Lecture 4 - Chapter 7: Molecular signaling within neurons Flashcards
What are the essential components of chemical signaling?
Signaling cells → release signal molecules → interact with specific receptors on targeting cells → production of intracellular effector molecules → subsequent cellular response.
What are forms of chemical communication?
- Synaptic transmission
- Paracrine signalling
- Endocrine signalling
Explain the following forms of chemical communication:
- Synaptic transmission
- Paracrine signaling
- Endocrine signaling
- Synaptic transmission: presynaptic release of neurotransmitters in synaptic cleft → interact with postsynaptic receptors → production of intracellular effector molecules
- Paracrine signaling: cell produces signalling molecules → interact with receptors on nearby target cells → production of intracellular effector molecules
- Endocrine signaling: secretion of hormones into the bloodstream, where they can affect targets throughout the body.
How can interaction of a signaling molecule with a single receptor lead to amplification of the signal?
(Wil be discussed in broader detail throughout this deck)
- Interaction of a signaling molecule with its receptor can lead to the activation of numerous intracellular G-proteins.
- These proteins can bind to other signaling molecules, such as adenylyl cyclase.
- This enzyme generates a number of cAMP molecules.
- cAMP binds and activates the enzymes kinases.
- Kinases can phosphorylate many target proteins.
Note: that not every step is an amplification step, but eventually it will lead to total amplification of the first signal.
What are the three classes of cell signaling molecules and also explain their characteristics.
- Cell-impermeant molecules, cannot cross the plasma membrane of the target cell and must bind to the extracellular part of the receptor.
- Cell-permeant molecules, are able to cross the plasma membrane and bind to receptors in the cytoplasm or nucleus of target cells.
- Cell-associated molecules, are present on the extracellular surface of the plasma membrane and can only activate receptors on target cells if they are directly adjacent to the signaling cell.
Name four different cellular receptors and explain what happens when a signal binds to the receptors.
- Channel-linked receptors → when a signal binds, the channel opens and ions flow across the membrane
- Enzyme-linked receptors → when a signal binds, the intracellular part of the receptor that has enzymatic activity is activated and the enzyme generates an activated effector.
- G-protein-coupled receptors → when a signal binds, the G-protein will bind intracellularly and is activated. This leads to signal amplification inside the cell.
- Intracellular receptors → cell-permeant signalling molecule crosses the membrane and bind to the intracellular receptor. The activated receptor regulates transcription.
We’ve just concluded that G-protein-coupled receptors are activated when a signal binds to the receptor, which will cause the G-protein to bind intracellular to the receptor, which can then amplificate the signal intracellular.
Only, G-proteins are not just ‘active proteins’, they need to be activated.
Explain the G-protein activation cycle.
Normally, G-protein is inactive due to the fact that GDP is bound to the protein. For activation, GDP needs to be exchanged for GTP. This is done by an activator protein GEF, which catalyses the exchange from GDP to GTP. When GTP is bound to the G-protein, the protein is activated.
G-proteins also need to be turned off, which is done by an inactivator protein called GAP. GAP removes the phosphate group (Pi), which changes GTP into GDP again.
There are two types of GTP-binding proteins (G-proteins):
- Heterotrimeric G-proteins
- Monomeric G-proteins
What are the characteristics of a heterotrimeric G-protein?
The heterotrimeric G-protein consists of three different subunits (α, β, and γ). When the receptor is activated, G-protein will bind to the receptor and the α-subunit will exchange its bound GDP for GTP. This will cause dissociation of the α-subunit from the other two subunits. Dissociation causes activation of both the α-subunit as the βγ-complex. The α-subunit can then interact with an effector protein.
The response is terminated by hydrolysis of GTP, which is enhanced by GAP.
There are two types of GTP-binding proteins (G-proteins):
- Heterotrimeric G-proteins
- Monomeric G-proteins
What are the characteristics of a monomeric G-protein?
In the picture, Ras is used as an example as a G-protein.
When the receptor is activated, an adaptor protein that is bound to the intracellular part of the receptor will bind to GEF. GEF is able to bind GTP to Ras, where Ras gets activated. The signal is again terminated by GAP, which hydrolyses GTP to GDP.
Name three examples of G-protein-coupled receptors. Also name the signal molecule that interacts with the receptor and the type of G-protein that is associated with the receptor.
- β-adrenergic G-protein coupled receptor, binds norepinephrine and is associated with G-protein Gs.
- mGluR G-protein coupled receptor, binds glutamate and is associated with G-protein Gq.
- Dopamine D2 G-protein coupled receptor, binds dopamine and is associated with G-protein Gi.
Explain the signaling cascade that occurs when norepinephrine binds to its β-adrenergic receptor (also think of what G-protein is involved)?
- Norepinephrine binds to its β-adrenergic receptor
- Gs-protein is activated
- Activates adenylyl cyclase
- Adenylyl cyclase produces cAMP
- cAMP activates protein kinase A
- Protein kinase A increases protein phosphorylation
Explain the signaling cascade that occurs when glutamate binds to its mGluR (also think of what G-protein is involved)?
- Glutamate binds to its mGluR
- Gq-protein is activated
- Activates phospholipase C
- Phospholipase C produces diacylglycerol and IP3
- Diacylglycerol activates protein kinase C and IP3 causes the release of Ca2+
- These actions cause an increase in protein phosphorylation and activation of calcium-binding proteins.
Explain the signaling cascade that occurs when dopamine binds to its dopamine D2 receptor (also think of what G-protein is involved)?
- Dopamine binds to its dopamine D2 receptor
- Gi-protein is activated
- This inhibits the activation of adenylyl cyclase
- Therefore no cAMP is produced
- Therefore no protein kinase A is activated
- This causes a decrease in protein phosphorylation
What is the most common intracellular messenger?
Calcium
How does calcium enter the cytosol?
Via calcium-permeable ion channels (either voltage- or ligand-gated Ca2+ channels)
There’s a calcium gradient inside the cell. Where the concentration of calcium is highest around the membrane and lowest deeper in the cell. What is done to maintain this gradient?
Two proteins reside in the cell (membrane) that are responsible for transporting calcium to the extracellular space:
- Ca2+ pump (ATPase that pumps Ca2+ out of the cell and protons (H+) into the cell)
- Na+/Ca2+ exchanger
Calcium is also transported to the endoplasmatic reticulum and mitochondria via gated calcium channels and are stored there for later signaling events.
What are examples of gated calcium channels on the endoplasmatic reticulum?
IP3 and ryanodine receptor
Just a picture that summarizes the previous questions.
Also study this picture, which also summarizes the previous questions.
Fill in:
- cAMP is converted from … (1) by an enzyme called …(2), while cGMP is converted from … (3) by an enzyme called …(4).
- cAMP can activate the protein …(5), while cGMP can activate the protein …(6).
- cAMP and cGMP can both be transported through a …(7).
- cAMP can be converted to AMP by an enzyme called … (8), while cGMP can be converted to GMP by an enzyme called …(9).
- cAMP is converted from ATP (1) by an enzyme called adenylyl cyclase (2), while cGMP is converted from GTP (3) by an enzyme called guanylyl cyclase (4).
- cAMP can activate the protein kinase A (PKA)(5), while cGMP can activate the protein kinase G (PKG) (6).
- cAMP and cGMP can both be transported through a cyclic nucleotide-gated channel (7).
- cAMP can be converted to AMP by an enzyme called cAMP phosphodiesterase (8), while cGMP can be converted to GMP by an enzyme called cGMP phosphodiesterase (9).
What are the functions of the two second messengers derived from PIP2 by phospholipase C; diaglycerol and IP3?
- Diaglycerol can activate protein kinase C (PKC)
- IP3 can activate ligand-gated calcium channels, where calcium can leave the E.R. and can enter the cytosol.
How is a protein phosphorylated and how can the phosphate group be removed again?
- A protein is phosphorylated by a protein kinase (second messengers can activate kinases). Kinases transfer phosphate groups from ATP to the target protein, which converts ATP to ADP.
- A protein is dephosphorylated by a protein phosphatase (second messengers can activate phosphatases). The phosphate group is then removed from the protein.
Explain how PKA is activated.
PKA is a heterotetramer (composed of different subunits) with two catalytic domains that are shielded by two regulatory domains. When cAMP binds to the regulatory domain, PKA changes in conformation and opens itself up. This way the two catalytic domains are active and PKA itself becomes active. PKA can then phosphorylate serine/threonine residues.
CaMKII is another kinase. Describe how this kinase is activated.
- When inactive, the kinase is in a closed configuration, where the catalytic domain is shielded by the regulatory domain.
- When Ca2+ binds to the regulatory domain Calmodulin on CaMKII, the conformation opens up and the catalytic domain is exposed and is activated.
Note: CaMKII is also a serine/threonine kinase
Describe how protein kinase C (PKC) is activated.
- In its inactive form, PKC is in a closed configuration. Here again, the catalytic domain is shielded by the regulatory domain.
- PKC contains different binding sites for DAG (diacylglycerol), Ca2+ and phosphatidylserine (PS). Since DAG is still part of the plasma membrane, it causes PKC to translocate to the membrane and bind with DAG and PS (also part of the membrane). This will cause PKC to change conformation and be activated.
What are dendritic spines and what’s interesting about them?
The dendrites of neurons have little branches, that are referred to as dendritic spines.
At the tip of these dendritic spines, are many excitatory synapses found that contain a lot of signaling components (e.g. second messengers).
Dendritic spines can occur in different forms, also depicted in the picture. What form of dendritic spines is considered as immature and what form as mature?
- Filopodia spines are considered as immature
- Mushroom spines are considered as mature
What form of dendritic spines is the most prevalent in dendrites?
Mushroom spines
Describe how mushroom spines look like.
Mushroom spines have a spine neck and a spine head, where the neck is connected to the dendrite.
What may be the function of the spine neck and spine head of a mushroom spine?
The neck biochemically isolates the head from the rest of the dendrite. This prevents diffusion of biochemical signals from the spine head to the rest of the dendrite. Most likely, spines serve as a reservoir for second messengers like calcium or IP3, so that these messengers can be concentrated here. So in the head, there’s the postsynaptic density that receives information for the presynaptic terminal.
What is the postsynaptic density?
The postsynaptic density is a protein dense specialization attached to the postsynaptic membrane. It consists of a scaffolding network (like PSD-95) and on the plasma membrane there are different receptors located (see picture).
What happens when an NGF dimer binds to its tyrosine-kinase receptor (TrkA)?
The receptor dimerizes, which causes autophosphorylation of the cytosolic part of the receptor. The cytosolic part of the receptor is then activated and can activate three different pathways: PI3 kinase, Ras and the phospholipase C (PLC) pathway.
Activation of the tyrosine-kinase receptor leads to activation of three pathways: the PI3 kinase, Ras and the phospholipase C (PLC) pathway.
What are the functions of these pathways?
- PI3 kinase → cell survival
- Ras and PLC pathway → neurite outgrowth and neuronal differentiation
Describe how DNA in chromatin form can lead to transcription of the DNA into RNA.
DNA is inactive when it is in chromatin structure. Decondensation of chromatin structure, leads to beads-on-a-string chromatin. This opens activates upstream activator sites (UAS) where transcriptional activator proteins and other proteins important for transcription can bind. This leads to the transcription of DNA to RNA.
How does CREB activation lead to transcription of mRNA?
CREB is normally bound to a binding site on the DNA, called CRE/CaRE element. CREB can be phosphorylated by multiple signaling pathways (PKA, MAP kinase or intracellular Ca2+).
