Tema 10 B (4).pdf

BIOSIGNALING Introduction. Extracellular chemical signals: hormones, neurotransmitters and growth factors. Properties of signal transduction mechanisms Main signal transduction systems: membrane and intracellular receptors Molecular mechanisms of signal...

BIOSIGNALING Introduction. Extracellular chemical signals: hormones, neurotransmitters and growth factors. Properties of signal transduction mechanisms Main signal transduction systems: membrane and intracellular receptors Molecular mechanisms of signal transduction. 1 BIOSEÑALIZACIÓN: 2ª PARTE 2  cAMP IS ABLE TO MEDIATE MULTIPLE SIGNALS DUE TO LOCALIZATION OF PROTEIN KINASE A PKA is localized to particular structures by anchoring protein Different anchors (AKAPs) are expressed in different cell types to determine the downstream effect of cAMP AKAP5 has binding sites for the β-adrenergic receptor, adenylyl cyclase, PKA, and a phosphoprotein phosphatase (PP2A), bringing them all together in the plane of the membrane. When epinephrine binds to the β-adrenergic receptor, Gsα triggers adenylyl cyclase produces cAMP, which reaches the nearby PKA quickly and with very little dilution. PKA phosphorylates its target protein, altering its activity, until the phosphoprotein phosphatase removes the phosphoryl group and returns the target protein to its prestimulus state. The AKAPs in this and other cases bring about a high local concentration of enzymes and second messengers, so that the signaling circuit remains highly localized, and the duration of the signal is limited. 3 4  cAMP IS A COMMON SECONDARY MESSENGER A large number of GPCRs mediate their effects via cAMP ✓ Both activating and inhibiting cAMP synthesis The human genome encodes about 1000 GPCRs ✓ With ligands such as hormones, growth factors, and neurotransmitters There are also hundreds of different GPCRs that can be responsible for similar processes ✓ Such as taste or smell Ligands for many GPCRs have yet to be identified 5  GPCRS CAN USE OTHER SECONDARY MESSENGER MOLECULES Inositol triphosphate (IP3) and Ca2+ as second messengers Two intracellular second messengers are produced in the hormone-sensitive phosphatidylinositol system: inositol 1,4,5-trisphosphate (IP3) and diacylglycerol are cleaved from phosphatidylinositol 4,5-bisphosphate (PIP2). Both contribute to the activation of protein kinase C. By increasing cytosolic [Ca2+], IP3 also activates other Ca2+-dependent enzymes, thus Ca2+ also acts as a second messenger. 6  Calcium modulates the function of many enzymes through calmodulin En varios tipos de células el aumento de la [Ca2+] desencadena respuestas como: exocitosis en neuronas, contracción muscular o reordenaciones del citoesqueleto. Llegada de impulso nervioso o de hormonas en estas células resulta en un aumento de la [Ca2+] (ya sea proveniente del retículo endoplasmático o del exterior de la célula a través de canales específicos de membrana) y éste aumento desemboca en una respuesta celular. El cambio en la [Ca2+] es detectado por la proteína calmodulina que al unir Ca2+ cambia de conformación y activa a la calmodulina quinasa (CaM quinasa), que fosforila a proteínas “diana” regulando sus actividades. A: Calmodulina: B: el importante cambio de conformación que sufre la calmodulina al unir Ca2+ y cómo se une a la proteína diana calmodulina quinasa). 7  II. Enzyme-linked membrane receptors II. 1. Receptor Tyrosine Kinases 8 II. 1. Receptor Tyrosine Kinases Many membrane receptors consists of: ✓ Extracellular ligand-binding domain, and of intracellular catalytic domain The most common catalytic domains have the tyrosine kinase activity ✓ Adds a phosphate group to itself; auto-phosphorylation leads to a conformational change allowing binding and catalytic phosphorylation of specific target proteins ✓ Adds a phosphate group to a tyrosine in specific target proteins Include those for insulin (INSR), Vascular epidermal growth factor (VEGFR), Platelet-derived growth factor (PDGFR), Epidermal growth factor (EGFR), High-affinity nerve growth factor (TrkA), Fibroblast growth factor (FGFR). All these receptors have a Tyr kinase domain on the cytoplasmic side of the plasma membrane (blue). The extracellular domain is unique to each type of receptor, reflecting the different growth-factor specificities. 9  INSULIN: THE HORMONE FOR GLUCOSE UPTAKE AND METABOLISM Insulin Interacts with the Cell via a Receptor Tyrosine Kinase Insulin is a peptide hormone that is produced by the -cells of islets of Langerhans in the pancreas Insulin is produced and released from the pancreas in response to nutrients such as glucose Insulin reaches target cells, such as liver, muscle, or fat tissue cells via bloodstream Binding of insulin to the insulin receptor initiates a cascade of events that leads to increased glucose uptake and metabolism Inability to make or sense insulin→ diabetes Insulin signaling cascade: ligand binding Insulin binding to the extracellular domains of the receptor activates the catalytic domain inside in the cell Catalytic domain in one receptor phosphorylates Tyr residues in another receptor Receptor auto-phosphorylation allows binding and phosphorylation of protein IRS-1 (Insulin Receptor Substrate-1) 10 Insulin Signaling Cascade Indirect interaction of phosphorylated IRS (Insulin receptor (INSR) with protein Ras initiates a series of protein phosphorylations ERK (extracellular regulated kinase); one of the phosphorylated protein kinases, enters the nucleus A transcription factor Elk1 becomes phosphorylated and stimulates the expression of specific genes ✓glucose transporter (GLUT4) 11  Two insulin signaling pathways 1- Activation of enzyme phosphoinositide kinase 3 (PI3K): 12 2- Inactivation of enzyme glycogen synthase kinase 3 (GSK 3) Glycogen sinthesis 13 Crosstalk Between a Tyrosine Kinase Receptor and a GPCR Is an Example of Integration of Signals 14  II. Enzyme-linked membrane receptors II. 2. Receptor guanylyl cyclase 15 II. 2. Receptor guanylyl cyclase Catalytic domain converts GTP to cGMP Works through activation of protein kinase G Two types of guanylyl cyclase that participate in signal transduction: One type is a homodimer with a single membrane spanning segment in each monomer, connecting the extracellular ligand-binding domain and the intracellular guanylyl cyclase domain. Receptors of this type are used to detect two extracellular ligands: atrial natriuretic factor (ANF; receptors in cells of the renal collecting ducts and vascular smooth muscle) and guanylin (peptide hormone produced in the intestine, with receptors in intestinal epithelial cells). The guanylin receptor is also the target of a bacterial endotoxin that triggers severe diarrhea. The other type is a soluble heme-containing enzyme that is activated by intracellular nitric oxide (NO); this form is present in many tissues, including smooth muscle of the heart and blood vessels. 16  IV/ GATED ION CHANNELS Regulate transport of ions across cell membranes Responds to: ✓ Changes in the membrane potential ✓ Ligand binding to specific receptor sites Membranes are electrically polarized The inside of the cell is typically negatively charged compared to the outside: –50 to –70 mV The membrane potential is largely due to electrogenic Na+K+ ATPase ✓ 3 Na+ out ✓ 2 K+ in Flow of ionic species across the membrane depends on its concentration gradient and overall electrical potential Blue arrows show the direction in which ions tend to move spontaneously across the plasma membrane in an animal cell, driven by the combination of chemical and electrical gradients. The chemical gradient drives Na+ and Ca2+ inward (producing depolarization) and K outward (producing hyperpolarization). The electrical gradient drives Cl- outward, against its concentration gradient (producing depolarization). 17 18  Voltage-Gated and Ligand-Gated, Ion Channels in Nerve Signaling Nerve signals within nerves propagate as electrical impulses. Propagation of the impulse involves opening of voltage- gated Na+ channels. Opening of voltage-gated Ca++ channels at the end of the axon triggers the release of neurotransmitter acetylcholine. Acetylcholine opens the ligand-gated ion channel on the receiving cell. 19