Cell Communication PDF

Cell Communication  Session Learning Outcomes (SLOs) SLO# 1: Describe the importance of cell signaling in biology. SLO# 2: Explain the basic principles of cellular signal transduction. SLO# 3: Compare the main types of cell communication that exist in multicellular organisms....

Cell Communication  Session Learning Outcomes (SLOs) SLO# 1: Describe the importance of cell signaling in biology. SLO# 2: Explain the basic principles of cellular signal transduction. SLO# 3: Compare the main types of cell communication that exist in multicellular organisms.  Definitions Ligand: the chemical signal that binds to a receptor. Receptor: a protein that can bind to the signal (in the case of a molecule) or detect a signal in the case of light or other non- molecule signals. Reception: when the receptor binds to the signal, causing the receptor to change shape. Kinase: an enzyme that attaches a phosphate to another protein (usually activating it) Phosphatase: an enzyme that removes phosphate from a protein (usually inactivating it) Second Messenger: a small non protein molecule that diffuses rapidly through the cell during signal transduction. Examples include cAMP, IP3, DAG, Ca2+. Signal Transduction: converting a signal into a cellular response.  Cell Communication The basics of cell communication are found in all living things. All living cells must respond appropriately to their environment. Importance of cell signaling in unicellular organisms In single-celled organisms, signal transduction pathways influence how the cell responds to its environment and signaling between organisms.  Unicellular organisms Example: Yeast cells identify their mates by * cell signaling  factor Receptor 1 Exchange of mating factors: Each cell type secretes a a  mating factor that binds to receptors on the other cell type. Yeast cell, a factor Yeast cell, mating type a mating type  2 Mating: Binding of the factors to receptors induces changes in the cells that a  lead to their fusion. 3 New a/ cell: The nucleus of the fused cell a/ includes all the genes from the a and α cells.  Importance of cell signaling in multicellular organisms Cell communication is necessary for the existence of multicellular organisms. Cell-cell signalling permits coordinated function of cells within and between tissues, up to the organism level Cells must interpret the multitude of signals they receive from other cells to help coordinate their behaviors. Cells must communicate in order to proliferate, differentiate, migrate and maintain a functional state. Most animal cells both send and receive signals. Signals from other cells or the environment can be stimulatory (turn on a gene or protein) or inhibitory (turn off a gene or protein multicellular organisms Example: During animal development, cells in the embryo exchange signals to determine which specialized role each cell will adopt, what position it will occupy in the animal, and whether it will survive, divide, or die. The ability of cells in multicellular organisms to perceive and correctly respond to their microenvironment is the basis of growth, development, tissue repair, and immunity as well as normal tissue homeostasis. Errors in cellular information processing are responsible for diseases such as cancer, autoimmunity, and diabetes.  General Principal of Cell Signaling - Cells in a multicellular organism communicate via chemical messengers. - Communication between cells requires: Ligand: Extracellular signaling molecules that are produced and released by signaling cells. Receptor protein: the molecule to which the receptor binds and recognize the signaling molecules on the surface of target cells where they cause a cellular response by means of a signal transduction pathway.  Signal transduction pathways A signal transduction pathway is a series of steps by which a signal on a cell’s surface is converted into a specific cellular response. Figure 16-2 Essential Cell Biology (© Garland Science 2010) External signal is received and converted to another form to elicit a response  Methods used by Cells to Communicate There are different basic mechanisms for cellular communication: 1- Direct contact: Cell-cell or cell-matrix contact using membrane bound receptors. 2- Indirect: there are three kinds of chemical signaling: Extracellular signal molecules can act over either long or short distance A. Autocrine signaling B. Paracrine signaling C. Endocrine signaling  Types of signaling ▪ Cells that maintain an intimate membrane-to-membrane interface can engage in contact-dependent signaling. ▪ Cells may communicate by direct contact, or cell-cell recognition 1- Direct contact Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells Plasma membranes Gap junctions Plasmodesmata between animal cells between plant cells  Types of signaling 2- Cell-cell recognition ▪ The cells make direct physical contact through signal molecules lodged in the plasma membrane of the signaling cell and receptor proteins embedded in the plasma membrane of the target cell. ▪ Signaling by direct (cell-cell or cell-matrix) interactions plays a critical role in regulating the behavior of cells in animals tissues. For example: The integrins and cadherins function not only as cell adhesion molecules but also as signaling molecules that regulate cell proliferation and survival in response to cell-cell and cell-matrix contacts. During embryonic development, such contact-dependent signaling allows adjacent cells that are initially similar to become specialized to form different cell types cell-cell recognition Cell to Cell Contact: used to distinguish “self” from “other” Animal cells have MHC proteins on the surface of the cell to distinguish “self” from “other” Antigen Presenting Cells present antigen to T cells by cell-cell contact. Recognition of foreign antigen causes T cell activation and signal to other immune cells to mature  B Cells are antigen presenting cells to T helper cells B cells present antigen to TH cell. If the T cell receptor recognizes the antigen it will release lymphokines The B cell matures into a plasma cell and releases antibodies into the blood plasma.  Three kinds of chemical signaling 1- Autocrine signaling: ▪ Cells respond to signaling molecules that they themselves produce. ▪ Examples: response of the immune system to foreign antigens, and cancer cells. Examples of an autocrine: 1- The cytokine interleukin-1 (IL-1) in monocytes: When interleukin-1 is produced in response to external stimuli, it can bind to cell-surface receptors on the same cell that produced it.  kinds of chemical signaling Short distance signaling: 2- Paracrine signaling: ▪ The signaling molecules released by one cell in the extracellular fluid act on neighboring target cells ▪ Examples: - Neurotransmitters at synapses in the nervous system. - Cytokines that cause an inflammatory response in the infection area. * - Growth factors that control cell proliferation in a healing wound function. Synaptic signaling: Neuronal signals are transmitted along axons to remote target cells. *  Three kinds of chemical signaling Long distance signaling: Long-distance signaling 3- Endocrine signaling: Blood Endocrine cell ▪ The signaling molecules are vessel hormones secreted by endocrine cells and carried through the circulation system to act on target cells at distant Hormone travels in bloodstream body sites. to target cells Target Examples: cell - Progesterone and testosterone. - Thyroid hormones.  Types of signaling The endocrine hormones FSH and LH are made by the pituitary gland in the brain. - These hormones coordinate the maturation of ovules and and Endometrium Figure 45.10 Glucose homeostasis maintained by insulin and glucagon Each Cell Responds to a Limited Set of Signals ▪ A typical cell in a multicullar organism is exposed to hundreds of different signal molecules in its environment. ▪ These may be free in the extracellular fluid, embedded in the extracellular matrix in which cell rest, or bound to the surface of the neighboring cells. ▪ Each cell must respond selectively to a mixture of signal, disregarding some and reacting to others, according to the cell specialized function. ▪ Cell responds to a signal molecule depends on whether it possesses a receptor for that signal. ▪ Limited range of signals can still be used to control the behavior of cell in complex ways. ▪ The complexity is of two sorts: 1. Binding to one type of receptor protein, can cause a multitude of effects in the target cell. It can alter the cell’s shape, movement, metabolism, and gene expression. The information conveyed by the signal depends on how the target cell receives and interprets the signal. 2. A typical cell possesses a collection of different receptors. Such a variety makes the cell simultaneously be sensitive to many extracellular signals. these signal molecules work in combinations to regulate the behavior of the cell. Each cell is programmed to respond to specific combinations of extracellular signal molecules Extracellular signals alter the activity of a variety of cell proteins to change the behavior of the cell. ▪The signal molecule binds to a cell-surface receptor protein. ▪The receptor protein activates an intracellular signaling pathway that is mediated by a series of intracellular signaling proteins. ▪Some of these signaling proteins interact with target proteins, altering them to change the behavior of the cell. Signaling cascades of intracellular signaling molecules have several crucial functions. They transform the signal into a molecular form suitable for passing the signal along or stimulating a response. Modulation They relay the signal from the point in the by other cell at which it is received to the point at factors which the response is produced. Signaling cascades amplify the signals The signaling cascades can also distribute the signal so as to influence several processes in parallel.  Session Learning Outcomes (SLOs) SLO# 1: List the major classes of signaling molecules and the receptor types upon which they act. SLO# 2: Compare the 2 types of receptors that are classified based on location in the cell SLO# 4: Describe how signal information is transduced into cellular responses in the cytoplasm and in the nucleus  Extracellular signals Extracellular signal molecules generally fall into two classes: 1- The most of the extracellular signal molecules are Hydrophylic molecules. Large polypeptide hormones (insulin, glucagon, growth hormones). Small charged compounds (Adrenaline). Unable to cross the plasma membrane of the target cell. Bind to cell surface receptors. Usually use a 2nd messenger. Generate one or more signaling molecules inside the target cell.  2- Some of the extracellular signal molecules are: Small or hydrophobic. Diffuses through the plasma membrane. Made from cholesterol, lipids, hydrophobic AAs, or Vit A. Very stable change = hours to days (slow and long-lasting cellular change). Activate intracellular enzymes directly or bind to intracellular receptors proteins in either the cytosol or the nucleus. Both the cytosolic and nuclear receptors are referred to as nuclear receptors. Most steroid receptors are transcription regulators, which bind to promoter and turn on specific genes. Regulates gene expression - Cells in multicellular organisms use hundreds of kinds of extracellular molecules to send signals to one another. - Ligand can be: Proteins Peptide Amino acids, Nucleotides, Steroids Fatty acid derivatives, Dissolved gases  Extracellular signal molecules Extracellular signal molecules bind either to cell-surface receptors or to intracellular enzymes or receptors.  Extracellular signal molecules a cell’s response to a signal can be fast or slow ▪ Cell responses that need not involve changes in gene expression (changes in cell movement, secretion, or metabolism) occur more quickly. ▪ Cell responses that involve changes in gene expression and the synthesis of new proteins (cell growth and division) occur relatively slowly.  Intracellular Receptors ▪ Intracellular receptors are cytoplasmic or nuclear proteins. ▪ Receptors are often in the cytoplasm until a ligand binds to them and then they move to the nucleus. ▪ Signal molecules are lipid-soluble molecules.  Intracellular Receptors ▪ A steroid receptor has 3 functional domains: 1. Hormone-binding domain 2. DNA binding domain 3. Domain that interacts with coactivators to affect gene expression ▪ Steroid Receptor Superfamily are transcription factors that function either as activators or repressors of transcription. Cell Signaling by hydrophobic signals Nitric oxide (NO) triggers smooth muscle relaxation in a blood-vessel wall - Nitric oxide (NO), is able to diffuse across the membrane. - Nitric oxide gas signals by binding directly to an enzyme inside the target cell, and alters the activity of intracellular target enzymes.. Ex. It signals the dilation of blood vessels. The mechanism : 1. Acetylcholine is released from the terminus of nerve cell in the blood vessel wall. 6. the activated cyclase catalyzes the production of cGMp 5. NO bound to from Gtp. guanylyl cyclase. 3. The endothelial cells are stimulated to produce 7. cGMP causes 2. Acetylcholine NO (from arginine). muscle cells diffuses to relaxation reach acetylcholine 4. The NO diffuses into receptors adjacent smooth muscle cells.  Cell Signaling by hydrophobic signals One important class of signal molecules that rely on intracellular receptor proteins is the steroid hormones (cortisol, estradial, and testosterone) and the thyroid hormones (thyroxine). Cell Signaling by hydrophobic signals  Steroid hormones bind to intracellular receptors 2. Hormone binds to a receptor protein in 5. The mRNA is the cytoplasm,. translated into a specific protein. 1. Hormone passes through the plasma 3. Hormone-receptor membrane. complex enters the nucleus. 4. Hormone-receptor complex binds to target gene and stimulates the transcription of the gene into mRNA.