Lecture 7 & 8 Transmembrane Signaling Flashcards
How is a cell regulated?
Signalling
What is spatial regulation?
regualtion to make sure that the processes happen only within the cells that are supposed to do what it is required
What is temporal regulation?
regulation to make sure that the processes happen only when they are required and for how long they are required for
What is kinase?
an enzyme that catalyses the phosphorylation of a substrate
- When the substrate is a protein the enzyme is specifically called a
PROTEIN kinase
- When the substrate is a lipid it is called a LIPID kinase
What happens when a protein gets phosphorylated?
- It becomes more hydrophilic
- It can change its structure (conformational change)
- It can change its activation status (i.e. it can become either active or inactive)
What is the name of the process which is started because the cell received the instruction to do so?
Signal Transduction
What is Signal Transduction?
Signals are converted into intracellular biochemical reactions that ultimately induce the required cellular functions
Examples of signal transduction:
Cell Cycle (DNA relication, Translation)
Assembly of microtubules
What are the steps for all signalling pathways?
- A signal is sent by one cell (or coming from outside)
- The signal is “received” by the cell that is supposed to respond
- The signal is converted into instructions
- The cell respond to the instructions by doing what it is supposed to do
Step 1 in signalling pathways: what is a signal?
Signal arrives from outside the cell
A molecule (growth factor, hormone etc) released by another cell or coming from outside
Step 2 in signalling pathways: what are the different processes?
2a. The signal can simply and freely go from one cell to the other one (i..e through gap junctions)
2b. The signal cannot simply and freely go from one cell to the other. The receiving cell need a “receptor” to capture the signal
What is a gap junction?
an example of structures that allow signals (ions or small molecules) to physically go from one cell to another one, with the cells connected through tunnels
Gap Junctions: What is the connection between two cells called?
Direct cytoplasmic communication
Gap Junctions: what is it composed of?
- gap junctions comprise of 2 connexons (hemichannels)
- Each connexon comprises of 6 connexin protein molecules
Gap Junctions: what are connexin proteins?
Each connexin protein has 4 transmembrane domains (M) with 2
extracellular loops (E), 1 intracellular loop (CL) and both intracellular N- and Ctermini
Gap Junctions: What are the different types?
- homotypic (both hemichannels have same composition)
- heterotypic (two different hemi-channels)
- Multiple channels comprise a gap junction plaque
Step 2 in signalling pathways: 2b?
The signal cannot simply and freely go from one cell to the other. The receiving cell need a “receptor” to capture the signal
Step 2 in signalling pathways: when a “signal” cannot pass freely from one cell to another. What is the signal and how does it work?
the receiving cell is equipped with a protein receptor, as an “antenna”
that can capture the signal (=ligand) by binding to it
Step 2 in signalling pathways: Define the types of signalling based on whether the cell sending the signal is distant/close/the same as the cell receiving the signal
ENDOCRINE
AUTOCRINE
PARACRINE
JUXTACRINE
Step 2 in signalling pathways: Endocrine Signalling
“Sending” cell (ligand) is distant from “receiving” cells (target cell)
Insulin (signal) from endocrine pancreas binds to insulin receptor on target cells
Define endocrine.
Refers to glands that secrete their products into the blood
Define exocrine.
Refers to glands that secrete their products “out” through ducts
Step 2 in signalling pathways: Autocrine Signalling
“Sending” and “receiving” are the same cell
A ligand can act on the same cell which produced that signal
Step 2 in signalling pathways: Paracrine Signalling
“Sending” and “receiving” cells are next to each other
A ligand can act on neighbouring cells
Step 2 in signalling pathways: Juxtacrine Signalling
“Sending” and “receiving” cells are next to each other
(when ligand is on the sending cell and cannot be released, so needs cell to cell contact)
- A ligand can be a plasma membrane protein, lipid or oligosaccharide on the sending cell
- This can bind a receptor on the receiving cell
- It requires cell-cell contact (juxtacrine)
Gap vs Tight Junction
Tight Junctions are there to connect cells, not for communication or for exchanging signals
Step 2 in signalling pathways: when a “signal” cannot pass freely from one cell to another. What is the receptor and how does it work?
Signal (ligand) binds to a receptor:
• inside the cell (cytosol/nucleus)
• at the plasma membrane
Step 2 in signalling pathways: What are the types of receptors?
Nuclear & Transmembrane
Step 2 in signalling pathways: Nuclear Receptors
- (inside the cell. Used by ligands that are hydrophobic and that can cross the plasma membrane)*
- -* Hydrophobic signals bind to receptors inside the cell (“nuclear receptors”)
- the complex then goes into the nucelus which activates the function of the cell
Step 2 in signalling pathways: Nuclear Receptors example
cortisol signalling
cortisol is an example of an hydrophobic ligand that can go inside the cell where it binds the receptor.
The complex cortisol/receptor then goes into the nucleus to activate
transcription of genes and therefore synthesis of new proteins
Step 2 in signalling pathways: Transmembrane Receptors
- inserted into the plasma membrane
- Used by ligands that are hydrophilic therefore they cannot cross the membrane.
- Hydrophilic signals need to relay message across* cell membrane (transmembrane signalling)
- Extracellular signal is converted into an intracellular signal
- new signal can instruct cell on what to do
Step 3 in signalling pathways:
The signal is converted into instructions
Step 3 in signalling pathways: What are the two ways in which the signal is converted into instructions and what is the result?
3a. A new molecule is generated inside the cell to carry the message
3b. The message is carried directly by the receptor, without the need of synthesis of a new molecule
Result: A cascade of events is activated that ultimately allows the cell to do what it is supposed to do
Step 3 in signalling pathways: 3a. What is the new molecule that is generated inside the cell to carry the message called?
Second Messenger: generated only when the ligand binds the receptor
Step 3 in signalling pathways: Primary vs. Secondary Messengers
Ligand (Extracellular signal: 1st messenger)
A new molecule is generated intracellularly to carry the instructions (2nd messenger)
Step 3 in signalling pathways: Blood Glucose Example - when the concentration of blood glucose is low
- If the concentration of glucose in the blood is low, no insulin is secreted by the pancreas.
- There is no insulin to provide instructions to muscle cells
In the muscle cells:
- The insulin receptor is switched off
- The protein IRS and the kinases PI3K and Akt are INACTIVE, in the cytoplasm
- The lipid PIP2 is at the membrane
- The transporter for the glucose is stored intracellularly
- Glucose cannot enter the cell
Step 3 in signalling pathways: Blood Glucose Example - when the concentration of blood glucose is high
- When the concentration of glucose in the blood becomes high, insulin is secreted by the pancreas.
- Insulin now can provide instructions to muscle cells
In the muscle cells:
- When insulin binds, the receptor gets activated (i.e. it undergoes a conformational change therefore it becomes active)
- The insulin receptor is a Tyrosine kinase receptor and it is a dimer: one subunit phosphorylates the other (autophosphorylation)
- The phosphorylated Tyrosines in the receptor recruit IRS (IRS can bind to the Tyrosines on the receptor only when they are phosphorylated)
- Tyrosine residues within IRS get phosphorylated by the receptor
- Because IRS now is phosphorylated in its Tyrosine residue, it can bind the lipid kinase PI3K (PI3K cannot bind to IRS when IRS is not phosphorylated)
- Once it binds to IRS and it is at the plasma membrane, PI3K becomes active and it converts the lipid PIP2 into the lipid PIP3
–>
PIP3 was not there before and it is synthesised only when insulin binds to the receptor: IT IS A SECOND MESSENGER
Blood Glucose Example - when the concentration of blood glucose is high. How does the second messenger work?
It can activate proteins. Once these proteins are activated they allow the cell to do what it is supposed to do.
- PIP3 binds the protein kinase Akt that then goes to the plasma membrane.
- Once at the membrane, Akt is phosphorylated.
- Phosphorylated Akt is ACTIVE (i.e. it can phosphorylate other proteins. A SIGNALLING CASCADE is activated)
PI3K and Akt would be examples of SIGNALLING MOLECULES, proteins that were inactive in the absence of insulin and become activated once insulin binds the receptor
- One of the proteins that are phosphorylated by Akt allows the relocation of the glucose transporter GLUT4 to the plasma membrane
- The transporter is inserted into the membrane and glucose can enter the cell
Step 4 in signalling pathways:
Carrying the message
Step 4 in signalling pathways: Carrying the message. How are “signalling proteins” activated?
Proteins can become active because:
• They bind to other proteins
• They change their structure (conformational changes)
• They bind to cellular membranes
• They are relocated to specific cellular compartments (compartmentalisation)
• They are modified (covalent modifications, for instance phosphorylation can activate some of them- NB: Phosphorylation does not always mean activation though! Some proteins are inhibited by phosphorylation)
Step 4 in signalling pathways: How is PI3K activated?
PI3K is activated because:
- It binds to another protein (IRS)
- It changes conformation
- It goes to the plasma membrane, where its substrate (PIP2) is
Step 4 in signalling pathways: How is Akt activated?
- It binds to the second messenger PIP3
- It changes conformation
- It goes to the plasma membrane
- It gets phosphorylated
Different cellular functions can be activated by ….
The same signal
Just the fact that in signalling, one protein, once it is activated by a signal, can activate many proteins and many cellular functions simultaneously
Cellular functions are controlled by …..
many signalling cascades
there are many cellular pathways
Step 4 in signalling pathways: types of plasma membrane receptors
- Ligand-gated ion channels
• Enzyme-linked receptors
• G protein-coupled receptors (GPCRs)
Types of plasma membrane receptors: Ligand-gated ion channels
These are receptors/ion channels that are regulated by ligands. Only when the ligand binds to the receptor/ion channel, the channel opens and the ion can pass through
Types of plasma membrane receptors: Ligand-gated ion channels example
GABA receptor
(Neurotrasmitter in the central nervous system)
Types of plasma membrane receptors: enzyme-linked receptors
These are receptors (they can bind a ligand through their ligand binding domain extracellularly) and they are also enzymes (intracellularly)
In the absence of the ligand, the enzymatic activity of the receptor is OFF
Types of plasma membrane receptors: enzyme-linked receptors examples
- Guanylyl Cyclase receptors
- Serine/Threonine Kinase Receptors
- Tyrosine Kinase Receptors
Enzyme-linked receptors examples: Guanylyl Cyclase receptors
When the ligand binds, the enzymatic activity is turned ON
This receptor then convert GTP into cyclic GMP.
cGMP was not present before and it is generated only when the ligand binds: it is a SECOND MESSENGER
cGMP then activates proteins that eventually activate cellular functions
Enzyme-linked receptors examples: Serine/Threonine Kinase Receptors
In the absence of ligand the receptor is OFF. When the ligand binds the receptor, the receptor becomes active and then it can phosphorylate proteins at Ser or Thr residues
Enzyme-linked receptors examples: Serine/Threonine Kinase Receptors example
Transforming Growth Factorβ receptor
Enzyme-linked receptors examples: Serine/Threonine Kinase Receptors example - Transforming Growth Factorβ receptor
The receptor autophosphorylates when the ligand binds and then it can directly phosphorylate a protein.
So in this example there is NO SECOND MESSENGER (no new molecule is generated).
The receptor itself initiates the signalling cascade, ultimately inducing transcription of genes/synthesis of new proteins
that control proliferation, chemotaxis etc
Enzyme-linked receptors examples: Tyrosine Kinase Receptors
In the absence of ligand the receptor is OFF. When the ligand binds the receptor, the receptor becomes active and then it can phosphorylate proteins at Tyrosine residues
Enzyme-linked receptors examples: Tyrosine Kinase Receptors examples
• Receptors for insulin and other growth factors (for instance the receptor for epidermal growth factor or platelet derived growth factor etc) have intrinsic tyrosine kinase activity
• Some of these receptors can dimerise (note: insulin receptor is already a dimer)
• Receptors autophosphorylate
• Important in several cellular functions (cell proliferation, survival, migration etc)
Enzyme-linked receptors examples: Tyrosine Kinase Receptors example - growth factor receptors
Growth factor receptors are usually monomer in the absence of the ligand. When the ligand binds, they dimerise and become active. NB: Insulin receptor belongs to the same family and signals in the same way, apart from the dimerization, as it is already a dimer
Once they dimerise and they become active, the receptors can phosphorylate their own Tyrosine residues (autophosphorylation)
Once the Tyrosine of the receptors are phosphorylated, they can bind and activate other proteins - SIGNALLING CASCADE
