Cell to Cell Communication

Cell-to-cell communication •Communication is essential for multicellular organisms •Cells must coordinate activities in a way that enables them to develop, survive and reproduce.

Evolution of Cell Communication/ Signaling • A signal transduction pathway is a series of steps by which a signal on a cell’s surface is converted into a specific cellular response

• Pathway similarities suggest that ancestral signaling molecules evolved in prokaryotes and were modified later in eukaryotes • Cell signaling evolved well before first multicellular organisms (evidence for a common ancestor)

Proteins in the membrane! • Proteins are embedded in the phospholipid bilayer– these play a major role in cell signaling

• Integral proteinstransmembrane proteins which span the bilayer. • Peripheral proteins- not embedded in the bilayer– on the surface.

Animal Cells intercellular junctions • Tight junctions- membranes of neighboring cells tightly pressed against each other bound by proteins to form continuous cells around cells (skin cells) • Desmosomes- cells are fastened together into strong sheets (muscle cells) • Gap junctions- membrane proteins that surrounds a port through which ions, sugars, amino acids and other small molecules may pass.

Plasma membranes

Gap junctions between animal cells

(a) Cell junctions

(b) Cell-cell recognition

Plasmodesmata between plant cells

Types of Communication in Animal Cells 1. If there are gap junctions, cytoplasm of adjacent cells connects. Signaling substances are dissolved in the cytoplasm and pass freely between cells 2. Animal cells may communicate by cell-to-cell recognition. Ex: immune response and development 3. Messenger molecules secreted by signaling cell can travel short distances. These are called local regulators– they influence cells in the vicinity. (paracrine signaling – growth of cells/ synaptic signaling– nervous system) 4. Hormones- long distance signaling ex: endocrine system (travels through circulatory system. Large variation of molecules)

Long-distance signaling

Local signaling Electrical signal along nerve cell triggers release of neurotransmitter

Target cell

Secreting cell

Local regulator diffuses through extracellular fluid

a) Panacrine signaling

Neurotransmitter diffuses across synapse

Target cell is stimulated

b) Synaptic signaling

Cell Signaling– three Stages 1. Reception- Detection of signaling molecule 2. Transduction (changing of signal)- binding of signaling molecule changes receptor protein in some way 1. Transduction converts signal to a form that can bring about a cellular response. 3. Response- Triggers response- can be many different things such as catalysis by an enzymes, activation of genes, etc.

Transduced= changed • Signal transduction pathways- signal of a cells’ surface is converted to a specific cellular response in a series of steps.

1. Reception

2. Transduction (changing signal)

Activation of Cellular Response


Relay molecules in a signal transduction pathway

Signaling Molecule

3. Response

Step 1. Reception(Signal transduction pathways) • Signaling begins with the recognition of a chemical messenger, a ligand, by a receptor protein. • Ligand- a molecule that binds to another molecule (signal! Molecule) • Receptor protein usually undergoes a change in shape when a ligand binds (similar to allosteric regulation in enzymes) which initiates transduction of the signal.

Receptor Protein Example • There are many types of receptor proteins! • Different receptors recognize different chemical messengers, which can be peptides, small chemicals or proteins, in a specific one-to-one relationship. • Example of on type of receptor protein: Receptor tyrosine kinases. • A kinase is an enzyme that catalyzes phosphate (Pi) group transfer.

• Many pathways may be triggered by one ligand!!

Step 2: Transduction • Signal transduction is process by which a signal is converted to a cellular response. • Transduction is usually a multi-step pathway (like falling dominoes!)--- can be called signaling cascades. • Benefit of multiple steps: 1. Possible to greatly amplify signal (many responses for one signal) 2. More opportunities for regulation

•Molecules that relay a signal from a receptor to responses are often PROTEINS!

•Protein interaction is a major theme in regulation at the cellular level (thanks proteins!) •Remember! The original signaling molecule does not enter the cell… is all a message of proteins.

In transduction, proteins change shape • Proteins are activated by adding energy (from ATP!!) • If we give a protein energy from ATP, it get a phosphate group and is phosphorylated. (What process gives us ATP??) • Kinases are enzymes that transfer a Pi. Protein kinases transfer Pi to a protein.

• Many of the relay molecules (dominoes) are protein kinases in transduction and they often act on other protein kinases. • Falling dominoes= protein kinase cascade.

Protein Kinase Cascade= falling dominoessignal transduction

Protein phosphatases remove Pi from proteins • Protein phosphatases preform dephosphorylation on proteins. • When they dephosphorylate, they inactivate protein kinases and turn of the signal transduction pathway when the initial signal is no longer there.

• Phosphorylation/dephosphorylation system act as a molecular switch in the cell, turning cell activities on or off.

Secondary messengers are often important in the cascade (dominoes) • Not all components of signal transduction pathways are proteins (thanks proteins)

• Many pathways involve small, water-soluble molecules called secondary messengers • Since they are small and water-soluble, they diffuse through the cell easily (but you guys already knew that).

Step 3: Response • Signal transduction pathways lead to one or more cellular activities– occurs in the nucleus or the cytoplasm.

• Many regulate protein synthesis (thanks proteins!) by turning on or off specific gene • Many regulate the activity of proteins (thanks proteins!)