Intro to Cell Signalling

Question 1

What are the 3 basic methods of communication between cells in multicellular organisms?

Answer 1

1. Direct Cytoplasmic Bridges e.g. Gap Junctions
2. Messenger Anchored in Cell-Surface (Cells signal to neighbours by messengers that are integral plasma membrane proteins, good example in the compound eye of Drosophila)
3. Secreted Messenger (Used for signalling over different distances)

Question 2

What are the 3 types of secreted messenger?

Answer 2

1. Hormones (Released by endocrine cells and carried through the bloodstream to its target cell)
2. Neurotransmitter (Released from nerve terminal into the synaptic cleft (<100nm) between the neurone and the target cell)
3. Local mediators (aka paracrine regulation)

Question 3

Examples of Local Mediators:

Answer 3

Nerve Growth Factor (NGF)

Platelet-Derived Growth Factor (PDGF)

Question 4

Structure of Gap Junctions:

Answer 4

• Connexons contain 6 subunits of the protein connexin, arranged around a central pore.
• Pore open at low Ca2+, closed at High Ca2+, to prevent leaking of cell contents (as Ca2+ extracellular concentration is normally high).
• Six connexin subunits (each with 4 transmembrane helices) assemble into a hemichannel
• Hemichannels from neighbouring cells then associate together to form full channels between cells, with central pores of about 1.4 nm diameter

Question 5

“Compound eye of Drosophila”: example of “Messenger Anchored in Cell-Surface” communication

Answer 5

• Each eye facet contains 8 different photo-receptor cells (R1-R8), which respond to different wavelengths of light
• R7 only develops if it receives a signal from R8
  the signal is sensed by the sevenless receptor
• Mutation in the sevenless gene causes a lack of R7 cell
• The ligand for this receptor (bride of sevenless, or boss) is a seven-span membrane protein embedded in the plasma membrane of R8

Question 6

Platelet-Derived Growth Factor (PDGF)

Answer 6

• PDGF is released by platelets, the blood components responsible for clotting at the site of an injury to a blood vessel
• PDGF stimulates migration of fibroblasts to a wound, and their proliferation
• The fibroblasts then secrete extracellular matrix (ECM) components like collagen, to which other cells attach to repair the wound:
• PDGF is retained locally because it binds tightly to the ECM

Question 7

Nerve Growth Factor (NGF)

Answer 7

• NGF is the 1st example of a “neurotrophin” (A molecule that induces the survival, development, and function of neurons)
• NGF acts in the PNS, but related neurotrophins act on neurones of the CNS
• Neurotrophins are released by target cells that require innervation by a neurone
• Neurotrophins are chemotactic, i.e. axons grow along concentrations of increasing neurotrophin concentration