Lesson 2.3 - Cell Communication 2023-2024.pdf

CELL BIOLOGY AND HUMAN GENETICS Ve más allá Academic Year 2023-2024 DAVID BALLESTEROS PLAZA Department of Pre-clinical Dentistry (Building A) E-mail address: davidalberto.ballesteros @universidadeuropea.es Ve más allá Academic Year 2023-2024 Lesson 2 Part III CELL COMMUNICATION AND SIGNAL Cellular c...

CELL BIOLOGY AND HUMAN GENETICS Ve más allá Academic Year 2023-2024 DAVID BALLESTEROS PLAZA Department of Pre-clinical Dentistry (Building A) E-mail address: davidalberto.ballesteros @universidadeuropea.es Ve más allá Academic Year 2023-2024 Lesson 2 Part III CELL COMMUNICATION AND SIGNAL Cellular communication and receptors TRANSDUCTION Cell signaling: Classification of signaling molecules Signal components Types of cell surface receptors Nuclear receptors Integration of signals EXPRESSION OF NUCLEAR RECEPTORS INMUNOPEROXIDASe + DAB CELL SIGNALLING Cells communicate with their environment and with other cells. Cell communication is essential for: Developmental control and tissue organization Growth control and cell division Coordination of celular functions In the absence of any signals most animal cells are programmed to kill themselves. Some examples:  Growth factors that interact with the cell membrane and can trigger receptors that powerfully affect chromatin structure and the modulation of gene expression.  Metabolites in the blood that can trigger cell receptors to cause the release of a hormone needed for glucose regulation. DIFFERENT TYPES OF CELL SIGNALLING PARACRINE , AUTOCRINE ENDOCRINE SYNAPTIC  AUTOCRINE signaling: The secreted molecules act on the same cells that secrete them.  PARACRINE signaling: The secreted molecules are transported to the immediate environment of the signaling cell (neighbouring cells).  ENDOCRINE signaling: The signal molecules, called hormones, are released into the bloodstream and carry the signal to target cells distributed widely throughout the body.  SYNAPTIC signaling: A rapid intercellular communication mechanism that depends on neurotransmitters. GENERAL PRINCIPLES OF CELL SIGNALLING SIGNAL SIGNAL TRANSDUCTION CELL RESPONSE END OF RESPONSE SIGNAL TRANSDUCTION BEGINS WHEN : THE RECEPTOR ON A TARGET CELL RECEIVES AN INCOMING EXTRACELLULAR SIGNAL AND CONVERTS IT INTO INTRACELLULAR SIGNALS THAT ALTER CELL BEHAVIOUR SIGNAL TRANSDUCTION THROUGH CELL SURFACE RECEPTORS The extracellular signal is RECEIVED, TRANSMITTED and AMPLIFIED 1- Signal recognition by a specific receptor on the plasma membrane 2 Transmission of the signal to effector molecules located on the inner side of the membrane or in the cytoplasm : generation of second messengers 3 Cellular Response : second messengers modulate enzymatic activities, ion concentrations … through a signaling cascade that could eventually lead to changes in gene expression 4 End of Response by regulation, destruction or inactivation of the receptor and/or 2nd messengers. SIGNAL TRANSDUCTION INVOLVES TRANSMISSION AND AMPLIFICATION OF THE SIGNAL A complex celular response can be achieved with a single signal. Intracellular Second Messenger SIGNALLING MOLECULES (LIGANDS)  HYDROPHILIC. They activate proteins (receptors) found at the cell surface. The binding of ligand-receptor starts a series of intracellular signalling events mediated by second messengers (cAMP, cGMP, PIP3, DAG..) Neurotransmitters Glycoproteins and peptide Hormones Local chemical mediators  HYDROPHOBIC : Molecules that go through the plasma membrane easily and reversibly bind to intracellular receptors, either in the cytosol or in the nucleus. Steroid and thyroid Hormones Vitamin D Retinoids Eicosanoids Carbon monoxide (CO) and nitrogen monoxide (NO) EXEPTION : Steroids can also bind to cell Surface receptors SIGNALS THAT BIND TO CELL-SURFACE RECEPTORS Signals that CANNOT cross the plasma membrane : High molecular weight molecules Hydrophilic molecules They bind to different types of membrane receptors :  Ion channel Coupled Receptors (ligand-gated ion channels) : neurotransmitter receptors  G protein-coupled receptors : receptors associated to heterotrimeric G proteins such as adrenaline receptors.  Enzyme-linked receptors or catalytic receptors such as Growth Factor receptors with Tyrosine Kinase activity. ION-CHANNEL-COUPLED RECEPTORS  Transmembrane proteins that act as ion channels regulated by a ligand (neurotransmitter)  When activated, these receptors change the permeability of the plasma membrane to selected ions and modify the membrane potential.  These receptors transform an extracellular chemical signal into an electrical signal.  They are abundant in nerve cells and other electrically excitable cells such as muscle cells. EXAMPLE : Nicotinic acetylcholine (Ach) receptor in muscle Acetylcholine Receptor Receptor Bound Acetylcholine Na+ Channel Closed Na+ Channel Open Ion channel receptors at a neuromuscular junction 1. The process is initiated when the nerve impulse reaches the nerve terminal and depolarizes the plasma membrane of the terminal. Voltage gated Ca++ channels open up and the increase in [Ca++ ] triggers the release of Ach from synaptic vesicles. 2. Released Ach binds to receptors in the muscle cell that open ligand-gated Na+ channels. Influx of Na+ causes depolarization of membrane. 3. Depolarization spreads 4. Generalized depolarization activates voltage-gated Ca++ channels in sarcoplasmic reticulum. 5. Ca++ is released from the SR into the cytosol producing contraction of myofibrils. G-PROTEIN-COUPLED RECEPTORS  The largest family of cell-surface receptors (more tan 700 in humans).  All G protein-coupled receptors (GPCR) share a similar structure and contain seven membrane spanning domains.  Once activated by the specific ligand the receptor will activate a trimeric G protein that will in turn stimulate or inhibit a plasma membrane effector protein.  Effectors can be enzymes or ion channels. The GPCR family includes receptors for hormones, neurotransmitters, light (rhodopsin in the eye) or odorants (in the mammalian nose). EFFECTOR (Enzyme or Ion Channel) G PROTEIN HETEROTRIMERIC G PROTEINS  G proteins are integral membrane proteins made up of three subunits : α, β, γ.  In the unstimulated state the α subunit is bound to GDP and to the other two subunits.  When the G protein is activated by a ligand-bound receptor, the α subunit undergoes a conformational change and GDP is exchanged for GTP.  The GTP-bound α subunit dissociates from the βγ dimer and can now interact with and regulate effector molecules on the plasma membrane.  The α subunit has GTPase activity and eventually will hydrolyze its GTP to GDP, returning to its original inactive state and associating back with the β γ subunits  Different G proteins (Gs, Gi, Go, Gq...) act on different effectors (adenylyl cyclase, phospholipase C...) EFFECTOR REGULATION THROUGH G-PROTEIN-COUPLED RECEPTORS EXAMPLE OF GPCR : THE β2 ADRENERGIC RECEPTOR Adrenaline binds β2 Adrenergic Receptor Gα subunit (in Gs) binds GTP and activates ADENYLYL CYCLASE cAMP Activated AC produces cAMP (Second Messenger) cAMP activates PKA (protein Kinase A) The active enzyme form phosphorylates several target proteins in the cytosol or in the nucleus where it can modulate gene expression CELLULAR RESPONSE β2 A rise in intracellular cAMP can also modulate gene transcription ENZYME-LINKED RECEPTORS (CATALYTIC RECEPTORS)  Transmembrane proteins with a single transmembrane domain (in most cases).  Most of them have a catalytic domain in the intracellular region.  The binding of an extracellular signal causes two receptors to come together in the plasma membrane and form a DIMER.  Dimerization activates their catalytic activity.  The lasgest family of enzyme-linked receptors are RECEPTOR TYROSINE KINASES (RTKs) , that are activated by growth factors, cytokines and hormones.  Other less common enzymatic activities associated to these receptors are : Tyrosine Phosphatase Serine / Threonine Kinase Guanylyl Cyclase TYROSINE KINASE RECEPTORS (RTKs) Ligand binding induces dimerization of the receptor and activation of its kinase activity. Each intracelular tail phosphorylates Tyr residues on its dimerized partner´s tail which leads to further activation of the kinase activity. The fully active tyrosine kinase will recruit intracellular signalling proteins and will actívate them either through direct binding or through phosphorylation on Tyrosine residues. Activation of a RTK stimulates the assembly of an intracellular signaling complex TYROSINE KINASE RECEPTORS (RTKs) Most growth factor receptors are RTKs. The Insulin Receptor is also an RTK. However, the Insulin Receptor is a permanent dimer and therefore DOES NOT DIMERIZE upon Insulin binding. This family of receptors mediate critical cell functions : Cell growth Cell proliferation Cell diferentiation Insulin Receptor The Receptor is an enzyme: eg GUANILATE CYCLASE NO (nitric oxide) activates Guanylate cyclase: arteria 24 NUCLEAR RECEPTORS AND HYDROPHOBIC LIGANDS  Ligands such as steroid and thyroid hormones, vitamin D can diffuse through the plasma membrane and enter the cell.  They usually bind to specific receptors in the nucleus, activating the GENOMIC PATHWAY  These nuclear receptors have DNA-Binding domains and act as TRANSCRIPTION REGULATORS  When the ligand binds to the receptor , the receptor undergoes a conformational change that activates its binding to DNA allowing it to stimulate or inhibit transcription of target genes. GENERAL MECHANISM BY WHICH STEROID AND THYROID HORMONES, RETINOIDS, AND VITAMIN D REGULATE GENE EXPRESSION  GENOMIC PATHWAY HIDROPHOBIC LIGANDS ARE MORE VERSATILE THEY CAN ACTIVATE THE MEMBRANE AND THE GENOMIC PATHWAY Steroid receptors can also be found in the cytosol... Activated receptorcortisol complex binds to regulatory región of target gene and activates or represses its transcription …and on the cell surface (membrane steroid receptors) INTEGRATION OF DIFFERENT SIGNALLING PATHWAYS Most signaling pathways are interconnected and often share common elements : SIGNALLING CROSSTALK Each cell integrates all the different signals and produces one or several cellular responses. WHAT DETERMINES HOW A CELL RESPONDS TO A SPECIFIC SIGNAL ?  Cells are exposed to hundreds of different extracellular signals but each cell responds selectively to this mixture of signals, disregarding some and reacting to others.  Whether or not a cell responds to a specific signal depends first on whether or not it expresses the corresponding receptor.  In addition, different cell types can respond differently to the same signal BASIC CONCEPTS: MEMBRANE COMPOSITION AND TRANSPORT  Cell membranes: plasmatic membrane, endomembranes, mb of organelles  Properties functions: self-sealing, fluidity and selective permeability. Is a bilayer, electrically polarized compartmentalization, selective barrier (transport), cell signaling and generation and mantainment of electrochemical gradients  The Fluid Mosaic model (lipids and embded proteins, with lateral diffusion)  Role of lipid composition in fluidity. Cholesterol (fluidity buffer) sphingolipids, saturated and longer fatty acid more rigid. Lipid rafts as microdomains of signallig  Mb proteins: integral (types), lipid-anchored, peripheral, amphitropic (= anchored or separated, depending on function).  Classification according to function: transport, anchorage, receptors, structural  Transport of low PM molecules according to energy consumption and according to molecules (uni, sin, antiport)  Facilitatedchannels (voltage-regulated, mechanical, by ligand) // transporters, (carriers = permeases) // porins  Active Transport  pumps: Na / K ATPase: E1 in +Na+ E1P  E2P out  +K+E2 E1 In Example Glucose transport in brush border (pump extract sodium,basal mbsimporter drives glucose absortion mediated by sodium gradient apical mb glucose exit throught GLUT,basal mb)  Transport of large molecules: endocytosis, exocytosis, transcytosis.  receptor mediated: clathrin.  Example, Tte LDL particle, phagocytosis, neurotransmitter release  Caveolas and caveolin 30 SIGNAL TRANSDUCTION AND COMMUNICATION            Endocrine, paracrine, autocrine, synaptic communication Types of ligands and location of their receptors (Hydrophilic, only plasma mb, hydrophobic, in the nucleus or in the cytosol, but they may also have R in the plasma membrane, eg estrogens) Concept of signal transduction: reception, transmission and amplification of the signal. Example of R for epinephrine Types of plasma mb receptors: R channels activated by ligand, GPCR and enzymes The nicotinic R Function of GPCR [Ligand activates R  G prot αβγ (GTP GDP) α-GTP activates (regulates) Adenylate Cyclase  ↑ cAMP –> ↑ PKA  effector] Muscarinic R (Lig-activated R coupled to G protein that opens ion channel) RTK, R insulin; Guanylate cyclase and nitric oxide How nuclear R works: ligand binding activates the receptor that act as a transcription factor Concept of Integration of signals crosstalk specificity of the response depending on the cell type (and on the expressed receptors) 31