Cell Signaling in Physiology PDF

Uruk University/ College of Pharmacy Physiology I/ Cell signaling in physiology Cell signaling in physiology Receptors  Several messengers exert their action by targeting a specific protein known as receptors (or receptor proteins) i...

Uruk University/ College of Pharmacy Physiology I/ Cell signaling in physiology Cell signaling in physiology Receptors  Several messengers exert their action by targeting a specific protein known as receptors (or receptor proteins) in a target cell.  The first step in the action is the recognition and binding to the receptor.  The receptor is specific  The receptor has a binding site for that messenger  The binding forces are weak  The binding is reversible  Second step is receptor activation: a conformation change in the tertiary structure of the receptor  Third step is the signal transduction which is a sequence of events in the cell leading to the cell’s response  Cells respond by: Types of Receptors (1) Plasma Membrane Receptors (2) Intracellular Receptors 1-9  Uruk University/ College of Pharmacy Physiology I/ Cell signaling in physiology Interactions between Receptors and Ligands  There are 4 major features that define the interactions between receptors and their ligands: (1) Specificity  The receptor binds a particular ligand and not others.  Only certain cell types express the specific receptor required to bind a given ligand  The same receptor for a giving ligand may produce different response in different tissues (Why?) (2) Affinity  Means the degree to which a particular ligand binds to its receptor.  When a receptor binds a ligand at lower conc. then it is of high affinity.  This is important in minimizing the therapeutic doses of medicines  Example: Paracetamol binds COX enzyme in platelets at 100mg dose; while inducible “inflammatory” require higher doses. (3) Saturation  When the conc. of a ligand increases, the cell’s response increase and reach maximal effect.  When no effect elicits by increasing the ligand’s conc. beyond the maximal effect, the receptors are then saturated, because only a finite number of receptors are available, and they become fully saturated at some point. (4) Competition  It means the ability of a molecule to compete with a natural ligand for binding to its receptor.  Competition typically occurs with ligands that have a similarity in part of their structures, and it also underlies the action of many drugs.  The Competition is either agonistic or antagonistic  Agonistic competition - A ligand binds a receptor and activates it as do natural ligand. - Example: phenylephrine that constrict blood vessels as do norepinephrine  Antagonistic competition - A ligand binds a receptor but do not activate it. It further blocks the binding of a natural messenger. - Example: Antihistamines that block the effect of histamine 2-9  Uruk University/ College of Pharmacy Physiology I/ Cell signaling in physiology Regulation of Receptors  Downregulation:  A decrease in the total number of receptors for a given ligand (by internalization) due to a chronic high extracellular conc. of the ligand.  Up-regulation:  An increase in the total number of receptors for a given ligand (increased expression) due to a chronic low extracellular conc. of the ligand.  Increased sensitivity:  The increased responsiveness of a target cell to a given ligand; may result from up-regulation of receptors (as in denervation supersensitivity of skeletal muscle) Signal Transduction Pathways  What are the sequences of events between receptor activation and appearance of cell response?  Signal transduction pathways are mechanisms linking the receptor activation to cell response and are differ between lipid-soluble and water-soluble ligands (How that?) (1) Pathways Initiated by Lipid-Soluble Ligands  Examples on these ligands: steroid hormones, vitamin D  These ligands target a specific gene  Some of these ligands have inactive receptor located in the cytoplasm and other in the nucleus  The active receptor (ligand-receptor complex) binds to DNA causing alteration in the rates of transcription of one or more genes in a particular cell.  More than one gene may be subject to control by a single receptor type.  Gene transcription either increase or decrease by the same ligand  Example:  Cortisol increase expression many enzymes. A single nuclear receptor causes expression of numerous genes involved in the coordinated control of cellular metabolism and energy balance.  Cortisol inhibits transcription of several genes whose protein products mediate inflammatory responses that occur following injury or infection; for this reason, cortisol has important anti- inflammatory effects. 3-9  Uruk University/ College of Pharmacy Physiology I/ Cell signaling in physiology (2) Pathways Initiated by Water-Soluble Messengers  Examples on these ligands: polypeptide hormones, neurotransmitters, and paracrine and autocrine compounds  They exert their actions by binding to the extracellular portion of membrane receptors (They cannot pass the membrane by diffusion through the lipid bilayer of the plasma membrane, so they do not interact directly with genes).  Terminology:  First messenger: is a drug, hormone or any ligand bind extracellular portion of the receptor.  Second messenger: a substance that enter (e.g.; Ca+2) or generated in the cytoplasm (e.g.; cAMP, IP3, DAG) as a result of receptor activation by the first messenger that ultimately causes phosphorylation of proteins or gating of ion channels.  Protein kinases: are enzymes (e.g.; PKA, PKC) that phosphorylate other proteins by transferring a phosphate group to them from ATP causing changes in protein’s tertiary structure and, consequently, alters its activity (activation or inhibition).  Cascade reactions: Means an active protein kinase phosphorylate another inactive one, and the latter phosphorylate another inactive one and so on. The series ended by phosphorylation of key proteins, such as transporters, metabolic enzymes, ion channels, and contractile proteins.  Protein phosphatases: are enzymes that dephosphorylate proteins. They also participate in signal transduction pathways; they can also serve to stop a signal once a cell response has occurred. (A) Signaling by Receptors that are Ligand-Gated Ion Channels  Here the protein that acts as the receptor is also an ion channel (e.g.; nicotinic receptors for acetylcholine (Ach) which by themselves are ligand gated Na+ channels)  Binding of Ach to these receptors causes their opening and influx of Na+ along its conc. gradient.  The opening is only brief because Ach is hydrolyzed rapidly by acetylcholine esterase (AchE) at the receptor site.  Apparent response caused by receptor activation is a change in membrane electrical potential. 4-9  Uruk University/ College of Pharmacy Physiology I/ Cell signaling in physiology (B) Signaling by Receptors That Function as Enzymes  Tyrosine kinases are integral proteins spanning the membrane.  The binding site of the enzyme is extracellular and the catalytic is intracellular.  Binding of a first messenger (e.g.; many growth factors) causes phosphorylation of cytoplasmic portion (residue) and initiate cascade of reactions inside the cell.  An exception to this concept is the membrane-bound or soluble guanylyl cyclase enzymes. Such receptor enzymes when activated cause the cyclization of guanosine monophosphate “GMP” nucleotide forming a second messenger cyclic GMP “cGMP”. The latter then activate cGMP- dependent protein kinase to initiate cascade of reactions inside the cell.  Notes:  E.coli enterotoxin activate membrane-bound guanylyl cyclase  water secretion (diarrhea)  Nitric oxide “NO” (from nitrite drugs or nitric oxide stimulating drugs) activates soluble guanylyl cyclase  smooth muscle relaxation (C) Signaling by Receptors That Interact with Cytoplasmic Janus Kinases  Here, the binding of a first messenger (e.g.; cytokines “interleukins”) to its receptor causes a conformational change in the receptor that leads to activation of the janus kinase ended by gene transcription.  Janus kinases: are separated cytoplasmic kinases family associated with the receptor (not a part of the receptor)  There is different members of the janus kinase family each phosphorylate different target proteins, many of which act as transcription factors (D) Signaling by G-Protein-Coupled Receptors  G proteins: are inactive proteins bounded to the cytoplasmic portion of membrane receptors. Such receptor called G-protein coupled receptor (GPCR)  G-protein composed of 3 subunits (α,β,γ)  α subunit has inherit GTPase activity and can bind GDP and GTP  β and γ subunits help anchor the α subunit in the membrane  When GPCR activated, this increase the affinity of α subunit to GTP.  When α subunit binds GTP it dissociates and activate nearby membrane-bound enzyme or ion channel. These enzymes and ion channels are effector proteins that mediate the generation of second messengers as next steps in the sequence of events leading to the cells response. 5-9  Uruk University/ College of Pharmacy Physiology I/ Cell signaling in physiology  Then G protein serves as a switch to couple a receptor to an enzyme or an ion channel in the plasma membrane  When α subunit activates its effector, GTP is hydrolyzed to GDP + Pi by the inherent GTPase activity. This causes α subunit inactive and to recombine with β and γ subunits.  Cell response to GPCR activation by either stimulation or inhibition depending on the type of G-protein whether stimulatory (Gs) or inhibitory (Gi). Major Second Messengers 6-9 Uruk University/ College of Pharmacy Physiology I/ Cell signaling in physiology 7-9  Uruk University/ College of Pharmacy Physiology I/ Cell signaling in physiology Calcium as second messenger  The Ca+2 ion functions as a second messenger in a great variety of cellular responses to stimulate, both chemical and electrical changes.  Ca+2 bind to various cytosolic proteins, altering their conformation and thereby activating their function.  In smooth muscle cells, Ca+2 binds calmodulin forming Ca+2-calmodulin complex. The complex then activates or inhibits a large variety of calmodulin-dependent protein kinases (e.g.; MLCK  smooth muscle contraction)  In skeletal and cardiac muscle cells, Ca+2 binds troponin in actin microfilament  actin-myosin cross bridge and muscle contraction 8-9  Uruk University/ College of Pharmacy Physiology I/ Cell signaling in physiology Eicosanoids as second messengers Cessation of Activity in Signal Transduction Pathways  Responses to messengers are often briefly and subside when the receptor is no longer bound to the first messenger.  There are numerous ways to prevent chronic overstimulation of a cell by a messenger, which can be very detrimental. These ways are (1) First messenger removal:  First messenger metabolism: by enzymes in its vicinity  First messenger uptake by cells and destroyed  First messenger diffuse away from its receptor site (2) Second messenger removal:  Decrease production by removal of first messenger  Second messenger metabolic degradation (3) Receptor inactivation:  Chemical alteration of receptor (usually by phosphorylation) that decrease its affinity for a first messenger which results in:  The messenger is released from its receptor  Prevent further G-protein binding to the receptor  Receptor removal by endocytosis 9-9