CELL BIOLOGY CELL COMMUNICATION CELL SIGNALING CELL COMMUNICATION

CELL BIOLOGY CELL COMMUNICATION & CELL SIGNALING

CELL COMMUNICATION • Do you think your cells are just simple building blocks, unconscious and static as bricks in a wall? • If so, think again! Cells can detect what's going on around them, and they can respond in real time to cues from their neighbors and environment. • At this very moment, your cells are sending and receiving millions of messages in the form of chemical signaling molecules!

CELL SIGNALING • It is part of a complex system of communication that governs basic activities of cells and coordinates cell actions. • The ability of cells to perceive and correctly respond to their microenvironment is the basis of 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. By understanding cell signaling, diseases may be treated more effectively and, theoretically, artificial tissues may be created

• Cells typically communicate using chemical signals. These chemical signals, ligands, which are proteins or other molecules produced by a sending cell, and released into the extracellular space to reach a target cell, with 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. • Thus, the original intercellular (between-cells) signal is converted into an intracellular (within-cell) signal that triggers a response.

STAGES OF CELL SIGNALLING • Reception – where the target cell detects a signalling molecule present in the exogenous environment. • Transduction – the conversion of the signal to a form that can bring about a specific cellular response. • Response – the specific cellular effect brought about by the signalling molecule.

• Reception: a signal molecule (ligand) binds to a receptor protein (receptor), causing it to change shape. • The interaction between a ligand receptor is highly specific. • A conformational change in a receptor is often the initial stage in the transduction of a signal. • Receptors are found in two places; – Intracellular proteins are found inside the plasma membrane in the cytoplasm or nucleus. The signaling molecule must cross the plasma membrane and therefore must be hydrophobic (for instance the steroid hormone testosterone), or very small (Nitric Oxide). – Cell-surface proteins are embedded in the plasma membrane, and these receptors bind to water-soluble ligands.

• Transduction: cascades of molecular interactions relay signals from receptors to target molecule in the cell. • Signal transduction pathways often involve a phosphorylation cascade. • At each step, enzymes called protein kinases phosphorylate and thereby activate many proteins at the next level. • This cascade of phosphorylation greatly enhances the signal, allowing for a large cellular response. • Not all components of signal transduction pathways are proteins, some are small non-protein watersoluble ions called second messengers

• Response: cell signaling leads to the regulation of transcription or cytoplasmic activities. • Many signaling pathways ultimately affect protein synthesis, usually by turning specific genes on or off within the nucleus. • Often, the final activated molecule in a signaling pathway functions as a transcription factor.

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. • The main difference between the different categories of signaling is the distance that the signal travels through the organism to reach the target cell.

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.

Paracrine signaling • Often, cells that are near one another communicate through the release of chemical messengers (ligands that can diffuse through the space between the cells). This type of signaling, in which cells communicate over relatively short distances, is known as paracrine signaling.

• Paracrine signaling allows cells to locally coordinate activities with their neighbors. • 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. • [Example: spinal cord development]

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). • 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.

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 faraway targets are known as hormones.

• In humans, endocrine glands that release hormones include 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.

Signaling through cell-cell contact • Gap junctions 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, such as calcium ions, are able to move between cells, but large molecules like proteins and DNA cannot fit through the channels without special assistance.

ASSESSMENT • 1. When cells respond to an extracellular signal, they most often convert the information from one form to another. This process is called: A. signal transformation. B. signal transduction. C. signal interference. D. signal amplification.

• 2. When the hormone insulin is released into the bloodstream, what form of cell-to-cell signaling is being used? • A. Endocrine B. Paracrine C. Neuronal D. Contact-dependent

• 3. Many of the extracellular signal molecules that regulate inflammation are released locally at the site of infection. What form of cell-tocell signaling is being used? • A. Endocrine B. Paracrine C. Neuronal D. Contact-dependent

• 4. What does a target cell require to respond to an extracellular signal molecule? A. Access to the signal molecule B. The presence of an appropriate receptor for the signal molecule C. Appropriate intracellular signaling pathways D. All of the above

• 5. Each type of extracellular signal molecule induces a similar response in different target cells. A. True B. False

USEFUL LINKS • http: //study. com/academy/lesson/anoverview-of-cell-communication. html • http: //slideplayer. com/slide/5969676/ • https: //www. youtube. com/watch? v=URUJD 5 NEXC 8