Principles of Cell Biology (BIOL2060) Department of Biology Memorial University of Newfoundland Signal Transduction Mechanisms: II Messengers and Receptors Cell communication occurs through chemical signals and cellular receptors by either the 1) direct contact of molecules on two cell’s surfaces or the 2) release of a "chemical signal" recognized by another cell (near or far). Hormones are carried by the circulatory systems to many sites. Growth factors are released to act on nearby tissues. Ligands are signals that bind cell surface receptors (as observed with insulin (a ligand) and the insulin receptor) or that can pass into the cell and bind an internal receptor (such as the steroid hormones). Signal Transduction Signal transduction is defined as the ability of a cell to change behaviour in response to a receptor-ligand interaction. The ligand is the primary messenger. As the result of binding the receptor, other molecules or second messengers are produced within the target cell. Second messengers relay the signal from one location to another (such as from plasma membrane to nucleus).
Transcript
Principles of Cell Biology (BIOL2060)
Department of Biology Memorial University of Newfoundland
Signal Transduction Mechanisms: II Messengers and Receptors
Cell communication occurs through chemical signals and cellular receptors by either the 1) direct contact of molecules on two cell’s surfaces or the 2) release of a "chemical signal" recognized by another cell (near or far). Hormones are carried by the circulatory systems to many sites. Growth factors are released to act on nearby tissues. Ligands are signals that bind cell surface receptors (as observed with insulin (a ligand) and the insulin receptor) or that can pass into the cell and bind an internal receptor (such as the steroid hormones).
Signal Transduction
Signal transduction is defined as the ability of a cell to change behaviour in response to a receptor-ligand interaction. The ligand is the primary messenger. As the result of binding the receptor, other molecules or second messengers are produced within the target cell. Second messengers relay the signal from one location to another (such as from plasma membrane to nucleus). Often a cascade of changes occur within the cell which results in a change in the cell’s function or identity. The signal transduction pathway can act to amplify the cellular response to an external signal. Messenger molecules may be amino acids, peptides, proteins, fatty acids, lipids, nucleosides or nucleotides. Hydrophilic messengers bind to cell membrane receptors. Hydrophobic messengers bind to intracellular receptors which regulate expression of specific genes.
A ligand binds its receptor through a number of specific weak non-covalent bonds by fitting into a specific binding site or "pocket". In situations where even low concentrations of a ligand will result in binding of most of the cognate receptors, the receptor affinity is considered to be high.
Low receptor affinity occurs when a high concentration of the ligand is required for most receptors to be occupied. The dissociation constant (Kd,) is the concentration of ligand required to occupy one half of the total available receptors. This measurement of receptor affinity is often in the range of 10-4 to 10-9 mM.
With prolonged exposure to a ligand (and occupation of the receptor) cells often become desensitized. Desensitization of the cell to a ligand depends upon receptor down-regulation by either 1) removal of the receptor from the cell surface (receptor-mediated endocytosis) or 2) alterations to the receptor that lower the affinity for ligand or that render it unable to initiate the changes in cellular function (such as phosphorylation). Desensitization may lead to tolerance, a phenomenon that results in the loss of medicinal effectiveness of some medicines that are over prescribed. Receptor binding activates a "preprogrammed" sequence of signal transduction events that make use of previously dormant cellular processes.
G Proteins
Activiated G Proteins bind to enzymes or other proteins and alter the target protein’s activity. G Proteins are guanine-nucleotide binding proteins. G Protein-linked Receptors have an extracellular N-terminus and a cytosolic C-terminus separated by seven transmembrane alpha helices connected by peptide loops. One of the extracellular segments has an unique messenger-binding site. The cytosolic loop between the 5th and 6th alpha helices specifically binds a particular G protein.
G Proteins bound to GTP are active, those bound to GDP are not. The two classes of G Proteins are large heterotrimeric G Proteins and small monomeric G Proteins. In heterotrimeric G proteins (G alpha, beta, gamma), when a messenger binds the G Protein-linked receptor, the receptor changes conformation to allow association of the trimeric G Protein with the receptor. G-alpha subunit binds the guanine nucleotide (GDP or GTP). This interaction causes the G alpha subunit to release the GDP, pick up a GTP and detach from the complex. Depending upon the G protein in question, either the GTP-G alpha complex, the G beta- G gamma complex or both bind target protein(s).
The G alpha will remain an activating messenger until the GTP is hydrolyzed by the G alpha subunit (GTP -> GDP +Pi). The "inactive" GDP-G alpha will then reassociate with the G-beta-G alpha complex to rapidly turn down this pathway when the original stimulatory signal is removed.
Large numbers of G proteins provide diversity for signal transduction events. Some bind potassium or calcium ion channels in neurotransmitters. Some activate kinases (enyzmes that phosphorylate). Some cause either the release or formation of major second messengers such as cyclic AMP (cAMP) and calcium ions.
cyclic AMP is a second messenger used by a major class of G proteins. cyclic AMP (cAMP) is generated by adenylyl cyclase which is embedded in the plasma membrane with the enzymatic activity in the cytoplasm. Adenylyl cyclase is activated by binding an activated alpha subunit of the Gs G-protein (GTP-Gs). Phosphodiesterase continally degrades cAMP so in the absence of the ligand and active G-Protein, cAMP levels are reduced. Protein kinase A (PKA), a cAMP-dependent kinase, is the main intracellular target of cAMP. PKA phosphorylates a number of proteins that bear the key short stretch of amino acids, the PKA phosphorylation site (PKA PO4 site). PKA transfers a phosphate from the ATP to a serine or threonine in the PKA PO4 site. cAMP activates the catalytic subunits by causing the release of the negative regulatory subunits.
Disruption of G Protein signaling causes several human diseases. Vibrio cholerae (causes cholera) secretes the cholera toxin which alters salt and fluid in the intestine normally controlled by hormones that activate Gs G-Protein to increase cAMP. The cholera toxin enzymatically changes Gs so that it is unable to convert GTP to GDP. Gs can not then be inactivated and cAMP levels remain high causing intestinal cell to secrete salt and water. Eventually dehydration can lead to death (cholera).
Many G proteins use inositol triphosphate and diacylglyceral as second messengers G Protein-linked Receptor. The Gp G-Protein is activated by a ligand binding its G Protein-linked receptor to activate phospholipase C.
Phosphatidylinositol-4,5-bisphosphate (PIP2) is cleaved by phospholipase C into two molecules cytosolic inositol-1,4,5-triphosphate (InsP3) and membrane-bound diacylglycerol (DAG). The InsP3 receptor, a ligand-gated calcium channel in the endoplasmic reticulum, binds InsP3 and calcium ions are released into the cytosol. Calcium binds a protein known as calmodulin, and the Ca-calmodulin complex act to activate an number of processes. DAG remains membrane-bound and activates protein kinase C (PKC).
Calcium as a signal
The release of calcium ions is a key event in many signaling processes. Intracellular concentrations can be followed by injection of calcium indicator fluorescent dyes. Presence of ligand or increase in InsP3 and monitoring the increase in fluorescence The calcium ionophore releases calcium from the intracellular stores that mimics effect of InsP3 activation. Calcium ions act to regulate many cellular functions. Calcium levels in the cytoplasm is normally kept low (10-4) by calcium pumps in the plasma membrane (out of the cell) and by sodium-calcium exchangers a) out of the cell, b) into the endoplasmic reticulum (ER) lumen and c) into the mitochondrion. Calcium stores can be released from the ER by the InsP3 receptor channel and ryanodine receptor channel which opens in the presence of calcium itself (calcium-induced calcium release).
Although other proteins bind calcium to control activity, most often binding to the protein calmodulin, to form a calcium-calmodulin complex, is an intermediate step. When calcium ions are present, two bind each globular end (4 in total), the helical arm region then changes conformation (the active complex) and then the wraps around the calmodulin-binding site of target proteins. These are often protein kinases and protein phosphatases which vary depending upon the target cell (different cells have different responses).
Fertilization of animal eggs reveals an important example of calcium-mediated signal transduction after a receptor-ligand interaction. Initially the sperm binds the egg’s surface at the membrane and within 30
seconds, a wave of calcium release spreads from the site of sperm contact. Two main events in fertilization rely on calcium release. 1) Calcium stimulates the fusion of the cortical granules with the egg’s plasma membrane to alter the coat surrounding the egg to help prevent the binding of another sperm cell to the egg (slow block to polyspermy). 2) Calcium initiates egg activation, the resumption of metabolic processes.
Nitric oxide as a Signal
Nitric oxide (NO) is a toxic, short-lived gas molecule and has been found to be a signaling molecule in the cardiovascular system. Nitric oxide couples G protein-linked receptor stimulation in endothelial cells to relaxation of smooth muscle cells in blood vessels. NO synthase converts arginine to citrulline and NO. The binding of acetylcholine causes the release of NO in vascular endothelial cells that causes the relaxation of the vascular smooth muscle (vasodialator). 1) binding of acetylcholine to G protein receptors causes InsP3 production. 2) InsP3 releases calcium ions from endoplasmic reticulum. 3) ca++ ions and calmodulin form complex which stimulates NO synthase to produce NO. 4) NO (g) diffuses from endothelial cell into adjacent smooth muscle cells. 5) In smooth muscle cell, NO activates guanylyl cyclase to make cyclic GMP (cGMP). 6) cGMP activates protein kinase G which phosphorylates several muscle proteins to induce muscle relaxation. ...
Receptor tyrosine kinases
When the ligand binds a protein kinase-associated receptors, the kinase activity is stimulated and a cascade of phosphorylation transmit the signal. The best studied examples are the receptor tyrosine kinases (RTKs). Protein kinases add a phosphate group to amino acids that have a hydroxyl (OH) group containing side chain. (aa-OH to aa-PO4). Receptor tyrosine kinases aggregate and undergo autophosphorylation to start chain reactions that lead to cell growth, proliferation or differentiation. The structure of receptor tyrosine kinases often have one transmembrane (TM) domain, an extracellular ligand-binding domain and a cytosolic tail that contains tyrosine residue targets of the tyrosine activity. The RTK can be comprised of either one protein or two proteins: a receptor and a tyrosine kinase.
The separate kinase is a nonreceptor tyrosine kinase. Activation of the receptor tyrosine kinases is started by ligand binding causing receptor aggregation, often as dimers. Once clustered the tyrosine kinase activity phosphorylates other RTK of the same type (autophosphorylation). With phosphorylation, cytosolic adaptor proteins bind to receptors phosphorylated tyrosine residues. The adaptor proteins recognize short stretches of amino acids which include a phosphotyrosine through specific recognition domains such as the SH2 domain. RTKs can activate several signal transduction pathways at once, including inositol-phospholipid-calcium pathway and the Ras pathway.
Receptor tyrosine kinases can initiate a Ras/MAP kinase signal transduction cascade Phosphotyrosine-containing sites bind SH2-containing proteins which result in Ras, a small monomeric G protein, becoming activated. Once the epidermal growth factor receptor (EGFR) is autophosphorylated in response to the EGF ligand, a complex of GRB2 (SH2 domain-containing) and Sos (guanine-nucleotide release protein: GNRP) binds the receptor. Sos is thus activated to cause Ras to release GDP that allows Ras to bind a new GTP and become active. Once active, Ras triggers a cascade (the Ras pathway) which includes the mitogen-activated protein kinases (MAPKs). Note: a mitogen is a growth factor signal. Receptor tyrosine kinases activate a variety ofsignaling pathways RTK can activate a form of phophlipase C and phosphatidylinositol-3- kinase (PI3K).
Growth Factors
Growth factors act as messengers In addition to nutrients, cell often need growth factors to grow including a) Platelet-derived growth factor (PDGF), b) insulin, c) insulin-like growth factor 1 (IGF-1), d) fibroblast growth factor (FGF), e) epidermal growth factor (EGF) and f) nerve growth factor (NGF). These RTK ligands function in much more than growth and cell division.
Disruption of growth factor signaling through receptor tyrosine kinases can have dramatic effects on embryonic development. The fibroblast growth factors (FGFs) and fibroblast growth factor receptors (FGFRs) function in both embryonic and adult signaling. FGFRs are important in the development of mesoderm, the embryonic tissue that eventually becomes muscle, cartilage, bone and blood cells. A mutant receptor that, due to dimerization with normal versions of FGFR, has a dominant inhibitory effect upon the normal activity is a dominant negative (dn) mutation. A dn mutant version of FGFR mRNA injected into frog eggs cause the failure of mesodermal tissue to develop and produces tadpoles with heads but no bodies. In humans, defects in FGFRs lead to achondroplsia (dwarfism) and thanatophoric dysplasia severe bone abnormalities (fatal in infancy).
Serine/threonine kinase receptors phosphorylate both serine and threonine residues (not tyrosine) and act to transduce other types of growth factor signals. Transforming Growth Factor Beta (TGFß) binding to receptor results in the clustering of type I and type II TGFß receptors. The type I receptors are phosophorylated by type II receptors. Activated Type I receptors phosphorylate specific receptor-mediated SMADs. The activated receptor-mediated SMADs bind to co-SMADs and enter the nucleus to interact with DNA binding proteins to regulate gene expression.
Hormones
Chemical signals known as hormones are secreted by one tissue to regulate another tissue, often over a distance. Hormones are often transmitted by the circulatory system. Hormones control many physiological functions including growth and development, rates of physiological processes, concentrations of sugars and minerals, and responses to stress. Hormones can be proteins, peptides, steroids and other molecules.
Hormonal signals can be classified by the distance that they travel to reach their target cells. An endocrine hormone travels through the circulatory system and a paracrine hormone acts only upon near by cells. A paracrine hormone is roughly equal to a growth factor. Endocrine tissues secrete directly into the blood-stream and exocrine tissues into ducts for transport of the secretions to other parts of the body. The pancreas has both endocrine (insulin and glucagon) and paracrine
(digestive enzymes) functions. Once in the circulatory system, the endocrine hormones will eventually reach their target tissue(s) such as heart and liver (epinephrine) or liver and skeletal muscles (insulin). In the target tissue, intracellular effects, such as the activation of the cAMP pathway can control a number of cell functions. One example, is by epinephrine binding to the beta-adrenergic receptor, activation of PKA to cause the stimulation of glycogen breakdown.
Chemical properties of animal hormones There are four (4) categories of endocrine hormones. 1) amino acid derivatives (epinephrine) 2) peptides (antidiuretic hormone [vasopressin]) 3) proteins (insulin) 4) lipid-like hormones including steroids (testosterone) Paracrine hormones include histamine (a histine derivative) and the prostaglandins (arachidonic acid derivatives).
Apoptosis
Cells regulate programmed cell death (PCD) or apoptosis which is a very ordered mechanism to get rid of cells. Apoptosis is very different from necrosis that results from massive tissue injury. Apoptosis is an important part of normal like (removal of webbing of fingers and toes in embryos, extra neurons in infants and old blood cells in adults). The cell death program involves the activation specific proteases known as caspases from the procaspases. The Fas ligand on the surface of lymphocytes bind the Fas receptors on the infected cell’s surface. This results in the clustering of Fas, the attachment of adapter proteins and assembly of the procaspases at this site. The procaspases activate each other to start a cascade of events that ends in apoptosis.
Notes prepared from The World of the Cell, 7th edition Becker, Kleinsmith. Hardin & Bertoni
Figures copyright of Pearson Education Inc. email me at [email protected]
HORMONAL FEEDBACK REGULATORY SYSTEMS
Feedback control, both negative and positive, is a fundamental feature of endocrine systems.
Each of the major hypothalamic-pituitary-hormone axes is governed by negative feedback, a
process that maintains hormone levels within a relatively narrow range . Examples of
hypothalamic-pituitary negative feedback include
(1) thyroid hormones on the TRH-TSH axis
(2) cortisol on the CRH-ACTH axis
(3) gonadal steroids on the GnRH-LH/FSH axis
(4) IGF-I on the growth hormone–releasing hormone (GHRH)-GH axis.
These regulatory loops include both positive (e.g., TRH, TSH) and negative (e.g., T4, T3)
components, allowing for exquisite control of hormone levels. As an example, a small reduction
of thyroid hormone triggers a rapid increase of TRH and TSH secretion, resulting in thyroid
gland stimulation and increased thyroid hormone production. When thyroid hormone reaches a
normal level, it feeds back to suppress TRH and TSH, and a new steady state is attained.
Feedback regulation also occurs for endocrine systems that do not involve the pituitary gland,
such as calcium feedback on PTH, glucose inhibition of insulin secretion, and leptin feedback
on the hypothalamus. An understanding of feedback regulation provides important insights
into endocrine testing paradigms.
Positive feedback control also occurs but is not well understood. The primary example
is estrogen-mediated stimulation of the midcycle LH surge. Though chronic low levels of
estrogen are inhibitory, gradually rising estrogen levels stimulate LH secretion. This effect,
which is illustrative of an endocrine rhythm , involves activation of the hypothalamic GnRH
pulse generator. In addition, estrogen-primed gonadotropes are extraordinarily sensitive to
GnRH, leading to amplification of LH release.
Paracrine and Autocrine Control
The previously mentioned examples of feedback control involve classic endocrine pathways in
which hormones are released by one gland and act on a distant target gland. However, local
regulatory systems, often involving growth factors, are increasingly recognized.
Paracrine regulation refers to factors released by one cell that act on an adjacent cell in the
same tissue. For example, somatostatin secretion by pancreatic islet delta cells inhibits insulin
secretion from nearby beta cells.
Autocrine regulation describes the action of a factor on the same cell from which it is
produced. IGF-I acts on many cells that produce it, including chondrocytes, breast epithelium,
and gonadal cells. Unlike endocrine actions, paracrine and autocrine control are difficult to
document because local growth factor concentrations cannot be measured readily.
Anatomic relationships of glandular systems also greatly influence hormonal exposure: the
physical organization of islet cells enhances their intercellular communication; the portal
vasculature of the hypothalamic-pituitary system exposes the pituitary to high concentrations
of hypothalamic releasing factors; testicular seminiferous tubules gain exposure to high
testosterone levels produced by the interdigitated Leydig cells; the pancreas receives nutrient
information and local exposure to peptide hormones (incretins) from the gastrointestinal tract;
and the liver is the proximal target of insulin action because of portal drainage from the
pancreas.
Gastrin Pada manusia, gastrin adalah hormon peptida yang merangsang sekresi asam lambung (HCl) oleh sel-sel parietal lambung dan membantu motilitas lambung. Hal ini dirilis oleh sel G di antrum dari lambung, duodenum, dan pankreas. Ini mengikat reseptor cholecystokinin B untuk merangsang pelepasan histamines di enterochromaffin-seperti sel-sel, dan menginduksi penyisipan K + / H + ATPase pompa ke dalam membran apikal sel parietal (yang pada gilirannya meningkatkan H + release). Rilis dirangsang oleh peptida dalam lumen lambung.
Sejarah
Keberadaannya pertama kali diusulkan pada tahun 1905 oleh John fisiologi British Sydney Edkins, [1] [2] dan gastrins diisolasi pada tahun 1964 oleh Roderic Alfred Gregory dan Tracy di University of Liverpool. [3] Pada tahun 1964 struktur Gastrin ditentukan [4].[Sunting] Fisiologi
[Sunting] GenetikaGen GAS terletak pada lengan panjang dari kromosom ketujuh belas (17q21) [5].[Sunting] SintesisGastrin adalah hormon peptida linier diproduksi oleh sel G duodenum dan dalam antrum pilorus lambung. Hal ini disekresikan ke dalam aliran darah. Gastrin ditemukan terutama dalam tiga bentuk:gastrin-34 ("big gastrin")gastrin-17 ("gastrin kecil")gastrin-14 ("minigastrin")Juga, pentagastrin adalah, artifisial disintesis lima urutan asam amino identik dengan urutan lima asam amino terakhir pada akhir C-terminus gastrin.Angka merujuk pada jumlah asam amino.[Sunting] RilisGastrin dilepaskan sebagai respon terhadap rangsangan tertentu. Ini termasuk:distensi perutvagal stimulasi (dimediasi oleh bombesin Neurocrine, atau GRP pada manusia)kehadiran protein yang dicerna sebagian asam amino terutamahypercalcemiaGastrin rilis dihambat oleh: [6] [7]Kehadiran asam (terutama HCl disekresikan) di perut (kasus umpan balik negatif).Somatostatin juga menghambat pelepasan gastrin, bersama dengan secretin, GIP (peptida gastroinhibitory), VIP (peptida vasoaktif usus), glukagon dan kalsitonin.[Sunting] FungsiKehadiran gastrin merangsang sel parietal lambung untuk mensekresikan asam klorida (HCl) / asam lambung. Hal ini dilakukan baik secara langsung pada sel parietal dan tidak langsung melalui pengikatan ke reseptor pada sel-sel ECL CCK2/gastrin dalam lambung, yang kemudian merespon dengan melepaskan histamin, yang pada gilirannya bertindak dengan cara parakrin pada sel parietal
mendorong mereka untuk mengeluarkan ion H +. Ini adalah stimulus utama untuk sekresi asam oleh sel parietal.Seiring dengan fungsi yang disebutkan di atas, gastrin telah terbukti memiliki fungsi tambahan juga:Merangsang pematangan sel parietal dan pertumbuhan fundus.Menyebabkan sel utama untuk mensekresikan pepsinogen, yang zymogen (tidak aktif) bentuk pepsin enzim pencernaan.Meningkatkan mobilitas otot antral dan mempromosikan kontraksi perut.Memperkuat kontraksi antral terhadap pilorus, dan melemaskan sphincter pyloric, yang merangsang pengosongan lambung.Berperan dalam relaksasi dari katup ileocecal [8].Menginduksi sekresi pankreas dan pengosongan kandung empedu [9].Dampak sphincter bagian bawah tone (LES) kerongkongan, menyebabkan ia untuk bersantai [10] Dengan ini menjadi pertimbangan, tingkat tinggi gastrin mungkin memainkan peran dalam pengembangan beberapa gangguan LES lebih umum seperti penyakit asam surutnya..[Sunting] Faktor-faktor yang mempengaruhi sekresi[Sunting] Lambung lumenStimulasi faktor: protein diet dan asam amino (daging), hiperkalsemia. (Yakni selama fase lambung)Faktor Hambat: keasaman (pH di bawah 3) - mekanisme umpan balik negatif, diberikan melalui pelepasan somatostatin dari sel dalam lambung, yang δmenghambat pelepasan gastrin dan histamin.[Sunting] parakrinStimulasi faktor: bombesinFaktor penghambat: somatostatin - bekerja pada somatostatin-2 reseptor pada sel G. dengan cara parakrin melalui difusi lokal di ruang antarsel, tetapi juga sistemik melalui rilis ke dalam sirkulasi darah lokal mukosa, menghambat sekresi asam dengan bertindak pada sel parietal.[Sunting] NervousStimulasi faktor: Beta-adrenergik agen, agen kolinergik, gastrin-releasing peptide (GRP)Faktor Hambat: Enterogastric refleks[Sunting] SirkulasiStimulasi faktor: epinefrinHambat faktor: penghambatan peptida lambung (GIP), secretin, somatostatin, glukagon, calcitonin[Sunting] Peran dalam penyakit
Dalam sindrom Zollinger-Ellison, gastrin diproduksi pada tingkat yang berlebihan, sering kali gastrinoma (gastrin-penghasil tumor, sebagian besar jinak) dari duodenum atau pankreas. Untuk menyelidiki untuk hypergastrinemia (darah tinggi tingkat gastrin), sebuah "pentagastrin test" dapat dilakukan.Pada gastritis autoimun, sistem kekebalan tubuh menyerang sel-sel parietal yang mengarah ke hypochlorhydria (keasaman lambung rendah). Hal ini menyebabkan tingkat gastrin meningkat dalam upaya untuk mengimbangi pH meningkat dalam
perut. Akhirnya, semua sel parietal hilang dan hasil achlorhydria mengarah ke hilangnya umpan balik negatif pada sekresi gastrin. Plasma gastrin konsentrasi meningkat pada hampir semua individu dengan mucolipidosis tipe IV (rata-rata 1.507 pg / mL; kisaran 400-4.100 pg / mL) (normal 0-200 pg / mL) sekunder untuk achlorhydria konstitutif. Temuan ini memfasilitasi diagnosis pasien dengan gangguan ini neurogenetik. [11]
