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Phosphotidylinositol signal way:

In the phosphotidylinositol signal way, extracellular signal molecules combine to the G protein linked receptor on membrane surface, and activate the phospholipase C (PLC-β) that hydrolyze 4,5-diphophotidylinositol (PIP2) into 1,4,5–triphophotidylinositol (inositol phosphate 3, IP3) and diacyl glycerol (DG) as two second messengers. Extracellular signals are exchanged as intracellular signals at this time. We call the signal system as double messenger system).

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Phosphatidylinositol signal way

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IP3 can combine to the IP3 ligand gate Ca2+ channel to open it, and the Ca2+ concentration in cell will be lifted up. The high Ca2+ concentration will activate every Ca2+ dependent protein. If you use Ca2+ carrier ionomycin to treat cultured cells, you will get same result as described above. DG can activate plasma membrane bound protein kinase C (PKC). PKC is distributed in cell plasma without activity usually, but when cell received some excitation, IP3 will make high Ca2+ concentration, then, PKC is translocated onto plasma membrane inside surface to be activated by DG. The activated PKC can phosphorate Ser/Thr residues of proteins causing different cell exhibited with different response. For examples, secretion of cell, contraction of muscle, proliferation and differentiation of cell. The role of DG can be mimicked by phorbol ester.

4141The roles of IP3 and DG

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Ca2+ signal level can be regulated by calmodulin (CaM). Ca2+ bound CaM can activate CaM-Kinase. So, the response of cell to Ca2+ is depended upon the Ca2+ bound proteins and CaM-Kinases in cell. For example, CaM-Kinase II is adjacent at the synapse of neuron that is associated with memory formation.

IP3 signal will be dephosphorylized or phosphorylized as IP2 or IP4 to be stopped. Ca2+ will be exported out from cell by the pumps of Ca2+ and Na+-Ca2+.

DG will be stopped by two ways: 1. DG is phosphorylized as phosphatidic acid. 2. DG is hydrolyzed as monoesterglycerol.

4343The cancellation of Ca2+ signal

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Other G protein linked receptors:

1 ． The G protein in chemical receptor: The gas molecules can combined to the G protein linked receptor in the chemical receptor and activate adenylate cyclase to form cAMP, and open cAMP-gated cation channel to depolarize the membrane to cause the neuron excitation. This excitation causes olfaction or taste sense. 2 ． The G protein in optic receptor: Rhodopsin (Rh) is the G protein linked receptor in optic receptor complex. Light can change the structure of Rh and degenerate Rh as retinene and opsin. The opsin can activate G protein. The activated G protein can activate cGMP phosphodiesterase to hydrolyze cGMP in retinal rod cells. The Na+ channel will be shut down and the retinal rod cells will be hyperpolarized, that causes visual sense. The serial changes described as above can be shown as the follows briefly: Light signal Rh activated G protein activated cGMP phosphodiesterase activated The level of cGMP decreased Na+ channel shut down Concentration of Na+ in retinal rod cell fallen down Retinal membrane hyperpolarized The secretion of neuron transmitters inhibited visual sense

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G protein in optic receptor

G protein linked receptor

Retinal rod cell

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Enzyme linked receptor:

Enzyme linked receptor can be sorted as two types: 1. The receptors are of kinase activity, such as, peptide growth factors (EGF ， PDGF ， CSF) receptors. 2. The receptors are not of kinase activity, and they can link to non-receptor tyrosine kinase. For example, the super receptor family of cytokine. The receptors above can be activated when they combine to their ligands by a dimerization way. Six types of enzyme linked receptor were identified so far (Because they have kinase activity, we call them as receptor kinase): Receptor tyrosine ①kinase. Receptor linked tyrosine kinase. Receptor tyrosine lipase. ② ③ ④Receptor Ser/Thr kinase. Receptor guanylate cyclase. Receptor linked ⑤ ⑥histidine kinase. Receptor tyrosine kinase: 1. Tyrosine kinase: Tyrosine kinase can be sorted as three types: Receptor tyrosine kinase (it ①is located in membrane as a transmembrane protein). More than 50 types have been found in vertebrates. Plasma tyrosine kinase, such as, Src family, Tec ②family, ZAP70 family, and JAK family. Nucleus tyrosine kinase, such as Abl ③and Wee. The extracellular part of receptor tyrosine kinase is to be bound by ligands (polypeptide or hormones). Intracellular part is catalytic domain. These receptors include EGF 、 PDGF 、 FGF and others.

4747The dimerization and self-phosphorylation of receptor tyrosine kinase

4848Receptor tyrosine kinases

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The recognition domain between signal molecules:

A 50 – 100 mer domains in signal molecules are homologous each other. These domains can mediate the signal recognition or be linked together to form the signal transduction pathways like computer connections to connect each parts as “Signal transduction network”. The domains include: SH2 domain (Src Homology 2 domain): 100mer. Mediate the combination of signal and the proteins that contain phosphate tyrosine. SH3 domain (Src Homology 3 domain): 50~100mer. Mediate the combination of signal and the proteins that contain prolines. PH domain (Pleckstrin Homology domain): 100~120mer. Combine to membrane surface phospholipids (PIP2, PIP3, IP3) to translocate the PH domain protein to membrane from plasma.

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Ras signal pathway: Receptor tyrosine kinase (RTK) will be activated after signal combined, structure dimerized, and molecule phosphorated by itself. The activated RTK can activate Ras. Ras is a proto-oncogene family associated with cancer development. Raf is another proto-oncogene family. It is Sr/Thr protein kinase and also called as mitogen-activated protein kinase (MAPKKK). The N terminal of Raf can bind to Ras to be activated. Activated Raf will cause a serial reactions of protein kinase phosphorylation to take effects on the growth and differentiation of cells. The statement above is important to understand tumor development.

RTK-Ras signal pathway (a serial of activations) can be described briefly as the follows:

Ligand RTK adaptor GEF Ras Raf （ MAPKKK ） MAPKK MAPK Activated MAPK enters nucleus Transcription factors (Elk-1and others) Enhance proto-oncogenes (c-fos, c-jun) expression Tumors.

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Ras signal pathway

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The signal transductions mediated by insulin receptor:

Insulin receptor is receptor tyrosine kinase too composed of α subunits and β subunits. The β subunit is of kinase activity by that the insulin receptor substrates (IRS) can be phophorated. IRS can activate the proteins containing SH2 domain, such as, phosphotidylinositol 3-kinase (PI3K).

PI3K can catalyze phosphotidylinositol (PI) to form other two PI molecules, PI(3,4)P2 and PI(3,4,5)P3, as the anchoring sites for the intracellular signal proteins containing PH domain, and activate these proteins. The signal pathways for that include: ① Activate Bruton's tyrosine kinase (BTK) and phospholipase Cy to lead phosphotidylinositol pathway. ② Activate phosphoinositol dependent kinase (PKD1). PKD1 activates protein kinase B (PKB). Activated PKB can phosphorate the BAD protein that is associated with apoptosis to inhibit BAD activity for cell survival.

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IRS

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The activation of protein kinase B

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Receptor serine/threonine kinases: Receptor Ser/Thr kinases are transmembrane receptors, and main ligands for it are the members of the family of transforming growth factor-β (TGF-β) including TGF-β1 - TGF-β5. TGF-βs are of complicated bio-functions including cell proliferation inhibiting, matrix synthesis enhancing, skeleton growing, and others.

Receptor tyrosine phosphatases: Receptor tyrosine phosphatases are transmembrane receptors too. They are coworkers for receptor tyrosine kinases probably to regulate cell cycle. CD45 is the one of these receptors. Like receptor tyrosine kinases, receptor tyrosine phosphatases can be translocated as the cytosol tyrosine phosphatases with two SH domains, SHP1 and SHP2. The blood cells of the mouse with SHP1 deficiency are abnormal. It indicates that cytosol tyrosine phosphatases are associated with the differentiation of blood cells.

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Receptor guanylate cyclase: Receptor guanylate cyclase is transmembrane protein too. Like the receptor enzymes above, receptor guanylate cyclase is composed of extracellular part for signal binding and intracellular part for its catalytic activity. The ligands for it include atrial natriuretic peptides (ANPs) and brain natriuretic peptides (BNPs). When the blood pressure is increased, the atrial muscle cells secret ANPs to enhance the exportation of water and Na+ from kidneys, and the vascular smooth muscle cells are relaxed, the high blood pressure will be relieved.

Receptor super family of cytokine: Cytokine receptors are tyrosine kinase associated receptors. IL, IFN, CSF (colony stimulation factor), GH (growth hormone) and others are important to the communication of blood synthesizing stem cells and immune cells. The signal pathway for that is JAK － STAT or Ras pathway.

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JAK (just another kinase or janus kinase) is a family that is not receptor tyrosine kinase including JAK1, JAK2, JAK3 and TYK1. The substrate for JAK is STAT (signal transducer and activator of transcription) with SH2 and SH3 domains. This signal pathway is called as JAK-STAT pathway. It can be briefly described as the follows: 1. Combination of ligand and receptor causes dimerization of the receptor. 2. The dimerized receptor activate JAK. 3. The activated JAK phosphorates STAT. 4. The phosphorated STAT is dimerized and explores its nucleus binding signal. 5. The STAT dimer enters nucleus and regulates gene expression.

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JAK-STAT signal pathway

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III. The signal transductions mediated by intracellular receptors

The intracellular receptors are the gene regulatory proteins that can be activated by hormones. These receptors can combine inhibiting proteins, such as Hsp90, to form complexes that are not activated yet. If they combine to their ligands, such as cortisol (hormone from adrenal cortex), the inhibiting protein will fall off from the complex, and the receptor will be activated because the DNA binding site is explored. Steroid hormones are hydrophobic micromolecules (around 300Da). They can pass through the plasma membrane and nucleus membrane. The activated receptors can combine to specific DNA sequences to regulate gene expression. The combination of receptor and DNA sequence has been proved. The specific DNA binding sequence is receptor dependent transcription enhancer. The gene activation induced by steroid hormone can be sorted as two stages:

Activate the primary reactions of transcription of some special genes fast. ① ②The gene products in the primary reactions activate other genes. The primary reactions will be enlarged in this step. That is why we say that steroid hormones have enlarged and very efficient function to regulate the long time biological effects, such as cell differentiation. The mechanism for regulation by thyroid hormone and estrogen is same to steroid hormones.

6060Intracellular receptors

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IV. Regulable protein degeneration and signal transduction

Many very complicated signal transductions are involved with the cell development, cell function, and other life events. Excepting the signal transduction pathways described as above, the regulable protein degeneration pathways are also important for these life events. These pathways include Wnt, Hedgehog, Notch, NF-κB, and others. These pathways can take effects on the differentiation of adjacent cells, so, they are called as lateral signaling.

Wnt signal pathway: Wnt is a type of secreted glycoproteins. If the oncovirus is integrated to Wnt, the breast cancer will be induced. This integration was named as Int1. Wnt signal pathway can cause the accumulation of β-catenin in cell. β-catenin is a multiple function protein involved with cell junction to form adhesion belt, but, the free β-catenin can enter nucleus to regulate gene expression. If Wnt is abnormally expressed or activated, it will cause tumors.

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the receptor for Wnt is frizzled (Frz), a transmembrane protein. Activated Frz can activate the dishevelled (Dsh) in plasma, activated Dsh can block off the degeneration of β-catenin to accumulate β-catenin in plasma. The accumulated β-catenin will enter nucleus to interact with T cell factor / lymphoid enhancer factor (TCF/LEF) and regulate the target gene expression. TCF/ LEF is the transcription factors with bidirectional regulatory function. Its combination to Groucho can inhibit gene transcription, but to β-catenin inhance gene expression. Wnt can bind to another receptor, LRP5/6, that is LDL-receptor-related protein (LRP). We do not know LRP5/6 how to interact with Frz to activate Dsh so far. Wnt signal pathway can be briefly described as the follows: Wnt Frz Dsh Inhibition of degeneration of β-catenin β-catenin accumulation Enter nucleus TCF/LEF Transcription of some proto-oncogenes (c-myc 、 cyclinD1).

6363Wnt signal pathway

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Notch signal pathway:

Notch gene was firstly found in fruit fly. Its absence can cause the abnormal wing development. During the embryo development, when the neuron has been differentiated out from the precursors of epithelial tissue, the adjacent cells will be inhibited to differentiate to the same direction by the combination of the Notch on neuron differentiated cell and the Delta (ligand for Notch) on the adjacent cell surface. This combination can start the Notch signal pathway. We call this inhibition as lateral inhibition. If Notch was mutated, the embryo will die during the development because too many neurons were formed. Notch signal pathway is composed of Notch, Notch ligand, and CSL (some DNA bound protein). When Notch ligand, such as Delta, is combined with the Notch on adjacent cells, the Notch molecule will be cleaved by protease and release out the ICN (intracellular domain of Notch) that is of nucleus location signal function. The ICN will enter nucleus to bind to CLS and regulate gene expression. The signal pathway can be described as the follows: Delta Notch Enzyme cleavage ICN nucleus CLS-IC Gene transcription.

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Notch signal pathway

h

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Notch gene absence can cause the abnormal wing development of fruit fly

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Hedgehog signal pathway:

Hedgehog is secreted protein combined with cholesterol that is important for embryo development. If this gene was mutated, the fruit fly will developed out many spines on its surface like a hedgehog, that is why the gene was named as Hedgehog. In vertebrates, there are three genes to encode Hedgehog at least. They are Shh (Sonic hedgehog), Ihh (Indian hedgehog), and Dhh (Desert hedgehog).

Patched (Ptc) and Smoothened (Smo) are transmembrane proteins that mediate Hedgehog signal transduction into cell plasma. Ptc can inhibit Smo when Hedgehog is absent. The combination of Hedgehog and Ptc can stop the inhibition of Smo by Ptc, then, the activated Smo will cause a serial of down stream reactions to finish the Hedgehog signal transduction.

The transcription factor for Hedgehog signal pathway is Ci or Gli.

6868Hedgehog signal pathway

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NF-κB signal pathway:

NF-κB is the transcription factor of Rel family. NF-κB is involved with the regulation of immunology, inflammation, and the transcription of the genes that are associated with cell differentiation. In the cells of mammalian, there are 5 NF-κB members of Rel family: RelA (P65), RelB, RelC, NF-κB1(P50), and NF-κB2 (P52). Usually, the dimer of NF-κB is combined with IκB, an inhibition protein, to be embedded in plasma. Extracellular excitation can promote the IκB degeneration and push NF–κB dimer to enter nucleus for the regulation of gene transcription.

The members of IκB family include IκBα, IκBβ, IκBγ, IκBδ, IκBε, and Bcl-3.

IKK (IκB kinase) is the key kinase for NF-κB signal transduction. The extracellular signals, such as TNFα and IL-1, can activate IKK to cause IκB phosphorylation. The phosphorated IkB is easy to be degenerated.

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TGFβ-Smad signal pathway:

TGFβ is a growth inhibition factor family. TGFβ receptors include receptor I and II on the membrane. TGFβ can activate its receptor II, then, activated receptor II activates receptor I. The activated receptor I can phosphate the Smad-1 in plasma. The activated Smad-1 forms a dimer with Smad-4, then the dimer enters nucleus, binds DNA, and starts transcription.

TGFβ-Smad signal pathway is important in the primary stage of embryo development because it can induce some specific tissue development. TGFβ-Smad signal pathway is also important in tumor development. In some cancer tissues, such as colic carcinoma, the TGFβ receptor is absent.

Receptor II

TGFβ

Receptor I

Smad 1

Complex of Smad 1 and 4

Nucleus

TGFβ-Smad pathway

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My presentation about the chapter of cell communication ends here. This chapter is important for your life scientific career but complicated to your test of cell biology. So, pay your attention to this chapter please because signal transduction is one of the hot research projects for past decades, and will be so in future.