MCB 8: Cell Signalling Flashcards
Why do cells need to communicate with each other?
- physiological processes, such as visual, auditory processing or homeostasis, require cellular communication
- cells need to interpret many different signals from other cells, in order to coordinate their behaviours
Where can cells receive information from?
- the extracellular matrix
- soluble factors from the extracellular fluid
- adjacent cells

Why is cell signalling crucial for cells?
- cells need multiple signals just for them to survive
- some include signals to grow and divide, or to differentiate
- if cells are deprived of signals, most undergo apoptosis

What types of cell signalling exist?
Endocrine Signalling:
- the signalling molecule is usually a hormone
- they are secreted molecules from distant sites of the body
- they travel through the bloodstream to a distant target cell
- endocrine cells produce these hormones
Paracrine Signalling:
- when a hormone (or another signalling molecule) is released from the cell it was produced from into the extracellular fluid
- usually when a hormone is used in neighbouring cells
Autocrine signalling:
- when the signalling molecule acts on the same cell
Synaptic Signalling:
- nerve signals are transmitted electrically along a nerve axon
- when it reaches the nerve terminal, it causes the release of neurotransmitters onto adjacent target cells
- conversion of an electrical signal to chemical
Contact-dependent Signalling (or called juxtacrine signalling):
- a cell surface-bound signalling molecule binds to a receptor on an adjacent cell
In the immune system, cell surface receptors on antigen-presenting cells interact with cell surface receptors on T cells. What type of signalling is this?
- contact-dependent

What is signal transduction?
- this is the process of signal conversion in cells
- target cells convert an extracellular signalling molecule into an intracellular signalling molecule that alters cell behaviour
What are signalling and target cells?
- a signalling cell is the cell that produces the extracellular signal molecule
- a target cell is one that detects the signalling molecule
- most cells produce and receive signals

What are the three strategies cells use to transfer information across the cell membrane?
- passive diffusion
- gated channels
- transmembrane receptors
Describe how passive diffusion allows cells to transfer
- only certain molecules are able to diffuse: lipid-soluble hormones or permeable gas
- once they are inside the cell, they will elicit a change in the cell’s behaviour

Describe how gated channels allows the transfer of information across the cell membrane
- they transfer solutes (most ions) across the membrane
- they can be closed or opened in response to a specific signal

Describe how transmembrane receptors allow the transfer of information across the cell membrane
- these transmembrane proteins have a ligand-binding domain on the extracellular region and one or more transmembrane segments
- the intracellular part couples to signalling effector proteins inside the cell

What are receptors?
- receptors are the protein with which the signalling molecule interact first with on a target cell
- receptors recognise and respond specifically to the signalling molecule
- each receptor usually only responds to one type of signal
- there are two types of receptors:
- cell surface receptors
- intracellular receptors
What are cell surface receptors?
- usually a protein that spans the cell membrane and has parts that stick out of the cell and parts in the cytosol
- extracellular signalling molecules bind to the extracellular part
- the intracellular part passes on the message in the form of an intracellular signalling molecule

What are intracellular receptors?
- only molecules that are small and hydrophobic enough to pass through the plasma membrane react with intracellular receptors
- the signalling molecule passes directly through the cell membrane and interacts with a receptor molecule located inside the cell

How can the same signalling molecule induce different responses in different target cells? Give an example
- even if they have a similar cell-surface receptor, they differ in the intracellular effector proteins and transduce a different signal
- or the molecule binds to a different receptor in the first place, so the signal is different
- acetylcholine can:
- decrease heart rate in heart pacemaker cells
- increase secretion in salivary gland cells
- cause muscle contraction in skeletal muscle cells
What are signalling cascades/pathways?
- a series of events triggered by a signalling molecule binding to its receptor
- the outcome is a change in cellular behaviour
Explain how signalling pathways work
- signalling cascades always starts with an extracellular signalling molecule binding to the extracellular region of its receptor
- then the primary step of signal transduction occurs: this is thereceptor relaying the signal to the inside of the cell
- the signal is relayed onto a series of intracellular signalling molecules
- sometimes one molecule activates the next molecule in a pathway, or generates a new molecule in the pathway
- this goes on until it hits an effector protein
- they could be a metabolic enzyme, a cytoskeletal protein or a transcriptional regular for example
- these trigger responses in the cells

How can signalling cascades modify the incoming signal and how can these explain the key functions of the signalling cascade?
- a scaffold protein holds several intracellular molecules together so that they can be activated in a particular cellular location with greater speed
- one of the key functions of signalling pathways is to relay a signal, so it can be spread throughout the cell
- another key function is to amplify the signal
- one receptor can only activate one enzyme, but the enzyme may be able to generate many different molecules as its product at any given time
- this way one molecule of receptor and enzyme can generate many intracellular signalling molecules, often messenger molecules
- these can induce a large intracellular response
- another function is to integrate signal
- at any given time, cells can have many different signalling pathways that are active
- signals can be integrated from more than one pathway before they can be relayed onward
- the change of behaviour the cell actually performs depends on how many different pathways are active and how these different pathways are regulated

What are molecular switches?
- these are intercellular signalling proteins that can convert from an inactive to active state upon receiving a signal
- once activated, they can stimulate other proteins in the pathway
- they persist in the active state until some other process switches them off
What two classes of molecular switch proteins are there?
- regulation by phosphorylation
- carried out by protein kinases
- regulation by G proteins
- also known as GTP-binding proteins
Describe how molecular switch proteins are switched on or off by phosphorylation
- molecular switch proteins can be activated by adding a phosphate group
- the phosphate group is added covalently by protein kinase
- they can be inactivated by the removal of the phosphate
- taken off by protein phosphatase

Describe what happens chemically when phosphorylation occurs
- phosphate from ATP is transferred to the protein to form a phosphoester
- proteins can be phosphorylated on serine, threonine and tyrosine residues

Describe signalling by GTP-binding proteins
- G proteins are activated when they exchange their bound GDP for a molecule of GTP
- GDP is released from the G proteins and the G protein binds to GTP
- once activated by GTP, the proteins have intrinsic GTP-hydrolysing (GTPase) activity
- they switch themselves off by hydrolysing their bound GTP to GDP

What are adaptor proteins?
- adaptor proteins contain protein domains that are important in molecular recognition
- they bind proteins containing particular recognition motifs and bring proteins together in space functioning like a molecular bridge
- it is a smaller protein so usually binds two proteins together






