Cell Signaling (Lecture 1)

1. Cell Signaling (Lecture 1)

2. Introduction • Cells can not live in an isolated environment. • Prokaryotes communicate with each other, other organisms and surrounding environment. • Eukaryotes e.g yeasts, slime molds, and protozoans mate, differentiate and respond to the environment by secreting pheromones • Cells are able to receive and process signals. • Individual cells receive many signals simultaneously, and they then integrate the information they receive into a unified action plan. • They also send out messages to other cells both near and far.

3. What kind of signals do cells receive? • Most cell signals are chemical in nature. • Prokaryotic organisms have sensors that detect nutrients and help them navigate toward food sources. • In multicellular organisms, growth factors, hormones, neurotransmitters, and extracellular matrix components are some of the many types of chemical signals cells use. • These substances can exert their effects locally, or they might travel over long distances. • Some cells also respond to mechanical stimuli. For example sensory cells in skin and ear.

4. Signaling in plants and animals • In plants and animals, extra cellular signaling molecules control • Metabolism • Growth and differentiation of tissues • Synthesis and secretion of proteins • Composition of intracellular and extracellular fluids

5. General principle signaling • Synthesis of signaling molecules by the signaling cells • Release of signaling molecules • Transport of the signal to the target cell • Detection of a signal by a specific receptor protein present on the target cell • A change in cellular metabolism, function or development triggered by the receptor-signal complex • Removal of the signal, which often terminate the cellular response

6. 1 3 2 Signaling cell 4 5 6

7. Cellular responses due to cell signaling • Changes in the activity or function of specific enzymes and other proteins present in the cells • Changes in the amount of protein produced by a cell e.g. modification of transcription factors that stimulate or repress gene expression

8. Types of signaling

9. Receptor protein exhibit ligand binding effect • Receptor present on Plasma or nuclear membrane has ligand binding sites • Signaling molecules (hormones, pheromones or neurotransmitters) act as ligands • Confirmational change occurs in the receptor that initiate a sequence of chemical reactions • Receptor proteins are specific for each horomone

10. Different cells have different sets of receptor for the same ligand and each of which induces a different response • Different cells respond in a variety of way to the same ligand (e.g. acetylcholine) • Different ligands can induce the same cellular response in some cells (glucagon/epinephrine) • In most receptor-ligand system, the ligand do not have any function except to bind to receptor • Upon binding it changes the properties of receptor which then produce signals to the cell that a specific product is present • Target cells often degrade or modify the ligand to terminate or modify their response

11. Types of receptors

12. Classification of hormones • Lipophillic Hormones with intracellular receptors e.g steroid, thyroxine, retinoic acid • Hydrophillic with cell-surface receptors e.g peptide hormones (insulin growth factor and glucagon), small charge molecules (epinephrine and histamine) • Lipophillic with cell surface receptor e.g. prostaglandins