Cell Communication

Cell signaling The mechanisms by which cells communicate with each other Absolutely essential in multicellular organisms Can communicate in multiple ways

Sending Signals 4 basic “styles” of signaling Endocrine Contact- dependent Paracrine Neuronal

Local Signaling Ex. Paracrine & Neurons Paracrine Signaling Signaling molecule that diffuses through the interstitial fluid Ex. Histamine Stored in certain cells of immune system Released in response to allergic reactions, injury, or infection Causes blood vessels to dilate and capillaries to become more permeable

Local Signaling Neuron Signaling Most signal by releasing chemical compounds called neurotransmitters Diffuse across synapses = tiny gaps between neurons Ex. Acetylcholine, dopamine, serotonin

Long Distance Signals Ex. Endocrine/Hormone Signaling Hormones are secreted by endocrine glands Secreted into extracellular fluid Diffuses into capillaries and transported by blood to target cell

Cell Signaling Overview

Step 1: Reception A signal molecule binds to a receptor protein Signal molecule = ligand Causes protein to change shape Most receptors are plasma membrane proteins

Intracellular Receptors Found in cytosol or nucleus Activated by small or hydrophobic molecules that can cross membrane Ex. Steroid and thyroid hormones Can act as a transcription factor, turning on specific genes

Step 2: Transduction Cascades of molecular interactions relay signals from receptors to target molecules in the cell Usually involves multiple steps Can amplify a signal

Transduction Pathways The molecules that relay signals are mostly proteins Activated from receptor, which activates another protein, which activates another, etc. until response is activated

Protein Phosphorylation The signal is transmitted by phosphorylations Protein kinases transfer phosphates from ATP to protein Protein phosphatases remove the phosphates, called dephosphorylation Acts as on/off switch

Second Messengers The ligand is considered the “1st messenger” Second messengers are small, nonprotein molecules that spread by diffusion Participate in pathways initiated by G protein coupled receptors & receptor tyrosine kinases Ex. Cyclic AMP

Cyclic AMP (cAMP) One of the most widely used second messengers Adenylyl cyclase, an enzyme in plasma membrane, coverts ATP to cAMP

Many ligands trigger formation of cAMP Usually activates protein kinase A Phosphorylates various other proteins

Step 3: Response Regulation of transcription or cytoplasmic activities Many pathways regulate the synthesis of enzymes or other proteins By turning on/off genes Other pathways regulate the activity of enzymes Can also affect physical characteristics of a cell

Fine-tuning the Response Multistep pathways have 2 important benefits: Amplifying the signal Contributing to the specificity of the response

Signal Amplification Enzyme cascades amplify the cell’s response At each step, the number of activated products is much greater than in the preceding step

Specificity of Signaling & Response Different kinds of cells have different collections of proteins Allow cells to detect and respond to different signals The same signal can have different effects in cells with different proteins & pathways

Termination of Signal Receptor and each of the components of the transduction pathway return to inactive states Failure to terminate can lead to undesirable consequences Ex. Cholera