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Year 3: Cell and Molecular Biology > Cell signalling > Flashcards

Cell signalling Flashcards

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1
Q

What is cell-to-cell contact signalling?

A
  • Occurs when a cell is able to interact with a neighbouring cell, e.g. via binding of surface proteins or release of molecules into neighbouring cell, and as a result produce a response in the neighbouring cell.
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2
Q

What is paracrine signalling?

A
  • Occurs when a cell produces a signal that targets and produces a response in a nearby cell.
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3
Q

What is endocrine signalling?

A
  • Occurs when a cell produces a signal that targets and produces a response in a distant cell.
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4
Q

What is autocrine signalling?

A
  • Occurs when a cell produces a signal that targets and results in a response occuring within itself.
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5
Q

What is meant by the term “signal transduction?”

A
  • The process by which a physical or chemical signal is transmitted through a cell.
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6
Q

Describe some of the general mechanisms that are seen during cell signalling

A
  1. A signalling molecule will bind to a receptor
  2. Binding of signalling molecule to the receptor will cause changes in the receptor which will lead to the transduction of the signal into the cell
  3. Signal transuction occurs via a series of relay molecules
  4. Signal transduction eventually leads to a cellular response
  5. Cellular response will result in activation of feedback mechanisms which modulate cell sgannling pathway
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7
Q

Briefly describe the structure of a G protein coupled receptor (GPCR)

A
  • Large extracellular domain - contains the ligand binding region
  • 7 transmembrane domains
  • Intracellular domain - contains phosphorylation sites
  • G protein binding domain - located on one of the loops of intracellular domain
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8
Q

What are the names of the 3 subunits that make up a G protein?

A
  • α (alpha) subunit
  • β (beta) subunit
  • γ (gamma) subunit
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9
Q

What is the function of the covalently attached lipid tails and which subunits of the G protein have them?

A
  • Function of the Covalently attached lipid tails is to help anchor the G protein to the plasma membrane
  • α and γ subunits have them
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10
Q

What is the name of the protein that accelerates the inactivation of the α subunit and how does it achieve this?

A
  • Regulator of G protein signalling (RGS)
  • It binds to the activated α subunit
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11
Q

Describe the process of G protein coupled receptor (GPCR) activation

A
  1. Ligand binds to the extracellular ligand binding domain
  2. This results in a conformational in the cytosolic regions of the GPCR which activate a G protein.
  3. Specifically, the activated receptor results in a conformational change in the α subunit of the G protein.
  4. This causes the GDP bound to the α subunit to dissociate and leads to GTP binding instead
  5. Binding of GTP activates both the α subunit and the β-γ complex of the G protein
  6. Both the activated α subunit and β-γ complex then go on to bind and regulate the activity of target proteins within the membrane resulting in signalling cascades
  7. Eventually, α subunit hydrolyses GTP back into GDP causing α subunit to deactivate
  8. Deactivated α subunit reforms G protein with the β-γ complex
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12
Q

What are the 2 main pathways that are regulated by GPCRs?

A
  • cAMP pathway
  • Ca2+ pathway
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13
Q

Describe the cAMP pathway cascade that occurs as a result of GCPR activation

A
  1. Activated α subunit of G protein activates adenylyl cyclase
  2. Adenylyl cyclase then catalyses conversion of ATP into cAMP
  3. cAMP then goes on to bind to one of its target proteins, e.g. protein kinase A (PKA), and modulates its activity
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14
Q

Describe the Ca2+ pathway cascade that occurs as a result of GCPR activation

A
  1. Activated α subunit of G protein activates phospholipase C
  2. Activated phospholipase C catalyses conversion of PIP2 into DAG and IP3 (inositol triphosphate)
  3. Inositol triphosphate binds to its receptor on the endoplasmic reticulum causing it to release Ca2+ into the cytoplasm
  4. Ca2+ will then go and bind to its target protein and modulate its activity
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15
Q

Give some examples of ligands that are able to activate GCPRs

A
  • Hormones e.g. Adrenaline and Noradrenaline
  • Neurotransmitters e.g. Serotonin or Aceytylcholine
  • Odors in smell perception
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16
Q

Briefly describe the structure of receptor tyrosine kinases

A
  • Single-pass transmembrane receptors - only 1 transmembrane domain
  • Very diverse extracellular domains - e.g. PDGF has immunoglobulin-like domain while EGF has cytesine-rich domain
  • Intracellular domain - contains one or two tyrosine kinase domains
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17
Q

Explain how a receptor tyrosine kinase (RTK) becomes activated

A
  1. Ligand dimer binds to both RTKs which brings them close together (dimerization)
  2. This results in the tyrosine kinase domains of each of the receptors phosphorylating large amunts of tyrosine on the adjacent receptor (cross-phosphorylation)
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18
Q

What is the name of the main protein activated as a result of RTK activation?

A
  • Ras GTPase
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19
Q

Explain how Ras is activated as a result of RTK activation

A
  1. Grb-2 adapator protein binds to one of the phophorylated tyrosine kinases on the activatated RTK
  2. This allows Ras GEF (Guanine exchange factor) to bind to Grb-2 and position itself next to the membrane where inactive Ras is present
  3. Ras GEF is then able to catalyse exchange of GDP for GTP within Ras causing inactive Ras to become activated
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20
Q

Describe the phosphorylation cascade that occurs as a result of the activation of Ras

A
  1. Activated Ras phosphorylates and activates MAP-Kinase-kinase-kinase
  2. MAP-Kinase-kinase-kinase phosphorylates and activates MAP-kinase-kinase
  3. MAP-Kinase-kinase phosphorylates and activates MAP-Kinase
  4. MAP-Kinase phosphorylates and activates its target proteins e.g. transcription factors
  5. This leads to cell proliferation/cell fate determination
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21
Q

Name some of the other proteins apart from Grb-2 adaptor protein that can bind to activated RTKs as part of other cell signalling pathways

A
  • Phospholipase C
  • PI3K (phosphoinostide 3-kinase)
22
Q

What is the main difference between a receptor tyrosine kinase and a tyrosine kinase-associated receptor?

A
  • A receptor tyrosine kinase itself has inherent tyrosine kinase activity while a tyrsone kinase-associated receptor instead is bound to other kinases that have tyrosine kinase activity.
23
Q

Serine-threonine kinases are also involved in cell signalling. Give some examples of proteins that are serine-threonine kinases

A
  • TGF-β superfamily
    • TGF
    • BMP
    • Activin
24
Q

Explain how Serine threonine kianses are activated and the casacde that occurs as a result of their activation

A
  1. ​​​Ligand dimer, e.g. BMP, binds to binding site on serine-threonine kianse receptor (BMP receptor in case of BMP)
  2. This causes the type 2 serine-threonine kinase to phophorylate and activate the type 1 serine-threonine kinase
  3. Activated type 1 serine-threonine kinase phoshorylates an intracellular smad protein e.g. Smad 1,5 or 8
  4. Phosphorylated Smad protein binds to another Smad protein e.g. Smad 4, to form a transcriptional regulatory compex
  5. Transcriptional regulatory compex enters the nucleus and can activate or rpress target genes
25
Q

Explain how the Hedgehog pathway works in invertebrates in the absence of Hedgehog (Hh)

A
  1. Absence of Hh causes membrane protein pathced (Ptc) to inhibit membrane protein smoothered (Smo) and cause its degradation
  2. This resulst in Cubitus interruptus (Ci) being sequestered and held in the cytoplasm by 2 proteins: Cos2 and Su(fu)
  3. This results in several kinases being recruited, e.g. PKA, GSK-3 and CK1 which all phosphorylate the Ci associated with Cos2
  4. This results in the ubiquitylation of Ci by Slimb
  5. This causes the activation domain of Ci to be targeted for degradtion and forms the Ci repressor (CiRep)
  6. CiRep enters nucleus and represses hedghog target gene
26
Q

Explain how the Hedgehog pathway works in invertebrates in the presence of Hedgehog (Hh)

A
  1. Hedgehog binds to patched (Ptc) which causes it to become inactive
  2. This prevents Smootherd (Smo) from being repressed
  3. Smo is phosphorylated is PKA, CK1 and Gprk2 causing it to become active
  4. Activated Smo recruits and dimerizes both Cos2 and Su(fu) which releases Ci
  5. Ci then travels to the nucleus where it induces the transcription of the hedgehog target gene
27
Q

What is the function of the hedgehog pathway in invertebrates?

A
  • In flys function of Hh pathway is to regulate cell proliferation and position of cells to form the wing
  • In vertebrates function of Hh pathway is to also regulate cell proliferation and cell position but this time it’s to form a limb
28
Q

State 3 differences in the way the Hedgehog pathway works between vertebrates and invertebrates

A
  • In vertebrates there are several genes that encode components of the Hh pathway including the Hh gene and Ci genes (Gli1-3 in vertebrates) however, in invertebrates there is only one copy of each of these genes
  • In vertebrates Su(fu) has a more important role in inactivating the Gli transcription factors compared to in invertebrates where this function is mainly performed by Cos2
  • In vertebrates Hh pathway occurs specifically in the primary cilium (PC) whereas in invertebrates the Hh pathway doesn’t occur in a specific location within the cell
29
Q

Explain what happens in the Canonical Wnt signalling pathway in the absence of Wnt

A
  1. β-catenin is bound by destruction complex. which is made up of protein kinases such as: Axin, GSK-3, APC and CK1γ
  2. These protein kinases phosphorylate β-catenin which targets it for ubiquitination and degradation by the proteosome
  3. This means there’s no β-catenin in the cytoplasm capable of moving into the nucleus
  4. This results in transcriptional co-repressors binding to TCF transcription factors
  5. This prevents the transcription of certain genes
30
Q

Explain what happens in the Canonical Wnt signalling pathway in the presence of Wnt

A
  1. Wnt binds to the Frizzled receptor which causes the co-receptor LRP5/6 to also bind forming a complex
  2. This causes the protein kinases CK1γ and GSK-3 to associate with the membrane complex
  3. CK1γ and GSK-3 phosphorylate the tail of LRP5/6
  4. Then Dishevelled and Axin bind to the intracellular domains of Frizzled and LRP5/6 which prevents the formation of the destruction complex
  5. This results in β-catenin accumulating in the cytoplasm and then moving into the nucleus
  6. Once in nucleus β-catenin induces transcription of specific genes.
31
Q

How exactly does β-catenin induce the transcription of specific genes in the nucleus?

A
  • In the nucleus TCF is usually bound to by Groucho
  • However, when β-catenin enters the nucleus it displaces Groucho and binds to TCF itself
  • This binding recruits lots of other proteins to TCF causing the formation of a protein complex
  • This complex is able to induce the transcription of specific genes
32
Q

What are the 2 main pathways that are collectively non as the Non-canonical Wnt signalling pathways?

A
  • PCP pathway
  • Ca2+ pathway
33
Q

How are Wnt levels modulated?

A
  • They’re modulated by Secreted Wnt antagonsists
  • 3 main Secreted Wnt antagonsists are:
    • Sfrp - Secreted frizzled related protein
    • Dkk1 - Dickkopf1
    • Wif-1 - Wnt inhibitory factor
34
Q

How does Sfrp prevent the Wnt pathway from occuring?

A
  • Sfrp has a very similar structure to the extracellular structure of Frizzled
  • This means it is able to bind to Wnt and take it away from Frizzled
  • Sfrp can also bind directly to Fizzled and prevent Wnt from binding
35
Q

How does Dkk1 prevent the Wnt pathway from occuring?

A
  • Dkk1 binds to the LRP5/6 co-receptor
  • This prevents LRP5/6 from forming a complex with Frizzled and Wnt meaning the pathway is stopped
36
Q

How does Wif prevent the Wnt pathway from occuring?

A
  • Wif can bind to Wnt and prevent it from binding to the Frizzled receptor
37
Q

EPH signalling is an example of cell-to-cell signalling and is mediated by EPH receptors. What type of receptors are EPH receptors?

A
  • Receptor tyrosibne kinases (RTKs)
38
Q

How are EPH receptors classified?

A
  • Classified as either type A or type B receptors depending on the type of Ephrin ligand they bind to
  • Type A receptors bind to Ephrin A ligands and Type B receptors bind to Ephrin B ligands
39
Q

What are Ephrin A and B ligands?

A
  • All Ephrin ligands are membrane-tethered ligands
  • Ephrin A ligands are attached to the membrane by lipid modification
  • Ephrin B ligands are transmembrane proteins
40
Q

Which Ephrin receptor is able to bind to both Ephrin A and B ligands?

A
  • EPHA4
41
Q

What are some of the functions of EPH signalling?

A
  • Cell segregation and positioning - leads to tissue boundary formation
  • Axon guidance
  • Topographic mapping
  • Vascular remodelling
  • Cell migration
  • Axon tracking
42
Q

EPH signalling can be bi-directional, what does this mean?

A
  • It means that an intracellular repsonse can occur in both cells expressing the Eph receptor (forward signalling) and cells expressing the Ephrin ligand (reverse signalling)
43
Q

Briefly explain the Eph forward signalling cascade

A
  • Typically involves activation of intrinsic tyrosine kinase activity
  • This results in tyrosine phosphorylation of intracellular effector proteins
  • These effector proteins regulate the activity of small Rho family GTPases such as Rac1 and RhoA
  • This in turn means regulation of cytoskeleton dynamics
44
Q

Briefly explain the Ephrin reverse signalling cascade

A
  • Ephrin B reverse signalling involves tyrosine phosphorylation of its intracellular domain
  • This leads to the recruitment of PDZ-domain containing proteins which leads to downstream signalling pathways being activated
  • Ephrin A reverse signalling involves interaction with co-receptors that can mediate either attraction or repulsion via unknown mechanisms
45
Q

What is the function of the Notch pathway?

A
  • Cells use Notch pathway to distinguish themselves as one of 2 types of cells from a homogenous group of cells
46
Q

Explain what happens in the Notch signalling pathway

A
  1. Ligand e.g. Delta from one cell will bind to Notch receptor on an adjacent cell
  2. This causes the extracellular domain of the Notch receptor be to be cleaved from the rest of the receptor (1st cleavage)
  3. This results in the intracellular domain of Notch being cleaved (2nd cleavage)
  4. Intracellular domain then travels to the nucleus and binds to CSL complex
  5. This causes the repressor on the CSL complex to dissociate
  6. Co-activator mastermind and other proteins then bind to CSL complex
  7. This allows target genes to be expressed
47
Q

Why does activation of the Notch signalling pathway allow for cells to differentiate themselves from a homogenous group of cells?

A
  • Because levels of Delta between cells are slightly different
  • This means cells with more Delta will activate more Notch and lead to the repression of Delta in adjacent cells
  • This will eventually lead to 2 types of cells being formed from the homogenous group:
    • Cells with high levels of Delta and low levels of Notch
    • Cells with high levels of Notch and no Delta
48
Q

What is the name of the process in which cells are able to differentiate into 2 types of cells from a homogenous group of cells?

A
  • Lateral inhibition
49
Q

In nuerogenesis when lateral inhibition occurs which type of cells become neurons?

A
  • Cells with high levels of Delta and low levels of Notch
50
Q

Complete the following table comparing FGF signalling and Notch signalling

A
51
Q

List 4 differences between Hedgehog signalling and EPH signalling

A

Year 3: Cell and Molecular Biology (5 decks)

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