Cell-Cell Interactions

Cell-Cell Interactions Cell-cell interactions: How cells communicate with each other: Exocytosis: vesicles are used in order to transport certain things out of the cell and can even be used to incorporate proteins into the cell membrane (images of both are located to the right) Endocytosis: brings molecules into the cell (like glucose) and there are 3 different types: 1. Phagocytosis: process similar to eating, where the cell engulfs a molecule in order to bring it into the cell (molecule binds to specific receptors on the cell membrane that cause the membrane to reshape and engulf the molecule) 2. Pinocytosis: “how the cell drinks”, engulfs dissolved ions/solutes in the liquid medium surrounding the cell and is not very specific for what is brought into the cell 3. Receptor-mediated Endocytosis: very specific with regards to what is being brought into the cell and is kind of a “lockand-key” model where only specific molecules will be brought into the cell in a similar manner to that of phagocytosis There are 3 different types of cell junctions (different ways that cells connect to one another): 1. Gap Junctions: these are essentially tubes that join two cells together (allows for transport of water and ions to/from the connected cells) and also helps with transmitting electrochemical signals in the nervous system (neurons) and in cardiac muscle that make your heart beat 2. Tight Junctions: connections that are formed when cells are squished together, meaning the cell membranes are connected but the contents of each cell are not connected in any way (“watertight seal”). These are important in certain parts of the body that need to contain certain fluids (i.e. the bladder, kidney, or intestines) 3. Desmosomes: cell membranes are connected through threadlike substances that connect the cells across the space in between the cells and physically hold the cells together but do not allow the exchange of fluid/materials between the cells. It does, however, allow for the flow of water/solutes in between the two cells and is important for high stress areas of the body like the skin or intestines because the space in between offers a flexibility that other options simply can’t Cell Junctions: Cell Junctions are simply things that connect cells to other cells and often occur in epithelial tissue Membrane Receptors: Membrane Receptors: important for cell communication with the outside world and without them our cells wouldn’t be able to work together. It is essentially an integral protein that takes part in communicating with the outside environment (ligands bind to these membrane receptors and trigger changes within the cell) Signal Transduction: when a ligand binds to a receptor and causes an intracellular response Remember, each specific receptor binds to specific ligands 3 Main Membrane Receptors: 1. Ligand-Gated Ion Channels: transmembrane ion channels that open or close in response to allosteric binding of a chemical messenger like a ligand and are commonly found in electrically excitable cells like neurons. Has a channel/hole through it which allows the ion to pass through. Therefore an intracellular electrical signal forms within the cell (not the same as voltage gated ions channels or stretch activated ion channels). 2. G-Protein Coupled Receptors: only found in eukaryotes and comprise the largest known class of membrane receptors (humans have more than 1000 known GPCRs), they can regulate a wide array of functions. It has 7 transmembrane alpha helices and interact with G proteins (which are known to bind GTP/GDP and have 3 subunits: alpha, beta, and gamma - the alpha/gamma subunits are attached to the cell membrane through lipid anchors). In the inactive state, the G protein is bound to GDP at the alpha subunit. When a ligand binds the GPCR, a conformational change is induced and the alpha subunit binds GTP instead of GDP. The alpha subunit then detaches from the beta/gamma subunits and goes to find a protein to regulate target proteins (which can produce secondary messenger or ion channels that allow the ions to be secondary messengers). The target protein then gives off an intracellular signal and once GTP is hydrolyzed to GDP and the alpha subunit reattaches to the beta/gamma subunits. For example, epinephrine binds to an adrenergic receptor where the alpha subunit binds to adenylate cyclase which takes ATP and turns it into cAMP (which increases heart rate, dilate blood vessels, and break down glycogen → glucose) 3. Enzyme Linked Receptors: has a ligand binding domain on the extracellular portion and an enzymatic domain on the intracellular portion of the protein. So basically the ligand causes the intracellular domain to act as an enzyme. Receptor Tyrosine Kinases are an example of this and they phosphorylate the tyrosines on the other kinase. Once phosphorylated, proteins can come and bind to these phosphorylated tyrosines which can activate a whole bunch of different signaling pathways (which often ends in regulating gene transcription).