LEC 1, 2025: Cell Signaling (PDF)

MEMBRANES & TRANSPORT (040817305) CELL SIGNALLING By: Aliaa A. Masoud, PHD Lecturer of Biochemistry, Faculty of Science, Alexandria University  No cell lives in isolation Although cells can be act as self-contained u...

MEMBRANES & TRANSPORT (040817305) CELL SIGNALLING By: Aliaa A. Masoud, PHD Lecturer of Biochemistry, Faculty of Science, Alexandria University  No cell lives in isolation Although cells can be act as self-contained units. Even unicellular organism must detect and respond to outside influences: chemicals, light, other cells. In multicellular organism, more complexity (cells in tissues & organs), the communication between cells is essential to enable full coordinated activities to function in an integrated way. Cells send and receive information (signals). Information is relayed within cell to produce response. Cell Signaling Fundamental process of all organisms. Biological mechanism occurs in the cells giving them ability to respond to the surrounding environment. Communicate between or within the cells. Signaling cell Target cell Initiating of biological response in target cell resulted from release and interaction of chemical compound (signaling molecule) Intercellular intracellular produced by host cell. The original intercellular (between-cells) signal is converted into an intracellular (within-cell) signal that triggers a response. Errors in cellular information processing are responsible for diseases such as cancer, autoimmunity, and diabetes. Cell signaling is important area of research for drug discovery. Why Do Cells Communicate???  Release, production hormones and other regulatory molecules.  Metabolic activities.  Maintenance of homeostasis.  Control cell growth, division, development, differentiation & death.  DNA Repair.  Inflammation & immunity  Adaptation to environmental conditions. Key Steps in Cellular Signaling (Talking, Listening, Responding)  Essential elements of cellular communication:  Signaling molecule released from Signaling (sending) cell, for Signal transmission  Receptor protein in/on the target (responding) cell, to receiving information A. Signal transmission: involves the synthesis of signaling molecule by signaling cell, its release, transport A. Signal transmission to target cell. B. Intracellular signaling: B. Intracellular signaling 1. Reception: recognition/detection of ligand by target cell. 1.Reception 2. Transduction: signal cascade process 2.Transduction (series of reactions which activated after reception). 3. Response: initiation of biological response (inhibitory/excitatory effect) by final molecules on target cells. C. Signal removal & response termination: when the 3.Response signal molecule leave R, reverting R in its inactive state. Modes of Signaling Transmission A. Direct Cell-cell Signaling: Contact Dependent Signaling  The cells must be in direct contact, may be via: a. Juxtacrine signaling:  interacting membrane proteins on two different cells.  Cell-cell recognition – molecules on the surface of one cell are recognized by receptors on the adjacent cell  is important in the immune system and during development. b. Gap Junctions: interacting through special cell-cell junctions that directly connect the cytoplasm of adjacent cells, that allow small signaling molecules or ions (Ca, cAMP) to diffuse in-betweens.  B. Indirect Cell-Cell Signaling:  Signal molecules are secreted from signaling cell and transmitted to target cell  according to the signal transmission ‘distance that the signal travels through the organism to reach the target cell’. i. Distant (Endocrine) Signaling  Endocrine signals (hormones) as signaling molecules are produced by distant endocrine cells (located in endocrine glands), but affect other body regions some distance away.  Hormones travel to their target cells via the bloodstream, producing slower response.  Because of their form of transport, hormones become diluted and present in low concentrations.  Have a longer-lasting effect: higher half-life time.  Hormones can be derived from amino acids or lipids.  ii. Local Signaling The signal molecules produced by signaling cell and diffuse into the extracellular matrix then received by target cell, signals act locally between cells: local mediators. a. Paracrine Signaling  release of ligand by signaling cell, binding to receptors of nearby target cells.  acts over very short distances (immediate area: local), Soooo….quick responses, high concentration of ligand.  But: last only a short period of time (short half-life: degraded or removed quickly).  Autocrine and neuronal signaling can be considered specialized types of paracrine signaling.  EX. Neurotransmitters in synapse, Wound healing& developmental signals b. Autocrine Signaling  Signaling cells release ligands that bind to their own surface receptors or neighboring cells of same type. i.e.., the signaling cell and the target cell can be the same or of same type cell.  occurs for :  early embryonic development to ensure direct differentiation of identical cells into the same cell type and proper developmental outcome and function.  regulating pain sensation and inflammatory responses.  Apoptotic signal due to viral infection of cell. c. Neuronal (Synaptic)Signaling  Signaling cells (neurons) produce signals (neurotransmitters) that transferred to its target cell (neurons or muscle cells). Synaptic Gap  Synaptic Signals within the nerve cells are propagated by fast- moving electrical impulses: rapid response.  Chemical synapse: very small synaptic gap: allows rapid diffusion of neurotransmitters  Quick recovery, short-lived molecules. Intracellular Signaling refers to the overall process of transmission of molecular extracellular signals (Primary messenger) into intracellular responses by Receptor binding which  initiates cascade of intracellular biochemical changes.  modifies membrane potential Key players are:  Signaling molecules (ligands),  Receptors: Ligand will produce response only in cells having its particular receptors  Second messengers (small intracellular mediators)  Signaling proteins; including: scaffold, relay, bifurcation, adaptor, amplifier, transducer, integrator, anchoring, modulator, messenger, target proteins. A typical signaling pathway will involve many of these components, interact leading to cellular responses. Signal transduction pathways are not simple chains, but highly complex, branching, multi-step pathways. Different cell types Different sets of proteins: different protein profile Different specificity in detecting & responding to signals. 11 Altered cell shape Altered Altered or movement metabolism gene expression  Different Responses To The Same Signaling Molecule autonomic nervous system Thirsty Nicotinic R Muscarinic R Serous saliva secretion Wetting response Na+/Ca+2 flux change Excitatory contraction response  Routes of Signal Transduction Pathways Diffusible second Recruitment of messenger proteins to the PM Most signal transduction pathways involve a combination of these mechanisms. i. Signaling Molecules (Primary Messenger: Ligand) Produced by signaling cells, bind to receptors in target cells, act as chemical signals. Types: 1. Small/Hydrophobic Ligands:  Can directly diffuse through the plasma membrane and interact with internal Rs.  Important members of this class of ligands are the steroid & thyroid hormones, CO, NO. 2. Water-Soluble Ligands:  They can't cross membrane, either: polar, charged  They bind to cell-surface receptors  Peptides & proteins: thrombin, insulin , Growth factors, cytokines, interleukins.  Biogenic amines & Neurotransmitters: Adrenaline, histamine, acetylcholine  Amino acids: Glutamate, Gamma-aminobutyric acid (GABA)  Ions: Ca+2  Nucleotides : Adenosine nucleotides, adenine nucleotides,  Lipophilic signaling molecules, i.e. prostaglandins, leukotrienes. 3.Physical Signals : Light, temperatures, pressure, odorants. i. Signaling Molecules (Primary Messenger: Ligand) Ligand Agonists & Antagonists in Medicine  Synthetic analogs of receptor ligands are widely used in medicine.  Agonists: compounds that mimic the function of the natural ligand by binding to the receptor and inducing the normal response.  Antagonists bind to the receptor but induce no response. Instead, they typically block binding and signaling by the natural ligand.  Examples:  Isoproterenol (epinephrine agonist) binds to bronchial smooth muscle cell epinephrine receptors with 10-fold higher affinity than epinephrine, and is used to treat asthma, etc.  Alprenolol (epinephrine antagonist) is a beta-blocker that binds to cardiac muscle cell epinephrine receptors, blocking epinephrine action and slowing heart contractions. It therefore helps treat cardiac arrhythmias ‫ اضطراب ضربات القلب‬and angina ‫ذبحة‬. ii. Receptors Proteins that recognize the primary ‘extracellular’ signal molecule [ligand]. Binding [non-covalent/weak] of signaling molecule to its specific receptor, causing a conformational change in the receptor which is the initial transduction of the signal. Leading to the transmission of an intracellular signal, they act as signal transducer, they become activated and generate various intracellular signals that alter the behavior of cell. A given receptor may exhibit binding specificity & affinity for a certain ligand or a group of closely related (structurally) ligands. Receptor Affinity low ligand concentrations bind to most of cognate receptors High High concentrations of ligand is required to occupied most receptors low  A given ligand may bind to a number of different types of receptors, that exhibit different effector specificity (different cell responses). Receptors are grouped, based on their:  Location,  Structure,  Mechanism Of signal transmission. a. Intracellular Receptors Globular protein receptors located inside the cell (cytosol or nucleus or an organelle) rather than on its cell membrane: Non-membrane protein receptors. Small or hydrophobic signal molecules able to diffuse passively across the PM and bind to receptor proteins inside the target cell. Short signal transduction pathways. Nuclear Rs, may either:  located in the cytoplasm of a cell, translocated to the nucleus after stimulation by an agonist.  Ex. steroid hormones receptors.  located in the nucleus of a cell, even in the absence of agonists, EX. Vit A (retinoids), thyroid receptors.  Directly affect expression level of specific genes (ON/OFF) Cytoplasmic Rs:  IP3 receptor located on the endoplasmic reticulum.  Some intracrine peptide hormones. b. Extracellular (Cell Surface) Receptors Integral transmembrane proteins on the target-cell surface. Make most of receptors. Most water-soluble signal molecules bind to specific sites on receptor proteins in the plasma membrane. Ion -channel-linked receptors: Ligand-Gated Ion Channels Main Classes G-protein-coupled receptors (GPCRs) bind a ligand , activate a membrane protein called a G-protein, which then interacts with either an ion channel or an enzyme in the membrane. Enzyme-coupled receptors are cell-surface receptors with intracellular domains that are associated with an enzyme, catalytic R iii. Intracellular Signaling Members a. Second Messengers:  are relatively few small diffusible non-protein water-soluble molecules  relay signals received by cell-surface receptors to effector proteins.  facilitate amplification of the original first messenger signal.  While there are a large number of extracellular receptor ligands "first messengers’, they are few types of 2nd messengers. Membrane derived: Inositol-1,4,5-trisphosphate (IP3) & diacylglycerol (DAG) Types Cyclic nucleotides: (cAMP & cGMP), Ions: Ca+2 20 Second Messengers Large& Diffuse, Regulate R-Ligand rapid functional activity of binding change in its change its target signal intracellular target proteins concentration iii. Intracellular Signaling Members b. Signaling Proteins On contrast to second messengers,  Their concentrations cannot fluctuate rapidly,  They cannot easily move within the cell. The conformation of many proteins is related to their activity. Allosteric regulation is a widespread mechanism of control of protein function; effectors bind to regulatory sites distinct from the active site, usually inducing conformational changes that influence the activity. Regulate R-Ligand Allosteric Target protein activity of binding regulation Conformational target signal change proteins 22  Signaling Proteins Can Act As Molecular Switches Most pathways are comprised of several different proteins behave like molecular switch, signaling proteins affects (activate ON/inhibit OFF) each other, temporarily, in a carefully controlled sequence of binding interactions. Mostly by Reversible Post-translational Modification. Phosphorylation/ dephosphorylation is the most common. 1.Protein Kinases/Phosphatases Kinases: use ATP to phosphorylate amino acid side-chains in target proteins. typically are specific for tyrosine or serine/threonine sites. Phosphatases hydrolyze phosphates off of these residues. Kinases & phosphatases act together to switch the function of a target protein on or off. Activation of many cell-surface receptors leads directly or indirectly to changes in kinase or phosphatase activity. 23 Some receptors are themselves kinases (e.g., the insulin receptor).  Catalytic activity of Protein kinases/phosphatases Alter the conformations Modify activities of the proteins Cellular response In this example, protein kinase 2 is activated by protein kinase 1. Once activated, protein kinase 2 phosphorylates protein kinase 3, activating the enzyme. Protein kinase 3 then phosphorylates a Target protein Like falling dominoes, the receptor activates another protein, which activates another, and so on, until the protein producing the response is activated  Signaling Proteins Can Act As Molecular Switches 2.GTPase Switch Proteins GTPase switch protein also play important roles in intracellular signal transduction GTPases are active when bound to GTP and inactive when bound to GDP. The timeframe of activation depends on the GTPase activity of these proteins. Proteins known as guanine nucleotide-exchange factors (GEFs) promote exchange of GTP for GDP and activate GTPases. Proteins known as GTPase-accelerating proteins (GAPs), stimulate the rate of GTP hydrolysis to GDP, inactivate GTPases. There are two classes of GTPase switch proteins  Monomeric (small) G proteins interact with receptors via adaptor proteins and GEFs.  Trimeric (large) G proteins interact directly with receptors 25 Cellular Response Last stage in cellular communication Responses vary by type of ligand, type of receptor, type of cell. Same ligand and receptor may have different cellular response in different types of cells Example Gene expression, Synthesis or Breakdown of something, stimulus transmission Body system response [sweating, immune response, hormone release] Very long & complex sets of signaling interaction Trigger short-term cellular changes Changing the ion flow, activity of specific proteins, enzymes, Fast Action potential generation In very few steps Have relatively long-term/ permeant effects on cells Changing expression levels of enzymes & cell components by gene regulation, Slower Control cellular development  Aspects of Cell Signaling Signal Termination/ Inactivation Despite the tremendous complexity of signaling networks, many share common features  Aspects of Cell Signaling 1.Specificity of Cell Signaling: A. Particular protein profile in each kind of cells , specificity in signal detecting & responding B. Branching, Crosstalk & Integration Hundreds of different Rs, signaling proteins, and effectors combine into a complex network of interacting pathways within a single cell. It is not always linear: signal from R affects specific relay effector (single response) Pathway branching and crosstalk help cells to coordinate, regulate, generate complex responses to incoming external signals. Convergence: signals from unrelated Rs can affect common effector. Divergence: when signal from R can affects multiple effectors.  Aspects of Cell Signaling 2. Amplification of Signal & Response Minimal activated receptor activation by small amounts of ligands produces significant cellular responses. At each step, the number of activated participants in the pathway increases : referred to as signal amplification. The overall amplification associated with epinephrine signaling is estimated to be ~108-fold. 3. Termination of Signal Providing many opportunity to regulate the signaling cascade and inactivation mechanism. When ligand leave its R, the R reverts to its inactive state Desensitization: receptor down-regulation, tolerance due to prolonged exposure to ligand. Desensitization is a phenomena results in the loss of medicinal effectiveness of some medications due to over prescribed.