Meeting 19, 20

  1. Endocrine vs. Developmental Signaling
    Hormonal Signaling: control homeostasis in an already developed organism

    Developmental Signaling: pattern formation is achieved by inductive signals that change the fate of cells or tissues
  2. Spemann Organizer
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    signaling center in the most dorsal part of the mesoderm cells in the early frog gastrula; when transplanted in a host embryo, it takes over part of the host embryo and causesthe formation of a nearly conplete new neural & mesodermal axis

    -Spemann & Mangold showed that the Dorsal lip functions as an organizer of the embryo, and can even induces the formation of a second embryo when transplanted
  3. Ligand
    • an ion, molecule, or a molecular group that binds to
    • another chemical entity to form a larger complex; any molecule, other than an enzyme substrate, that binds tightly and specifically to a macromolecule (usually a protein) forming a macromolecule-ligand complex

    *have to bind tightly enough to cause a change but not tightly enough so it never leaves
  4. Binding specificity vs. effector specificity
    R + L <---> RL (Receptor + Ligand <--> RL)

    KA = [RL] / [R][L]

    KD = [R][L] / [RL]

    [RL]/Rt = 1/(1 + KD/[L])

    -drugs v. ligands: can change ligand….replace it with a drug possibily, and prevent normal binding there; alter funcitons of receptors
  5. Types of Signaling
    • • secretion signaling: produces signals (ex. like those made by dorsal lip) that can travel long distances & act on recipient (ex. pigmented) tissue
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    • • direct cell-cell signaling: when tight signaling is needed; molecules move very short range, ex. only talks to neighbor & doesn’t spread
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  6. Signaling by secreted molecules
    (a) Paracrine: secretory cell releases extracellular signal, which binds to receptor on adjacent target cell, induces change

    *Autocrine: target cell is the same as secretory cell; cell secretes extracellular signal, which binds to receptors on ITS p.membrane

    (b) Synaptic: nerve cell depolarizes, releases NT into synapse, causes target cell to either depolarize or hyperpolarize, etc. etc.

    (c) Endocrine: endocrine gland secretes hormone molecule, which travels via blood stream to distant target cell (binds to rcptr, activates or represses, blah blah blah)
  7. types of changes that can be induced by secretion of an extracellular signal molecule:
    proteins target by signlaing pathways range from:

    • (1) metabolic enzymes: alters the target cell's metabolism
    • (2) gene regulatory proteins: like a Tx factor; can enter nucleus and change cell's gene expression
    • (3) cytoskeletal proteins: can change cell shape or movement

    -cell can:

    • differentiate

    • • divide: signal binds to receptor tells the cell to produce cyclin/CDK to initiate mitosis (cell division)
    • * this happens before differentiation; often cells cannot divide once differentiated

    • surivive

    • • DIE
    • - caspase: protease important for apoptosis b/c it degrades protein & DNA
  8. Mechanism of Signal Transduction: Types of Receptors & Activation
    • - Receptor can be:
    • • Ion channel-linked (ex. acetylcholine or NMDA rcptr; ligand binding opens channel)
    • • Enzyme-linked: triggering a phosphorylation cascade
    • • G-protein-linked

    • - Activation can involve:
    • • Proteolytic processing (ex. Notch: binds to ligand, cuts off the protein inside the cell, protein enters nucleus & activates transcription)
    • • Oligomerization (ex. RTKs [receptor tyrosine kinases]; ligand binds to receptor, attracts another ligand, forming a dimer --> causes activation)
    • • Conformational change (ex. GPCR)
  9. Enzyme-linked Receptors
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    • example of enzyme-linked receptor: tyrosine kinase
    • •adds a phosphate to a tyrosine (*kinases phosphorylate)
    • • enzymes here can phosphorylate molecules, ex. serine, tyrosine etc.
    • •ligand can bind to 1 receptor, bringing two together cause causing a chain of activation
    • • OR binding recruits a protein
  10. guess the 2nd messenger:
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    • cAMP
    • cyclic AMP

    • activates protein kinase A (PKA)
  11. guess the 2nd messenger:
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    • cGMP (cyclic GMP)
    • • activates protein kinase G (PKG) & opens cation channels in rod cells
  12. who am I?
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    • DAG (diacylglycerol)
    • • activates protein kinase C (PKC) by FIRST activating phospholipase C (PLC)
    • -PLC is the linker between DAG and PKC
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    • IP3 (Inositol-triphosphate)
    • • opens Ca2+ channels in endoplasmic reticulim

    • *relevant here: Ca2+ ions also act as 2nd messengers
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    -often times there is a molecule (or channel) that activates the 2nd messenger, so it can activate it's target molecule

    • -cAMP
    • • linking enzymes (2)
    • (1) adenyl cyclase: makes cAMP from ATP
    • (2) phosphodiesterase (enzyme): destroys cAMP

    • -cGMP
    • • linking enzyme (1)
    • (1) guanylyl cyclase: makes cGMP from GTP

    • -Ca2+
    • • linking enzyme (1)
    • (1) Ca2+ channel
  15. PLC (phospholipase C)
    PLC is an enzyme that cleaves phospholipids just before their phosphate group

    • one such phospholipids, PIP2 is cleaved by PLC into DAG & IP3

    • • DAG stays bound to the membrane
    • -Ca2+ & DAG together activate PKC (protein kinase C)
    • -PKC phosphorylates molecules, leading to changes in cell activity

    • • IP3 is released into the cytosol
    • - it diffuses through the cytosol & binds to IP3-receptors, (ex. calcium channels in the endoplasmic reticulum)
    • - this causes increase in cytosolic concentration of Ca2+, causing changes in cell activity
  16. Nuclear Receptors (ex. Steroid hormones) As Developmental Signals
    • • retinoic acid, thyroid hormones and Vitamin D
    • are liposoluble steroid hormones that work to maintain homeostasis
    • • they're secreted/transported by carriers proteins
    • • the (small, hydrophobic) signal enters the cell where it binds to a “nuclear receptor” (ex. RAR)
    • • the steroid-receptor complex --> moved into the nucleus, where it activates transcription of genes containing response element (RARE)
  17. Can you memorize the shape of these steroid hormones?
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    • • big rings are hydrophobic & allow transport through membrane
    • • all have some charged/hydrophilic activity that allows transport through the membrane (so they don't get stuck there, ex. hydroxyl group, OH-)
  18. how can ligand binding induce activation of the receptor?
    one way = GPCR (G-protein coupled receptor); mostly a conformational change in response to binding of a ligand or when light activates a receptor
  19. Oligomerization (ex. receptor tyrosine kinases (RTKs)
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    • • activation of a receptor can just occur by dimerization
    • • the binding of 2 ligands to 2 seperate receptors can sometimes cause the complexes to dimerize

    • mutation
    • • a constitutively active receptor; this can occur when a ligand binding domain is replaced w/ a dimerization binding domain
    • -this is the kind of mutation that exists in life & can cause oncogenes
  20. Conformational Change (I don't know)
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    • depending on the membrane potential or ligand channel, K+ channel will open or close

    • •this is a 7 g-protein coupled receptor; when ligand binds (red +) changes conformation and causes activation of the G-protein
    • -this can be done simply using light
  21. G Protein-Coupled Receptors
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    • • all GPCRs have the same orientation in the membrane and contain:
    • -7 transmembrane α-helical regions (H1-H7)
    • -4 extracellular segments (E1-E4)
    • -4 cytosolic segments (C1-C4)
    • -C4 (the carboxyl-terminal segment) & the C3 loop (also sometimes the C2 loop) interact w/ G-proteins

    • •6-7% of genome is made of GPCR coding genes
    • •they're the largest family of cell surface receptors, & largest family of protein in our genome
    • -ex. GPCRs code for olfactory genes/olfaction (how you smell)
    • •all domains use the same mechanisms to talk to the G-protein
    • •they respond to a wide variety of mediators (hormones, NTs, local mediators, epinephrine, serotonin)
  22. G-Proteins
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    • • G-proteins are trimeric, & made of α, β, and γ subunits
    • -α & βγ subunits (respectively) are attached to the membrane via covalently bound lipid molecules
    • -α subunit: binds & hydrolyzes GTP; codes for GTPase (protein that hydrolyzes, deactivates, GTP)
    • -βγ subunit: holds onto the α subunit, brings it to the membrane

    • • G-protein is:
    • -Inactive when bound to a GDP
    • -Activate when bound to GTP
    • -in active form (GTP bound to α subunit) --> it to dissociate from βγ subunit

    •peak of activation is immediately followed by a drop in activation
  23. Mechanism of the Activation of Effector Proteins Associated w/ GPCRs
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    • (1) binding of a ligand to a GPCR induces a conformational change in the receptor, activating it
    • (2) the activated receptor (+ bound ligand) binds to the Gα subunit
    • (3) this causes a conformational change in the Gα subunit, making it release GDP
    • (4) now GTP binds to Gα, dissociating it from both the receptor & Gβγ
    • (5) the free Gα+GTP now binds to & activates an effector protein (enzyme in the figure)
    • -in other news, the ligand bound to the receptor is released when Gα dissociates
    • (6) hydrolysis of GTP ends signaling & leads to reassembly of G-protein subunits, returning the system to its resting state
    • -binding of another ligand molecule causes the cycle to repeat
  24. FUN FACT!
    Gβ is the only subunit of the G-protein NOT attached to membrane via covalent interactions w/ lipid molecules! how cool! remains in place based on association w/ γ subunit
  25. rhodopsin
    • is a photoreceptor: molecule that can detect light; type of GPCR
    • -collects energyof the photon and uses it to change conformation of the receptor
    • -its ligand is LIGHT
  26. When G-protein target is Adenylyl Cyclase
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    • • uses ATP to make cAMP; this is done b/c G-protein signaling pathway activates adenylyl cyclase

    • • cAMP then activates PKA
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    • • PKA (protein kinase A) is a serine kinase; cAMP binds to its regulatory subunit & releases active PKA
    • • PKA can then go into nucleus & phosphorylate transcription factors (ex: CREB)
    • • CREB can only bind to DNA when phosphorylated; activates or represses DNA once bound
  27. PLC, IP3 & DAG Pathway in a little more detail
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    • • in this situation, the effector protein is PLC
    • • everything in the pathway is the same: ligand binds to receptor, activates G-protein, Gα dissociates, & activates PLC
    • (1) once activated, PLC cleaves PIP2 (phosphatidyl inositol) ester bond, yielding IP3 and DAG; these 2 molecules induce different pathways
    • (2) IP3 diffuses through cytosol and binds to receptors on ER, opening Ca2+ channels
    • (3) Ca is released into cytosol
    • (4) one of several responses to an increase in cytosolic Ca is recruitment of PKC to the membrane, where it is activated by DAG (DAG activates a kinase)
    • (5) the kinase (C) can now alter the activity of numerous enzymes, altering activity in the cell
  28. Human Rod Cell & Rhodopsin (type of GPCR)
    • • two types of photoreceptors, rods & cones:
    • -rods: allow detection of light & are very sensitive
    • -cones: detect color; humans have 3 types b/c we are trichromads (red, green, & blue)
    • • in the human rod cell, rhodpsin, a light-sensitive GPCR is located in the flattened membrane disks of outer segment
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    • • opsin has no ligand binding (light, the activator, cannot be 'bound')
    • -it's already pre-bound to a 'ligand': 11-cis-retinal
    • -when exposed to light, it changes conformation and becomes active or deactivated
    • • in the dark: cis isomer; receptor is inactive, ion channels are open
    • • in the light: trans isomer; receptor is active, ion channels are closed

    • Reviewing the Opsin GPCR when exposed to light (b/c that's when G-protein is activated):
    • (1) when opsin is exposed to light, its convered from inactive to active state (all the following is the SAME, binds to inactive GDP bound G-protein, mediates release of GDP & binding of GTP)
    • (2) active GTP*Gα binds to PDE, activating it
    • -PDE (phosphodiesterase) hydrolyzes cGMP to just GMP
    • (3) active PDE turns cGMP to GMP
    • (4) the resulting decrease of cytosolic cGMP causes it to dissociate from the NT gated channels in the membrane
    • -the channels CLOSE
    • (5) membrane becomes 'permanently' hyperpolarized for a lil

    - light causes hyperpolarization -
  29. How to Deactivate Opsin
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    • -activated opsin can be a target for phosphorylation by GRK (G-protein-linked receptor kinase)
    • -once tagged with phosphate, this is a signal for arrestin to come bind
    • -arrestin binds to opsin & sticks to it, preventing it from subsequently activating a G-protein
  30. Overview
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Card Set
Meeting 19, 20
2nd Messenger/Nuc Rcptrs, G-proteins