General Principles of Cell Signalling

Study Notes

Cell signaling is a process that allows cells to communicate with each other and their environment.
Four main stages: reception, transduction, response, and termination
Reception: cells receive signals from their surroundings using receptors
Transduction: the signal is converted into a form that can be understood by the cell
Response: the cell responds to the signal, often by changing its behaviour or gene expression
Termination: the signaling pathway is shut off, preventing an overreaction

Specificity: signals are recognized only by specific receptors
Amplification: a single signal molecule can trigger a cascade of events, amplifying the signal
Network: signaling pathways can interact with each other, creating complex networks
Feedback: signaling pathways are regulated by feedback mechanisms, ensuring proper control

Juxtacrine: cells signal each other through direct contact
Paracrine: cells signal nearby cells through secreted signaling molecules
Synaptic: specialized form of paracrine signaling used by neurons to communicate across synapses
Endocrine: cells signal distant cells through hormones that travel in the bloodstream

Hormones: secreted signaling molecules that travel throughout the body
Types:
- Proteins: like adrenocorticotrophin
- Glycosylated proteins: like thyroid-stimulating hormone
- Peptides: like vasopressin
- Monoamines: like norepinephrine
- Amino acid derivatives: like melatonin
- Steroids: like cortisol
- Lipids: like prostaglandins
Receptors: proteins that bind to hormones and initiate the signaling cascade
Receptor Location:
- Cell membrane: for water-soluble hormones
- Nucleus: for lipid-soluble hormones

Protein and peptide hormones: synthesized through a classical DNA-mRNA-protein pathway
Lipid-based hormones: synthesized from precursors, like cholesterol

Steroid hormone-secreting cells: contain abundant smooth endoplasmic reticulum (SER)
Peptide hormone-secreting cells: contain abundant rough endoplasmic reticulum (RER)

Most ligands: bind to cell surface receptors
Exceptions: steroid hormones and thyroid hormones, which bind to intracellular receptors

Fast responses: occur within minutes, usually involving changes in protein activity
Slow responses: take hours or days to occur, usually involving changes in gene expression

Location: in the nucleus or cytoplasm
Function: regulate gene transcription
Structure: usually have three domains:
- Ligand-binding domain: binds to the hormone
- DNA-binding domain: binds to DNA
- Activation function domain: regulates gene transcription
Ligands: primarily steroids and lipid-based hormones
Exceptions:
- Triiodothyronine (T3)
- Oestrogen