Introduction to Cell Communication Packet #17 Chapter #11

Why Communication Between Cells is Important A typical free-living cell must be able to Sniff out nutrients Sense the difference between light and dark Detect and avoid poisons and predators Cells must also be able to communicate to provide long range integration of metabolism. Therefore, cells must be able to communicate with each other. Example {Plants} Respond to temperature, conditions of light and dark, guide the cycle of growth, flowering and fruiting

Players of Cell Signaling

The Players The Signal Signaling Cell Target Cell Ligand Cell sending the message Target Cell Cell receiving the message Ligand Molecule such as a hormone or a neurotransmitter that binds to a specific site on a protein (receptor protein) Receptor Protein* Recognizes and responds specifically to the signal molecule Performs the first step in a series of transduction processes at the receiving end

Molecules Used As Signals Proteins Peptides Amino Acids Nucleotides Steroids Fatty Acid Derivatives Dissolved Gases

Target Cell Contain receptor proteins in the cell membrane Recognizes and responds specifically to the signal molecule Performs the first step in a series of transduction processes at the receiving end

General Steps of Cell Signaling

Three Steps Three Steps Signal reception Transduction Response

Signal Reception & Types of Receptors

Reception Cells are stimulated by an extracellular signal Signal binds to, and activates, a receptor protein Each receptor protein recognizes a specific signal molecule Once signal is recognized, a new intracellular signal is generated. This is the first step of transduction

Receptors I There are two basic mechanisms by which chemical signals cause a biologic effect within the cell Hence there are two types of receptors Intracellular receptors Inside the cell Cell surface receptors Found on the surface of the cell

Classes of Cell-Surface Receptors Three classes Ion-channel linked receptor G-protein linked receptor Enzyme-linked receptor

Intracellular Receptors Used for molecules that are sufficiently small and hydrophobic to diffuse across the membrane Best known hydrophobic signal molecules Steroid hormones Cortisol Estradiol Testosterone Thyroid hormones Thyroxine Vitamin D Retinoic Acid

Intracellular Receptors Receptors lie in the interior of the target cell Cytosol or nucleus When steroid hormone binds, receptor undergoes large conformational change that enables it to bond to its corresponding sequence in the DNA Promotes or inhibits transcription of a set of genes Generally regulatory proteins or enzymes More details to come later.

Cell Surface Receptors Largest class of receptors Used for signals that are too large or too hydrophillic to cross the plasma membrane Lie in the plasma membrane of the cell Relays message across the membrane

Signal Transduction & Transduction Pathways

Signal Transduction Signal transduction is the continuation and/or conversion of signals from one form to the next. The signals, although sometimes in different forms, represent the same information.

Signal Transduction II After reception, when the original signal molecule binds on the receptor protein, the message is passed through the cell membrane through a set of intracellular signaling molecules—transduction.

Pathways Two classifications Catabolic pathway Anabolic Degradable pathway Reactions serve to capture chemical energy {ATP} from the degradation of energy-rich fuel molecules Reactions that break down complex molecules, such as proteins, polysaccharides, and lipids, to a few simple molecules Anabolic Synthetic pathway Combine small molecules, such as amino acids to form complex molecules such as proteins Require energy ATP is broken down to ADP + P

Introduction of Pathways Multistep Pathways Can amplify a signal Provide more opportunities for coordination and regulation At each step in a pathway The signal is transduced into a different form, commonly a conformational change in a protein Many signal pathways Include phosphorylation cascades In this process A series of protein kinases add a phosphate to the next one in line, activating it Phosphatase enzymes then remove the phosphates

Phosphorylation Cascades

Cascade Utilizing G-proteins

Regulation of Pathways How does the organism regulate metabolism? Regulatory signals inform individual cells of the metabolic state of the body as a whole Hormones Nervous system Availability of nutrients These influence signals that are generated within the cell itself

Signals Within the Cell Regulatory signals Substrates Allosteric activators Allosteric inhibitors Product Cause a rapid response

Signaling Cascades The relay chain during transduction Physically transfer the point of reception to the end point where the cell machinery will make the response Transform the signal into a form that stimulates the response Amplify the signal received Distribute the signal so that many processes are influenced Each step is open to interference from other factors so that the signal of the cell can me modulated according to conditions prevailing inside or outside the cell

Questions How do cells receive the signal? Where do the cells receive the signal? What do cells do once they have received the signal? How do cells respond to the signals received? Among all the hundreds of signals being sent, and received, how do cells determine which signal was sent to them?

“Types” of Communication

Private vs. Public Private Messages Public Messages Only a select few cells receive the message Public Messages Message sent throughout entire body

Long Distance vs. Short Distances Long Distance Messages Are messages that are sent over a long path from the point of origin location up until the signal's final spot.** Short Distance Messages Are messages that are sent over a short distance from the point of origin up until the signal’s final stop.** Sometimes within the immediate area.

Example Public & Long Distance Hormonal Signaling Hormones may be sent throughout the bodies of plants and animals In animals, hormonal signals are secreted into the bloodstream In plants, hormonal signals are secreted into plant sap. Hormones, produced in animals, are produced in endocrine cells and help make up the Endocrine System.

Example Public and Short Distance Paracrine Signaling Signal molecules diffuse locally through the extracellular medium Signal remains in the neighborhood of the cell that secretes them Signal acts as local mediators Regulate inflammation at sites of infection Regulate cell proliferation in wound healing Growth factors Stimulate target cells to grow and multiply Part of the bigger Immune System Used in the healing process

Private & Short Distance Synaptic Signaling Signals sent via the nervous system are sent over long distances. However, the signal must travel along multiple individual nerve cells and the communication between individual nerve cells is called synaptic signaling.

Private & Short Distance Intimate Messaging Most short range of all Face to face conversation No molecules are released Different in comparison to the other three. Cells make direct contact, through signaling molecules, in their plasma membrane Message is delivered by the binding of a signal molecule anchored in the plasma membrane of the signaling cell to a receptor molecule embedded in the plasma membrane of the target cell Cell to cell connection Remember from earlier packet the functions of proteins in cell membrane.

Private & Short Distance Intimate Messaging II In embryonic development, this type of signaling has an important role in tissues where adjacent cells that are initially similar have to become specialized in different ways Plays role in determining what type of cell a stem cell becomes.

Types of Cell to Cell Connection Gap Junctions Tight Junctions Desomosomes Plasmodesmata*

Cell to Cell Connections Intimate Messaging Cell to Cell Connections

Gap Junctions Communicating cell to cell junction that allows ions and small molecules to pass from the cytoplasm of one cell to the cytoplasm of the next Found in animal cells

Tight Junctions Cell to cell junction that seals adjacent epithelial cells together, preventing dissolved molecules in the extracellular medium from passing from one side of the epithelial sheet to the other Found in animal cells.

Desmosomes Specialized to cell to cell junction, usually formed between two epithelial cells, mediated by cadherin molecules and characterized by dense plaques of protein into which intermediate filaments in the two adjoining cells insert

Plasmodesmata Organelle found in plant cells that allow adjacent cells to communicate, relay signals and move materials between them

Private & Short Distance

Yeast Cells When a yeast cell is ready to mate, it secretes a small protein called a mating factor to which other yeast cells are sensitive They detect the mating factor They put out a protrusion toward the source of the factor Halt the cell cycle