Unit 3. Cell Communication

Why do cells need to communicate?

• Coordinate cell growth and movement

• Maintain homeostasis

• Respond to the environment

• Receive chemical signals

• Initiate cell death (apoptosis)

What is cell communication?

• How cells send, receive, and respond to messages - signals - sent by other cells

• Allows cells to function as a community or as part of a larger organism

Types of Cell Communication/Signaling

“Local” Signaling:

• Direct

• Short-distance

And Long-distance signaling

What are receptors?

• Proteins that receive and bind to external signals like ligands

• Triggers response inside of the cell

• Cell-surface receptors (on the membrane) vs. intracellular receptors (in the cytoplasm/nucleus)

What are ligands?

• Signaling molecules that actively move around the body and bind to specific receptors on target cells

• Highly specific (lock and key) - cell-surface receptors bind hydrophilic ligands that can’t pass through the phospholipid bilayer, internal receptors bind hydrophobic ligands that can’t

• Acts as a chemical messenger to trigger responses (hormones, neurotransmitters)

-crine

Autocrine: signals itself

Paracrine: signals neighboring cells (like neurotransmitters)

Endocrine: signals distant cells via bloodstream (hormones)

Direct signaling

• Cells “talk” through physical connection and share signals and molecules immediately by touching or using tunnels to pass along messages

“Fusing together” - gap junctions (animals) and plasmodesmata (plants) mean cells are directly touching and protein channels between cells allow molecules to pass through

“Cell-cell recognition” - external process where receptors bind together (ex. Immune cells), surface molecules like ligands and receptors bind

Short-Distance Signaling

• part 2 of “local signaling”

“Paracrine signaling” - cell releases molecules and secretes them along a short distance where they bind to the receptors of nearby target cells

• common Paracrine factors are neurotransmitters, growth factors

Ex. Synaptic signaling - neurons transmit signals across a tiny gap called the synapse using messengers called neurotransmitters - speedy and specific progress across a short distance

Long Distance Signaling

• primarily endocrine signaling - hormones travel through the bloodstream to distant target cells

Ex. Epinephrine (Adrenaline)

Where are receptors located?

• OUTside - cell-surface receptors for hydrophilic ligands (ions, neurotransmitters)

• INside - intracellular receptors for hydrophobic ligands (steroids, like estrogen)

Three Stages of Cell Communication

1. Reception - ligand binds, receptor changes shape (conformational change) and causes the signal/message to be passed

2. Transduction - relay molecules pass signal inwards; series of receptors

3. Response - cell does something, like gene expression, enzyme activation, secretion

Cell Signaling Reception

• the target cell detects a specific external signal (ligand) by binding it to a specific receptor protein, triggering the receptor to change shape and initiate the signal transduction process

1. Ligand/signaling molecule releases (signal is released)

2. Ligand-receptor binding (signal is detected and receptor binds to ligand)

3. Receptor location (intracellular vs cell-surface)

4. Conformational change (receptor changes shape)

5. Initiation of transduction (process begins)

G-Protein Coupled Receptors (GPCRs)

• Surface-receptors (embedded in the membrane)

• Detects molecules outside of the cell like hormones and neurotransmitters and triggers intracellular responses

• GPCRs change shape when a signaling molecule (ligand) binds, which activates the internal G-protein and triggers intracellular responses

Ligand-Gated Ion Channels

• Found at synapses in neurons

• Transmembrane proteins that allows ions to pass through

• Ligand (neurotransmitter) binds, causing a shift in the protein’s shape that opens the channel, allowing ions to pass through and initiate transduction

Transduction

• Signal transduction pathway: convert an external signals (like a hormone) through a step-by-step process by relaying the message via secondary messengers to the ultimate cell response

• Activated receptor initiates a chain reaction, relaying the signal through intracellular molecules that amplify the signal

What are relay molecules and the phosphorylation cascade?

• Relay proteins are intracellular molecules that pass a signal from the cell’s receptor to the interior, forming a cascade where one protein activates the next, like a baton pass

• Phosphorylation cascade: chain reaction in cells where one enzyme (a protein kinase) activates the next by adding a phosphate group from ATP, amplifying the signal to trigger a major cellular response

Kinases: enzymes that phosphorylate other proteins

G proteins: activated by receptors, used to activate other enzymes

Epinephrine Transduction Pathway

• Ligand binds to a membrane receptor

• Receptor activates a G proteins that allows, activating a second messenger (relay protein cAMP)

• cAMP activates kinase cascade, which leads to a glucose release for fight or flight response

Cell Signaling Response

Cell responds to the intent of the signal - activates an enzyme, gene expression, etc

• the final goal of cell signaling

Interruptions to Cell Signaling (Mutations)

• Disrupted cell communication - lost signal, blocked transmissions, defected cascade (kinase are too slow/fast/not working) - because of mutations

• Leads to diseases like cancer and diabetes

Ex. Diabetes - insulin pathway is disrupted because signal is blunted through damages receptor

Type 1: lacks enough insulin for signal to start

Type 2: cell does not hear signal properly (insulin resistance), glucose can’t get in

If pathway is interrupted, cellular response will be affected