Introduction to Cell Communication

1. Introduction to Cell Communication Packet #17 Chapter #11

2. 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.

3. Players of Cell Signaling

4. The Players • The Signal • Signaling Cell • 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

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

6. 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

7. General Steps of Cell Signaling

8. Three Steps • Three Steps • Signal reception • Transduction • Response

9. Signal Reception & Types of Receptors

10. 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

11. 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

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

13. 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

14. 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.

15. 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

16. Signal Transduction & Transduction Pathways

17. 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.

18. 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.

19. Pathways • Two classifications • Catabolic pathway • 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

20. 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

21. Phosphorylation Cascades

22. Cascade Utilizing G-proteins

23. 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

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

25. 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

26. 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?

27. “Types” of Communication

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

29. 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.

30. ExamplePublic & 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.

31. ExamplePublic 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

32. Private & Short DistanceSynaptic 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.

33. Private & Short DistanceIntimate 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.

34. Private & Short DistanceIntimate 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.

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

36. Intimate Messaging Cell to Cell Connections

37. 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

38. 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.

39. 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

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

41. Private & Short Distance

42. 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