Cell Membrane Signalling Systems PDF

BMED 214 EU 2023 CELL MEMBRANE SIGNALLING SYSTEMS A cell within a multicellular organism may need to signal to other cells that are at various distances from the original cell. Not all cell...

BMED 214 EU 2023 CELL MEMBRANE SIGNALLING SYSTEMS A cell within a multicellular organism may need to signal to other cells that are at various distances from the original cell. Not all cells are affected by the same signals. Cells typically communicate using chemical signals. These chemical signals, which are proteins or other molecules produced by a sending cell, are often secreted from the cell and released into the extracellular space. There, they can float – like messages in a bottle – over to neighboring cells. Sending cell: this cell secretes a ligand. Target cell: this cell has a receptor that can bind the ligand. The ligand binds to the receptor and triggers a signaling cascade inside the cell, leading to a response. Nontarget cell: this cell does not have a receptor for the ligand (though it may have other kinds of receptors). The cell does not perceive the ligand and thus does not respond to it. Not all cells can “hear” a particular chemical message. In order to detect a signal (that is, to be a target cell), a neighbor cell must have the right receptor for that signal. When a signaling molecule binds to its receptor, it alters the shape or activity of the receptor, triggering a change inside of the cell. Signaling molecules are often called ligands, a general term for molecules that bind specifically to other molecules (such as receptors). The message carried by a ligand is often relayed through a chain of chemical messengers inside the cell. Ultimately, it leads to a change in the cell, such as alteration in the activity of a gene or even the induction of a whole process, such as cell division. Thus, the original intercellular (between-cells) signal is converted into an intracellular (within-cell) signal that triggers a response. Forms of signaling Cell-cell signaling involves the transmission of a signal from a sending cell to a receiving cell. However, not all sending and receiving cells are next-door neighbors, nor do all cell pairs exchange signals in the same way. There are four basic categories of chemical signaling found in multicellular organisms: paracrine signaling, autocrine signaling, endocrine signaling, and signaling by direct contact. The main difference between the different categories of signaling is the distance that the signal travels through the organism to reach the target cell. 1 BMED 214 EU 2023 Paracrine signaling Paracrine signaling is a mechanism in which one cell secretes a molecule that acts on a second cell in close proximity. The signaling molecule may never enter the bloodstream. In contrast, endocrine signaling involves the secretion of a molecule by one cell into the bloodstream. The signaling molecule can travel in the blood and bind to the receptor on the effector cell. Autocrine pathway functions by the secretion and reception of a messenger molecule by a single cell. Juxtacrine signaling is a form of cell communication by direct contact. All these signals influence the behavior of the effector cells. These behaviors include regulating physiologic processes such as metabolism, transport, motility, division, and growth. Paracrine signaling allows cells to locally coordinate activities with their neighbors. Although they're used in many different tissues and contexts, paracrine signals are especially important during development, when they allow one group of cells to tell a neighboring group of cells what cellular identity to take on. The synapsis between neurons or in the neuromuscular junction is a type of paracrine signaling. A neuron releases a neurotransmitter in the synaptic cleft, and nearby cells (other neurons or muscle cells) receive signals through receptors to neurotransmitters. Paracrine signaling involves a cell secreting a molecule into the blood or the lumen. The signaling molecule can diffuse over a short distance and bind to the cells. Synaptic signaling One unique example of paracrine signaling is synaptic signaling, in which nerve cells transmit signals. This process is named for the synapse, the junction between two nerve cells where signal transmission occurs. When the sending neuron fires, an electrical impulse moves rapidly through the cell, traveling down a long, fiber-like extension called an axon. When the impulse reaches the synapse, it triggers the release of ligands called neurotransmitters, which quickly cross the small gap between the nerve cells. When the neurotransmitters arrive at the receiving cell, they bind to receptors and cause a chemical change inside of the cell (often, opening ion channels and changing the electrical potential across the membrane). 2 BMED 214 EU 2023 Synaptic signaling. Neurotransmitter is released from vesicles at the end of the axon of the sending cell. It diffuses across the small gap between sending and target neurons and binds to receptors on the target neuron. The neurotransmitters that are released into the chemical synapse are quickly degraded or taken back up by the sending cell. This "resets" the system so they synapse is prepared to respond quickly to the next signal. Autocrine signaling In autocrine signaling, a cell signals to itself, releasing a ligand that binds to receptors on its own surface (or, depending on the type of signal, to receptors inside of the cell). This may seem like an odd thing for a cell to do, but autocrine signaling plays an important role in many processes. For instance, autocrine signaling is important during development, helping cells take on and reinforce their correct identities. From a medical standpoint, autocrine signaling is important in cancer and is thought to play a key role in metastasis (the spread of cancer from its original site to other parts of the body). In many cases, a signal may have both autocrine and paracrine effects, binding to the sending cell as well as other similar cells in the area. Endocrine signaling When cells need to transmit signals over long distances, they often use the circulatory system as a distribution network for the messages they send. In long-distance endocrine signaling, signals are produced by specialized cells and released into the bloodstream, which carries them to target cells in distant parts of the body. Signals that are produced in one part of the body and travel through the circulation to reach far-away targets are known as hormones. In humans, endocrine glands that release hormones include the thyroid, the hypothalamus, and the pituitary, as well as the gonads (testes and ovaries) and the pancreas. Each endocrine gland releases one or more types of hormones, many of which are master regulators of development and physiology. For example, the pituitary releases growth hormone (GH), which promotes growth, particularly of the skeleton and cartilage. Like most hormones, GH affects many different types of cells throughout the body. However, cartilage cells provide one example of how GH functions: it binds to receptors on the surface of these cells and encourages them to divide. 3 BMED 214 EU 2023 Endocrine signaling: a cell targets a distant cell through the bloodstream. A signaling molecule is released by one cell, then travels through the bloodstream to bind to receptors on a distant target cell elsewhere in the body. Signaling through cell-cell contact Gap junctions in animals and plasmodesmata in plants are tiny channels that directly connect neighboring cells. These water-filled channels allow small signaling molecules, called intracellular mediators, to diffuse between the two cells. Small molecules and ions are able to move between cells, but large molecules like proteins and DNA cannot fit through the channels without special assistance. The transfer of signaling molecules transmits the current state of one cell to its neighbor. This allows a group of cells to coordinate their response to a signal that only one of them may have received. In plants, there are plasmodesmata between almost all cells, making the entire plant into one giant network. Signaling across gap junctions. A cell targets a neighboring cell connected via gap junctions. Signals travel from one cell to the other by passing through the gap junctions. In another form of direct signaling, two cells may bind to one another because they carry complementary proteins on their surfaces. When the proteins bind to one another, this interaction changes the shape of one or both proteins, transmitting a signal. This kind of signaling is especially important in the immune system, where immune cells use cell-surface markers to recognize “self” cells (the body's own cells) and cells infected by pathogens 4 BMED 214 EU 2023 SIGNAL TRANSDUCTION Signal transduction is defined as the response of a cell to the application of an external stimulus. Stimuli can be chemical or physical, typical examples being light, oxygen, nutrients, hormones. Modules in signal transduction Signal transduction always involves the following phenomena: A receptor of the stimulus Receptors need a receiver domain (e.g. a ligand- binding domain for chemotaxis, or a light-absorbing chromophore in phototaxis). Their activation is forwarded by a transducer protein that contains a signalling domain. The architecture of receptors and transducers is as heterogenous as the perceived signals. The receiver and signalling domain may be present in distinct proteins that form a complex or may be fused in a single polypeptide chain. While the signalling domain functions inside of the cell, the receiver domain is frequently on the outside. Consequently, the receptor/transducer is located in the membrane. A signal transduction chain or a signal transduction network: This chain is responsible for amplification, integration, and adaptation. A typical network are the phosphorylation cascades of the MAP kinase pathway in eucaryotes. Integration indicates that several receptors activate/deactivate one and the same catalyst which thereby acts as a signal integrator. Amplification typically consists of activation of a catalyst, such as a protein kinase, which amplifies the input of a single unit (photon or molecule) into the phosphorylation of many target molecules. Adaptation is defined as return of the signalling system to the pre-stimulus level while the stimulus persists. This enables cells to perceive changes in stimulus size rather than absolute stimulus levels for example adaptation of the eye to bright sunlight or dim moonlight. A target that is affected and produces the cellular response: Typical targets are the genome that results in regulation of gene expression and the cytoskeleton in eukaryotes Basic mechanism of intracellular signal transduction Once a receptor protein receives a signal, it undergoes a conformational change, which in turn launches a series of biochemical reactions within the cell. These intracellular signaling pathways, also called signal transduction cascades, typically amplify the message, producing multiple intracellular signals for every one receptor that is bound. When a ligand binds to a cell-surface receptor, the receptor’s intracellular domain (part inside the cell) changes in some way. Generally, it takes on a new shape, which may make it active as an enzyme or let it bind other molecules. The change in the receptor sets off a series of signaling events. For instance, the receptor may turn on another signaling molecule inside of the cell, which in turn activates its own target. This chain reaction can eventually lead to a change in the cell's behavior or characteristics 5 BMED 214 EU 2023 Because of the directional flow of information, the term upstream is often used to describe molecules and events that come earlier in the relay chain, while downstream may be used to describe those that come later (relative to a particular molecule of interest). Phosphorylation is one of the most effective ways of changing the structure of proteins due to the large size negative charge of the phosphate group; effective as a recognition marker for other proteins. Adding a phosphate group attaches a big cluster of negative charge to the surface of the protein. This negative charge may attract or repel amino acids within the protein itself, changing its shape. Consequently, changing the shape of the protein may alter its ability to work as an enzyme, either increasing or decreasing activity. Also, phosphorylation provides a docking site for an interaction partner (positive charges), or prevent another partner from binding. Phosphate groups can’t be attached to just any part of a protein. Instead, they are typically linked to one of the three amino acids that have hydroxyl (-OH) groups in their side chains: tyrosine, threonine, and serine. The enzymes that perform phosphorylation are called protein kinases, and many types exist. In signal transduction, a reaction that removes the phosphate group takes place. This reaction is called dephosphorylation, and the enzymes that perform it are called protein phosphatases. 6 BMED 214 EU 2023 For signal transduction by phosphorylation to take place, kinases need to be activated. First, receptors themselves may have kinase activity. In such cases, receptors on the plasma membrane surface become bound with a signaling molecule (i.e., a first messenger) and form dimers, thereby activating the kinase domain of the receptor proteins within the cell. Kinases activated in the cell may induce chain reactions. This phenomenon is called a kinase cascade, since a series of reactions occur in the same way as a waterfall flowing over a cliff. A well-known example is that of MAPK (mitogen-activated protein kinase), which is activated by extracellular stimuli, such as growth factor signaling. When growth factor ligands bind to their receptors, the receptors pair up and act as kinases, attaching phosphate groups to one another’s intracellular tails. The activated receptors trigger a series of events (G-proteins). These events activate the kinase Raf (rapidly accelerated fibrosarcoma). RAF kinases are a family of three serine/threonine- specific protein kinases that are related to retroviral oncogenes Active Raf phosphorylates and activates MEK, which phosphorylates and activates the ERKs. MEK (MAP kinase kinase) is a key signal amplification point in the Ras-MEK-ERK (extracellular signal-related kinase) transduction pathway driving growth and survival signaling in mammalian cells. The Extracellular signal-Regulated Kinase (ERK) is a Mitogen-Activated Protein Kinase (MAP kinase) that promotes cell migration and invasion. Growth factors, adhesion, and oncogenes activate ERK. The ERKs phosphorylate and activate a variety of target molecules. These include transcription factors, like c-Myc, as well as cytoplasmic targets. The activated targets promote cell growth and division. 7 BMED 214 EU 2023 Together, Raf, MEK, and the ERKs make up a three-tiered kinase signaling pathway called a mitogen-activated protein kinase (MAPK) cascade. (A mitogen is a signal that causes cells to undergo mitosis, or divide.) Because they play a central role in promoting cell division, the genes encoding the growth factor receptor, Raf, and c-Myc are all proto-oncogenes, meaning that overactive forms of these proteins are associated with cancer. Second messengers Although proteins are important in signal transduction pathways, other types of molecules can participate as well. Many pathways involve second messengers, small, non-protein molecules that pass along a signal initiated by the binding of a ligand (the “first messenger”) to its receptor. Second messengers include Ca2+ ions; cyclic AMP (cAMP), a derivative of ATP; and inositol phosphates, which are made from phospholipids. Calcium ions Calcium ions are a widely used type of second messenger. In most cells, the concentration of calcium ions (Ca2+) in the cytosol is very low, as ion pumps in the plasma membrane continually work to remove it. For signaling purposes, Ca2+ may be stored in compartments such as the endoplasmic reticulum. In pathways that use calcium ions as a second messenger, upstream signaling events release a ligand that binds to and opens ligand-gated calcium ion channels. These channels open and allow the higher levels of Ca2+ that are present outside the cell (or in intracellular storage compartments) to flow into the cytoplasm, raising the concentration of cytoplasmic Ca2+. How does the released Ca2+ help pass along the signal? Some proteins in the cell have binding sites for Ca2+ ions, and the released ions attach to these proteins and change their shape (and thus, their activity). The proteins present and the response produced are different in different types of cells. For instance, Ca2+ signaling in the β-cells of the pancreas leads to the release of insulin, while Ca2+ signaling in muscle cells leads to muscle contraction. Cyclic AMP (cAMP) Another second messenger used in many different cell types is cyclic adenosine monophosphate (cyclic AMP or cAMP), a small molecule made from ATP. In response to signals, an enzyme called adenylyl cyclase converts ATP into cAMP, removing two phosphates and linking the remaining phosphate to the sugar in a ring shape. Once generated, cAMP can activate an enzyme called protein kinase A (PKA), enabling it to phosphorylate its targets and pass along the signal. Protein kinase A is found in a variety of types of cells, and it has different target proteins in each. This allows the same cAMP second messenger to produce different responses in different contexts. 3',5'-Cyclic AMP (abbreviated cAMP), is used by cells as a transient signal. Adenylate Cyclase (Adenylyl Cyclase) catalyzes cAMP synthesis: ATP  cAMP + PPi. 8 BMED 214 EU 2023 The reaction is highly spontaneous due to the production of PPi, which spontaneously hydrolyzes. Phosphodiesterase catalyzes hydrolytic cleavage of one of the phosphate ester linkages (in red), converting cAMP  5'-AMP. This is a highly spontaneous reaction, because cAMP is sterically constrained by having a phosphate with ester linkages to two hydroxyls of the same ribose. The lability of cAMP to hydrolysis makes it an excellent transient signal. Adenylate cyclase (AC) converts adenosine triphosphate (ATP) into cAMP, which stimulates cAMP-dependent protein kinase A (PKA). Subsequently, specific proteins are phosphorylated by PKA to evoke cellular reactions. The phosphorylation of the cAMP response-element binding-protein (CREB), a transcription factor, is important in the regulation of gene transcription. Extracellular signals activate the transcription of a variety of target genes via alterations in CREB phosphorylation, thereby, resulting in multiple physiological functions. A ligand binds to a receptor, leading indirectly to activation of adenylyl cyclase, which converts ATP to cAMP. cAMP binds to protein kinase A and activates it, allowing PKA to phosphorylate downstream factors to produce a cellular response. Inositol phosphates Although we usually think of plasma membrane phospholipids as structural components of the cell, they can also be important participants in signaling. Phospholipids called phosphatidylinositols can be phosphorylated and snipped in half, releasing two fragments that both act as second messengers. One lipid in this group that's particularly important in signaling is called PIP2. Phospholipase C (PLC) enzymes are responsible for the hydrolysis of the inner membrane component phosphatidylinositol-4,5-bisphosphate (PIP2), generating the second messengers inositol-1,4,5- triphosphate (IP3) and diacylglycerol (DAG). These fragments made can both act as second messengers. 9 BMED 214 EU 2023 Inositol triphosphate is a second messenger that helps in transmission of chemical signals such as growth factors, neurotransmitters, hypertrophic stimuli called Angiotensin-II, and hormones for the transduction networks of the cell. IP3 is a negatively charged water-soluble molecule that can rapidly diffuse into cytosol to bind with IP3 receptor; it is opened to release Ca2+ out of endoplasmic reticulum. The production of IP3 is therefore capable of coupling the activated receptor in the plasma membrane to the Ca2+ ions released from an intracellular store. The basic structure of the IP3 receptor is comprised of three domains: an IP3 binding domain near the amino terminus, a coupling domain in the center of the molecule, and a transmembrane domain near the carboxyl terminus. Various cellular responses can depend on this pathway, which includes contraction of smooth muscle and secretion of enzymes by the acinar cells of the pancreas. Additionally, calcium ions are capable of initiating or inhibiting signaling pathways in the cells. Hence, stimulation of IP3 signaling cascade controls the enzymatic activity within eukaryotic cells. DAG stays in the plasma membrane and can activate a target called protein kinase C (PKC), allowing it to phosphorylate its own targets. A diglyceride, or diacylglycerol (DAG), is a glyceride consisting of two fatty acid chains covalently bonded to a glycerol molecule through ester linkages. The DAG pathway is a message generating pathway that is involved in the activation of enzymes and in turn produces various biological events, including transcription of DNA. Similar to other lipids, DAG also diffuses through the membrane surface where it can interact with some other enzyme called protein kinase C and hence activating them. 10 BMED 214 EU 2023 11