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