CELL BIOLOGY - CLUTCH CH. 14 - CELL SIGNALINGlightcat-files.s3.amazonaws.com/packets/admin_cell-biology-16-clutc… · CONCEPT: OVERVIEW OF CELL SURFACE RECEPTORS ... Eicosanoids

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CELL BIOLOGY - CLUTCH

CH. 14 - CELL SIGNALING

CONCEPT: OVERVIEW OF CELL SURFACE RECEPTORS

● There are three types of cell surface _______________________________ □ Ion channel coupled receptors produce and respond to different electrical gradients stemming from ions

- Can also convert chemical signals into electrical signals (Ex: neuronal signaling)

□ G-protein coupled receptors activate G proteins in the cytosol

- G proteins then act on enzymes or ion channels to cause signaling cascades

□ Enzyme coupled receptors (protein kinase receptors) act as enzymes, usually in a receptor complex

- C-terminal domain contains enzymatic characteristics

EXAMPLE: Three types of cell surface receptors

● Commonalities exist between the three main receptors and their ___________________________ pathways □ Usually, these receptors are activated by binding to a ligand

- If the ligand remains for long time periods the cell responds by down-regulating (removing) the receptor

□ Protein kinases (add phosphates) and phosphatases (remove phosphates) are involved downstream of activation

□ Second messengers are molecules that act to sign ________________________ of the receptor/ligand binding

- Make up signal transduction pathways which is the collection of stepwise signaling events

- The final step to a signal transduction pathway is usually activation or inhibition of a transcription factor

Ion Channel Coupled Receptor

G Protein Coupled Receptor

Enzyme Coupled Receptor(Protein Kinase Receptor)



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EXAMPLE: Example of a signal transduction pathway



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PRACTICE: 1. Which of the following is not a type of cell surface receptors?

a. Ion Channel Receptors b. G-Protein Coupled Receptors c. Signal Coupled Receptors d. Enzyme Coupled Receptors

2. Which of the following receptors responds to an electrical gradient across a membrane? a. Ion Channel Receptors b. G-Protein Coupled Receptors c. Signal Coupled Receptors d. Enzyme Coupled Receptors



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CONCEPT: OVERVIEW OF EXTRACELLULAR SIGNALING MOLECULES

Types of extracellular signaling molecules

● There are many types of extracellular signaling molecules, with numerous different functions □ Examples of signaling molecules include:

□ Signaling molecules have a variety of functions

● Signaling molecules act over different ___________________________________ □ Endocrine molecules are transported through the circulatory system to distance cells

- Ex: Hormones

□ Paracrine molecules signal to cells in close proximity (local mediators)

- Ex: Neurotransmitters or growth factors

□ Autocrine molecules signal to themselves

EXAMPLE:

Signaling molecules types Steroid hormones and nuclear receptors

Gasses like NO Neurotransmitters

Peptide Hormones and Growth factors (Ex: insulin and endorphins)

Eicosanoids – lipids that bind cell surface receptors

Signaling molecules functions Relay signals in the cell

Act as scaffolds to bring signaling proteins together Transduce signals into a different form

Amply signal through signal cascades which produce large amounts of a small intracellular signaling molecule

Spread, anchor, or modulate other signals

Endocrine

Paracrine

Autocrine

Cells



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Cell Response to Signaling Molecules

● Receptor transmit signals in two main ways 1. Receptor transmits a signal from its cytoplasmic domain to a nearby enzyme

- Generates a second messenger that will continue to signal in the cell

2. Receptor transforms its cytoplasmic domain into a recruiting station for signaling proteins

● Cells must be able to respond to a ____________________________ of signaling molecules and pathways □ Different cell types respond differently to signaling molecules

- Ex: Acetylcholine decreases heart muscle contraction and stimulates skeletal muscle contraction

EXAMPLE: Two methods of receptor signaling

1. 2.

Signaling Enzyme



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PRACTICE: 1. Which of the following is not a classification of signaling molecules?

a. Autocrine b. Paracrine c. Endocrine d. Lyoncrine

2. True or False: Signaling molecules work only through signaling cascades. a. True b. False



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CONCEPT: G PROTEIN COUPLED RECEPTORS

Structure and Signaling

● G Protein Coupled Receptors are the largest family of cell surface receptors (700+) and signal through ________ proteins □ GPCRs are composed of a single polypeptide chain that snakes back and forth through the bilayer seven times

- Three extracellular loops that bind ligands

- Three intracellular loops that bind signaling proteins

□ The cytosolic side of the GPCR is bound to a G protein which acts as a molecular ______________________

- G protein is a trimeric protein (three subunits) activated upon GTP binding and inactivated with GDP

- When activated a G protein can couple ligand/receptor binding to other enzymes or ion channels

EXAMPLE: G protein activation through GPCRs

● Regulation of GPCR signaling involves regulating the G protein □ Signaling can be effected by proteins that effect GTP ______________________________

□ Desnsitation is a process that blocks active receptors from activating G proteins

- G protein coupled receptor kinase 2 (GRK2) bind to GPCRs and compete for binding

□ The GPCR can be regulated via receptor inactivation, sequesteriation, or down-regulation



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Common GPCR Signaling Pathways

● Cyclic AMP (cAMP) concentration is regulated by G protein _________________________________ □ cAMP is a signaling molecule present in every cell that has been studied

□ cAMP is kept at a low concentration in the cell

- Extracellular signals can cause a huge increase in intracellular concentration

□ cAMP is made by the enzyme adenylyl cyclase and degraded by phosphodiesterase

- G proteins can stimulate or inhibit adenylyl cyclase which therefore controls cAMP levels

EXAMPLE: cAMP signaling pathway

● Calcium is an important intracellular signaling molecule □ G proteins can trigger ______________________________ in cytosolic calcium concentration

□ Calcium effects many other enzymes and proteins

- CaM kinases are protein kinases that phosphorylate proteins that inhibit and activate gene transcription

□ Calmodulin is a protein which mediates animal cell responses to calcium

cAMPadenylyl cyclase

G protein



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EXAMPLE: Various methods of calcium signaling

● Inositol phospholipid signaling pathways are ____________________________________ by GPCRs □ G protein Gq activated adenylyl cyclase – which in this pathway cleaves a lipid called PIP2

□ This cleavage creates two molecules: IP3 and DAG which are both second messengers

- IP3: Binds to ER and opens calcium channels

- DAG: Acts in a variety of signaling pathways

□ Calmodulin is a protein which mediates animal cell responses to calcium

- Binds calcium, undergoes conformational change, binds to other proteins and influences their functions

EXAMPLE: Inositol phospholipid signaling pathway



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PRACTICE:

1. Which of the following is true regarding G-protein coupled receptors? a. It contains three transmembrane sections b. The G protein binds the intracellular side of the G protein coupled receptors c. There are four intracellular and extracellular loops d. G proteins respond only to calcium

2. True or False: cAMP levels are kept at high levels in the cell. a. True b. False



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3. Calmodulin is an important signaling molecule because it does what? a. Cleaves PIP2 into IP3 and DAG b. Creates cAMP c. Mediates the animal cell response to calcium d. Blocks GPCR activation



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CONCEPT: PROTEIN KINASE RECEPTORS

Structure and Activation

● Protein kinase receptors (enzyme coupled receptors) are transmembrane proteins that are activated via ligand binding □ There are two main _____________________:

- Receptor kinases are receptors that contain kinase activity on the cytosolic surface

- Include receptor tyrosine kinases (largest class) and receptor serine-threonine kinases

- Named based on the amino acids they phosphorylate

- Non-receptor kinases are kinases that only bind to a receptor when its bound to a ligand

EXAMPLE: Two types of receptors

● Activation of this pathway is triggered through receptor ____________________________ □ Ligand binding causes two receptor molecules to bind and form a dimer

- One receptor phosphorylates (activates) the other receptor’s kinase domains (transautophosphorylation)

□ Once phosphorylated, other intracellular signaling molecules and complex are recruited to cytosolic tails

- Adaptor proteins can be recruited to link signaling proteins together to form a signaling complex

- Docking proteins can be recruited to serve as docking sites for other proteins

- Transcription factors can be recruited for activation and subsequent transfer to the nucleus

- Other signaling enzymes can also be recruited

□ SH2 domain is a common amino acid domain on signaling molecules which bind phosphorylated ____________

1. Ligand Binding

2. Kinase BindingPP

Cytoplasm

Extracellular

Receptor Kinases Non-Receptor Kinases



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EXAMPLE: Example of intracellular signaling molecule recruitment

Inhibiting the Receptor Activation and Signaling

● Down regulation of signaling occurs in ___________________ ways □ Receptor mediated endocytosis internalizes the receptor so it can no longer signal from the plasma membrane

- 20-50% of the receptor is degraded, with the rest being eventually returned to the plasma membrane

□ Lysosomal degradation can destroy the receptor so it cannot signal

□ Phosphotyrosine phosphatases will remove phosphates from activated RTKs to inactivate them

□ SOCS proteins terminate signals from special receptors that bind to cytokine signaling molecules

EXAMPLE: Methods of receptor inhibition

PP

Activated

PP

Endosome

Degraded in lysosome

P

Receptor mediated endocytosis

PhosphotyrosinePhosphotases

1.

2

3.



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Common Receptor Protein Kinase Signaling Pathways

● Activation of Ras, which is a small GTP binding protein that acts as a major signaling _______________ □ Virtually all receptor tyrosine kinases (RTKs) activate Ras

- Ras is mutated in around 30% of all cancers

□ Ras cycles between an active (GTP-bound) form an inactive (GDP-bound) form

- Autophosphorylation of the RTK GRB2 results in Ras activation (GDP to GTP)

□ When activated, it phosphorylates and activates a _______________ of serine threonine protein kinases

- MAP kinase signaling pathway:

- MAP kinase kinase kinase is phosphorylated by Ras à MAP kinase kinase à MAP kinase

- Map kinase kinase kinase can phosphorylate nuclear proteins which regulate gene expression (Ex: Jun)

EXAMPLE: Ras MAPK Signaling Pathway

● RTKs also activate phosphoinositide 3-kinase by phosphorylating inositol phospholipids in plasma membrane □ Phosphorylation serves as a docking site for other signaling proteins

- Protekin kinase B (Akt) – inhibits the Bad protein, which prevents apoptosis to promote survival

- Phospholipiase C – results in formation of IP3 and DAG

GTPRas

Gene Expression

MAP3K

MAP2K

MAPK



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EXAMPLE: PI3KàAKTàBAD Pathway

● Transforming growth factor beta (TGFb) is activated by serine-threonine ________________

□ TGF binds to the TGF receptor resulting in dimerization of two types of serine-threnoine kinases

- Type II phosphorylated Type I

- Type I initiates signal transduction cascade

□ Results in phosphorylation and activation of SMADs (transcription factors)

EXAMPLE: TGF beta signaling pathway

Bad

Promotes Survival Prevents Apoptosis

Akt PI3K

RTK

Extracellular Space

Cytoplasm

Nucleus

Gene Expression

SMADs

TGF beta



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Non-Receptor Protein Kinase Signaling Pathway (JAK/STAT)

● The JAK/STAT signaling pathway is a great example of non-receptor tyrosine kinases □ Cytokine signaling molecules bind to a __________________________ on the plasma membrane

- Recruits the Janus kinase (JAK) to the receptor and activates it

- JAK then recruits, phosphorylates, and activates STATS a family of transcription factors

- Once phosphorylated STAT disassociates from JAK and can travel to nucleus

□ There are four JAKs and six STATS which each regulate different signaling pathways

EXAMPLE: JAK/STAT Pathway



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PRACTICE: 1. Which of the following is not a common example of protein kinase signaling cascades?

a. TGFb b. Inositol Phospholipid pathway c. MAP Kinase pathway d. GTP signaling

2. Ligand binding to a receptor kinase causes what to happen? a. Immediate activation of the single receptor kinase b. Dimerization and inactivation of two receptor kinases c. Dimerization and activation of two receptor kinases d. Binding of the kinase to the receptor



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3. Which of the following is not a way to inhibit receptor activation? a. Phosphatases removing phosphates from the receptor b. Receptor mediated endocytosis c. Lysosomal Degradation d. Autophosphorylation

4. Which pathway can activate the MAP kinase pathway? a. TGFb b. Inositol Phospholipid pathway c. Ras signaling d. JAK/STAT



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CONCEPT: INTRACELLULAR MESSENGERS: HORMONES AND NITRIC OXIDE

● Hormones cross the plasma membrane and bind to intracellular _________________________ □ Steroid hormones bind to nuclear receptors (in cytosol or nucleus) which act as transcriptional regulators

- If the nuclear receptor is in the cytosol, upon hormone binding it is transferred into the nucleus

□ Hormones activate receptors bind triggering a conformational change

□ Each hormone binds to a ___________________ nuclear receptor

- Each nuclear receptor recognizes different DNA regulatory sites

EXAMPLE: Nuclear receptor activation via a hormone



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● Nitric oxide (NO) can dissolve across the plasma membrane and bind to intracellular signaling _____________ □ NO regulates a variety of different pathways

- NO can be released into blood vessels in response to neurotransmitters to cause muscle relaxation

- NO binds guanyl cyclase to stimulate the formation of cylic GMP from GTP (ex: viagara)

EXAMPLE: NO signaling in cells

cGMP

GTP

guanyl cyclase

NO



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PRACTICE: 1. Steroid hormones signaling through binding….

a. GTPases b. Nuclear receptors c. Kinases d. Phosphatases

2. Nitric Oxide signals by binding to which of the following molecules? a. GTPases b. Nuclear Receptors c. Adenyl cyclase d. Guanyl cyclase



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CONCEPT: PHOSPHOINOSITIDE SIGNALING PATHWAY

● The phosphoinositide signaling pathway (inositol phospholipid pathway) can be _________________ by GPCRs or RTKs □ GPCRs activate Gq which activates the enzyme phospholipase C

- Phospholipase C cleaves an inositol phospholipid from the cytosolic side

- Cleavage generates 1,4,5 triphosphate (IP3) and diacylglycerol (DAG)

- IP3 binds and opens ER calcium channels causing calcium to rush into cytoplasm

- DAG recruits and activates protein kinase C which activates other proteins when Ca2+ is present

□ Calcium surges into the cytoplasm trigger a lot of signaling events

- Ex: Egg development, muscle contraction, nerve cell secretion

EXAMPLE: Signaling pathway of phospholipase C

□ RTKs and cytokine receptors can also activate phospholipase C for a __________________ purpose

- Phospholipase C recruit’s phosphatidylinositol-3 (PI-3) kinase to the membrane

- Phosphatidylinositol-3 (PI-3) kinase adds phosphates onto the phosphatidylinositol lipid

- Can trigger cell division and prevent cell death

- Also activates Akt which can regulate a variety of other proteins

- PTEN phosphatase removes the phosphate groups from PI-3 kinase to inactivate it



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EXAMPLE: PI3K signaling pathway



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PRACTICE: 1. True or False: The inositol signaling pathway can be activated by both GPCRs and RTKs.

a. True b. False

2. Which of the following molecules removes phosphates from PI-3 kinase? a. Phospholipase C b. Phosphatidylinositol-3 kinase c. PTEN Phosphatase d. Protein Kinase C



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CONCEPT: INTEGRATION OF MULTIPLE SIGNALING PATHWAYS

● Signaling pathways do not work independently of other signaling pathways and are not ________________ pathways □ Signaling networks are pathways that are connected via crosstalk between individual signaling molecules

- Numerous extracellular signals

- Protein kinase overlapping in multiple pathways

- Cross-talk between second messenger

□ There can be positive and negative interactions

- Feedback loops – when the end product mediates activity of an early product

- Feedfoward relays – activity of one component stimulates a distant downstream component

□ The signaling networks in the cell are ________________________

- 1500 receptors, 700 kinases/phosphatases, and 2000 transcription factors

EXAMPLE: Complexity of signaling networks



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Insulin Signaling ● Insulin and glucagon work together to maintain stable blood ____________________ level in the cells □ Two hormones: insulin and glucagon

□ After a meal – there is lots of glucose in the blood stream

- Triggers the cell to create and release insulin

- Insulin binds to insulin receptors

□ Insulin binding triggers a variety of pathways including:

- Protein kinase B phosphorylation – triggers intracellular vesicle fusion, and import of glucose into the cell

□ When blood glucose levels drop – insulin receptors stop being activated

- triggers the cells to increase secretion of glucagon

- Glucagon binds its receptors and stimulates a variety of other signaling pathways

EXAMPLE: Variety of insulin signaling pathways

Insulin

ReceptorExtracellular

IntracellularTyrosine Kinase

PI3-K pathway

Lipid synthesis

cell survival

MAP kinase

Cell Growth



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PRACTICE: 1. True or False: A signaling network is usually linear, with little interaction with other signaling pathways.

a. True b. False

2. When insulin binds to insulin receptors what happens to glucose? a. Glucose is brought into the cell b. Glucose is created by the cell c. Glucose is exported into the bloodstream d. Glucose is degraded



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CONCEPT: SIGNALING IN PLANTS

● Signaling evolved ___________________________ in plants and animals □ Plants have receptor kinases but their largest class are serine/threonine kinases

□ No plant homologs exist for important signaling molecules like JAK/STAT, Notch, Wnt, Hedgehog

● There are six classes of plant signaling molecules

Signaling Molecule Function Auxins Stimulate plant growth

Gibberelins Stimulate stem elongation Cytokinins Stimulates cell division

Absicsic acid Stimulates cell dormancy Ethylene Stimulates fruit ripening

Phytochromes Sense light and signal for some function

PRACTICE:

1. True or False: Plant and Animal signaling evolved differently. a. True b. False



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2. Which signaling molecule stimulates fruit ripening? a. Auxins b. Ethylene c. Phytochromes d. Gibberelins



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CELL BIOLOGY - CLUTCH CH. 14 - CELL SIGNALINGlightcat-files.s3.amazonaws.com/packets/admin_cell-biology-16-clutc… · CONCEPT: OVERVIEW OF CELL SURFACE RECEPTORS ... Eicosanoids