Cell Communication & Signaling

What is signal transduction?

Conversion of one type of signal into another; begins when a receptor receives an extracellular signal and produces intracellular molecules that alter cell behavior.

What are the main types of cell signaling?

• Endocrine

• Autocrine

• Neuronal

• Paracrine

• Contact-Dependent

Two classes of extracellular signaling molecules?

• Large/hydrophilic → bind cell-surface receptors.

• Small/hydrophobic → cross membrane, bind intracellular receptors.

Fast vs slow cellular responses?

• Fast (seconds–minutes): changes in protein activity (movement, secretion, metabolism).

• Slow (hours): changes in gene expression (growth, division, differentiation).

Functions of intracellular signaling pathways?

• Relay

• Amplify

• Integrate, 

• Distribute signals

• Feedback regulation ensures optimal response.

What does GCPRs stand for?

G‑Protein‑Coupled Receptors

Structure of GPCRs?

• Largest receptor family (~700 in humans) 

• 7 transmembrane helices

• Extracellular ligand-binding domain

• Cytosolic G-protein binding loop

How are G-proteins activated?

Ligand binds GPCR → conformational change → α subunit exchanges GDP for GTP → α and βγ dissociate → both activate downstream targets.

How is G-protein inactivated?

α subunit hydrolyzes GTP → GDP, reassociates with βγ → inactive G-protein reforms.

Key second messengers in GPCR pathways?

cAMP, IP3, Ca²⁺, DAG, NO (gas).

Example of cAMP pathway in fight-or-flight?

Adrenaline → GPCR → G-protein → adenylyl cyclase → cAMP → PKA → glycogen breakdown ↑, glycogen synthesis ↓ → glucose mobilization.

How does caffeine affect cAMP signaling?

• Inhibits phosphodiesterase (PDE) → cAMP persists → sustained PKA activation → ↑ metabolism, heart rate, dopamine release → “buzz.”

How do receptor tyrosine kinases (RTKs) work?

Ligand binding → receptor dimerization → kinase domains phosphorylate each other → phosphorylated tyrosines act as docking sites → downstream signaling.

Ras/MAPK cascade steps?

Activated Ras → MAP3K → MAP2K → MAPK → phosphorylates targets → changes in gene expression, proliferation.

Why is Ras important in cancer?

30% of human cancers have Ras mutations; constitutively active Ras drives uncontrolled proliferation.

Signaling Cell: Endocrine Cell

Signaling Molecule: Hormones 

Communication distance: Long 

Communication mode: Public, broadcast to the whole body 

Endocrine Signaling

Signaling Cell: Many Cell Types 

Signaling Molecule: Cytokines, Interferons 

Communication distance: Local 

Communication mode: Semi-public

Paracrine Signaling

Signaling Cell: Many Cell Types 

Signaling Molecule: Cytokines, Interferons 

Communication distance: Local

Communication mode: Semi-public 

Autocrine Signaling 

Signaling Cell: Nerve Cells (Neurons)
Signaling Molecule:  Neurotransmitter 

Communication distance: long 

Communication mode: Private

Neuronal Signaling

Signaling Cell: Several
Signaling Molecule: Several

Communication distance: Very short 

Communication mode: Private 

Contact-Dependent Signaling