Chapter 27 Signaling Through TGF-Beta Receptors, Guanylyl Cyclases, and Ion Channels Flashcards
1
Q
Receptor Serine/Threonine Kinases (TGFβ Receptor Family)
A
- These are the receptors for a related family of about 33 ligands in humans including TGFbs (transforming growth factor βs), BMPs (bone morphogenetic proteins), anti-Mullerian hormone (AMH, MIS), inhibins, and the activins.
- During development, these proteins regulate pattern formation, influence proliferation, differentiation, tissue remodeling, and cell death
- In adults, they are involved in cell homeostasis, tissue repair and remodeling, and immune regulation
2
Q
Transforming Growth Factor Betas
A
- Members of the TGFβ Subgroup of the TGFβ Superfamily
- Inhibition of cell proliferation
3
Q
Bone Morphogenetic Proteins (BMPs)
A
- Members of the BMP subgroup of the TGFβ superfamily
- Osteogenesis
- Chondrogenesis
4
Q
Processing of signaling molecules in TGFβ superfamily
A
- TGFβ superfamily members are synthesized in a precursor form
- Undergoes dimerization, cleavage, and secretion
- Once secreted, TGFβ superfamily ligands that remain associated with the pro-region and/or other interacting proteins are biologicaly inactive
- Undergoes activation by proteases
- A variety of diverse mechanisms (most poorly understood) result in production of an active form TGFβ superfamily ligand
- Mature TGFβ is 112 amino acids long
- Signaling is typicallly paracrine, can be autocrine, particularly in cance
5
Q
Ligands in TGFβ Superfamily
A
- Can heterodimerize making interpreting signaling quite difficult
- Ligands are procwessed from a longer precursor to a mature peptide
- May dimerize into homodimersor heterodimers
- Many of the peptides have similar functions and can compensate for each other, making understanding function difficult
6
Q
Signaling Through TGFβ Superfamily Requires 2 Types of Receptors
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- Type I Receptors
- Type II Receptros
7
Q
Type I Receptors
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Phosphorylate R-Smads
8
Q
Type II Receptors
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Bind the ligand, phosphorylate Type I receptors
9
Q
The TGFb/Smad Signaling Pathway
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- Receptors for the TGFb superfamily of ligands are composed of two different subunits, i.e. they are heterodimeric. Actually, there are 2 Type I receptors and 2 Type II receptors. Thus, the receptor is a tetramer.
- The type II receptor dimer binds ligand first and then forms a complex with the type II receptor dimer.
- Both receptor subunits possess serine/threonine protein kinase activity.
- After ligand binding, the type II subunit phosphorylates a specific site on the type I subunit to activate its kinase activity
- The type I subunit then phosphorylates latent transcription factors known as Smads.
10
Q
The TGFb/Smad Pathway
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11
Q
TGFβ
A
- Originally described as a tumor-promoting protein (transforming growth factor β).
- However, normally, it is a tumor suppressor and inhibits cell proliferation.
- As tumors progress to metastasis, they become TGFb-resistant.
- Eventually in tumors, TGFb becomes oncogenic, which is why it was called ‘transforming’ growth factor.
- So the consequences of TGFβ signaling are dependent on the cell context
12
Q
Even in the absence of mutations, TGFβ switches from…
A
tumor suppressor to oncogenic protein as carcinomas progress
13
Q
The Function of TGFβ as a
Tumor Suppressor Protein Can Be Altered in Two Major Ways
A
- Loss-of-function mutations in the core components of the pathway disable the tumor suppressor effects
- Mutated receptors: Ovarian, esophageal, head & neck, GI, colon, stomach, lung cancers
- Mutated SMAD4: Pancreas, colon, esophageal cancers
- Downstream alterations that usurp the normal functions of the pathway to cause growth promotion, i.e. this is what happens when TGFβ changes from a tumor suppressor protein to a tumor promoter protein
14
Q
Because TGFβ Affects so Many Functions, It Can Become Tumor Promoting as a Cancer Progresses
A
15
Q
The Receptor Guanylyl Cyclases: Atrial Natriuretic Peptide (ANP) Receptors
A
- ANPs are hormones secreted primarily by the heart in response to high blood pressure
- ANPs primarily act by relaxing smooth muscle cells in blood vessels and stimulating excretion of Na+ and H2O. They regulate salt and water balance and thus blood pressure
- Natriuresis is the excretion of sodium in the urine via action of the kidneys, and usually refers to the excess excretion of sodium (diuretic, anti-diuretic)
- The ANP receptors have guanylyl cyclase activity in their cytosolic domains
16
Q
The ANPs
A
ANPs are synthesized as precursor proteins that are later proteolytically processed to the final active peptide.
17
Q
ANP
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atrial natiuretic peptide
18
Q
BNP
A
brain natiuretic peptide
19
Q
CNP
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cardiac natiuretic peptide
20
Q
The Guanylyl Cyclase Receptor Family
A
21
Q
Receptor Guanylyl Cyclases Make cGMP, Which Activates PKG
A
22
Q
cGMP Functions as a Second Messenger in a Manner Similar to cAMP
A
- Most of the actions of cGMP are mediated by binding to PKG (cGMP-dependent protein kinase)
- PKG has both the regulatory and catalytic functions in a single protein
- cGMP is quickly hydrolyzed to 5’ GMP by cyclic nucleotide phosphodiesterases (PDEs), as was cAMP, which helps turn the signaling pathway off
23
Q
The Membrane Guanylyl Cyclase Receptor Signaling Pathway
A
- One target protein of PKG is the endoplasmic reticulum IP3-gated Ca2+ channel, which is inhibited by phosphorylation.
- Consequently, less Ca2+ is released from the ER and less smooth muscle contraction occurs when PKG is active. Therefore, there is smooth muscle relaxation.
24
Q
Effects of Inactivating Mutations in NPRs
A
- Mutations in NPRA
- Hypertension and heart failure
- Mutations in NPRB
- Short-limbed dwarfism known as Maroteaux syndrome, which demonstrates its primary role in bone
25
Q
Nitric Oxide (NO)
A
- NO is a highly reactive and toxic, free radical gas with a half life of ~5 seconds.
- It functions as a signal molecule in biological systems when at low concentrations (pM = 10-12 M)
- It functions as a killer at higher concentrations (nM = 10-9 M)
- NO is synthesized in cells by a family of enzymes - nitric oxide synthases (NOS) - which use arginine as the N donor and O2 to oxidize N to NO
- Discovery of NO as a biological signal molecule came from studies on how acetylcholine acts to relax smooth muscle cells surrounding blood vessels (vasodilation)
26
Q
Deamination of Arginine to Citrulline by NOS Releases NO
A
27
Q
A Family of Nitric Oxide Synthases Make NO
A
- Neuronal constitutive NOS (nNOS)
- Endothelial constitutive NOS (eNOS)
- Induced NOS (iNOS)
28
Q
Neuronal constitutive NOS (nNOS)
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- Signaling in the nervous system
- Enzyme activity is regulated.
29
Q
Endothelial constitutive NOS (eNOS)
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- Signaling in the vascular system
- Enzyme activity is regulated
30
Q
Induced NOS (iNOS)
A
- Killing of microbes, viruses (and surrounding cells)
- Gene transcription is regulated (induced).
31
Q
Regulation of NOS Activity
A
- nNOS and eNOS are activated by the binding of Ca2+-CaM.
- Ca2+ is released by neurotransmitter action, sheer stress, or pressure and binds to CaM (calmodulin). Both nNOs and eNOS release small pulses of NO (pM).
- iNOS transcription is induced in phagocytes (macrophages, monocytes, and neutrophils) primarily by invading microbes or inflammation
- Which elicit the release of interferon-γ or TNFα. As a result, the phagocytes make large amounts of NO (nM).
32
Q
Pathway of NO Signaling in Vasodilation
A
- Acetylcholine stimulates blood vessel dilation by binding to a GPCR (a muscarinic acetylcholine receptor) on the surface of endothelial cells. This activates Gq.
- Ca2+/calmodulin binds to eNOS to activate it, leading to synthesis of NO.
- NO acts as a paracrine mediator to influence nearby cells
- Binding of NO to intracellular (soluble) guanylyl cyclase activates the guanylyl cyclase, which then makes cGMP. cGMP activates protein kinase G (PKG).
33
Q
Smooth Muscle Relaxation by NO
A
34
Q
Pathway of NO Signaling in Vasodilation
A
- NO is small and both hydrophobic and hydrophilic so that it freely diffuses across the membrane of endothelial cells and enters smooth muscle cells
- Inside smooth muscle cells, NO binds to its intracellular ‘receptor’, cytosolic (soluble) guanylyl cyclase, by covalently binding to the Fe of the heme group in the guanylyl cyclase
- This increases activity of the guanylyl cyclase and increases formation of cGMP in smooth muscle
- Increased cGMP works by activating PKG
35
Q
NO Covalently Binds to Fe in…
A
- Cytosolic (Soluble) Guanylyl Cyclase
- This covalent binding is not readily reversible, so this differs from other signaling systems.
36
Q
Signaling via NO
A
37
Q
Signaling Through Soluble and Receptor Guanylyl Cyclases
A
38
Q
Killing by NO: iNOS Is induced in Response to Viruses and Microbes
A
39
Q
Physiological Effects of NO
A
40
Q
Pathological Effects of NO
A
41
Q
Membrane Channels Form Pores Across Membranes
A
- Channels are membrane proteins that form pores that let the passive movement of ions (ion channels), water (acquaporins), or other solutes to passively across the membrane along their electrochemical gradient.
- Two important properties distinguish ion channels from aqueous pores:
- They show ion selectivity, i.e. they only let a specific ion(s) pass
- Ion channels are not continuously open. Instead, they are gated (regulated), which allows them to open briefly and then close. In addition, with prolonged chemical stimulation they can go into a closed, desensitized state until the stimulus is removed.
42
Q
Ion Channels
A
- Ion channels are integral plasma membrane proteins that possess hydrophilic pores highly specific for the transport of inorganic ions. They are selective for specific ions.
- Transport by ion channels is always passive (in the direction of the concentration gradient).
- The permeating ions have to shed most of their water as they pass through a selectivity filter.
- Passage through an ion channel is regulated, i.e. they are not always open.
43
Q
Voltage-gated channels
A
Respond to a voltage change across the membrane
44
Q
Ligand-gated extracellular ligand channels
A
typically bind neurotransmitters
45
Q
Ligand-gated intracellular ligand channels
A
typically bind ions or nucleotides
46
Q
Mechanically-gated channels
A
are often linked to the cytoskeleton and respond to mechanical stress
47
Q
Signaling Across Synaptic Junctions Is Controlled by Ligand-Gated Channels
A
- In response to an action potential, secretory vesicles containing neurotransmitter fuse with the membrane and the neurotransmitter is released into the synapse.
- The released neurotransmitter binds to and opens the transmitter-gated channels on the postsynaptic target cell.
- The resultant ion flow alters the membrane potential of the target cell, thereby transmitting a signal from the excited nerve to the target
48
Q
The Acetylcholine Receptors at Neuromuscular Junctions Are Transmitter-Gated Channels
A
- Acetylcholine binds to 2 structurally and functionally distinct classes of receptors
- One class of receptors are GPCR and activate Gi or Gq. These are often referred to as “muscarinic acetylcholine receptors” because they bind the mushroom toxin muscarine.
- The second class of acetylcholine receptors are ion channels. The channel-forming class of acetylcholine receptors are referred to as “nicotinic acetylcholine receptors” because they bind nicotine.
49
Q
The Nicotinic Acetylcholine Receptor
A
- Has been studied extensively because it is readily accessible to study because it is at neuromuscular junctions, and there are ~20,000 receptors per μm2 with few other receptors in the same membrane.
- It is composed of five polypeptide subunits that associate to form a channel; each subunit contributes a single transmembrane a helix to form the channel.
- The channel opens when two molecules of acetylcholine bind cooperatively to sites on the extracellular surface of the receptor.
- This causes rearrangement of the a helices to open the pore and allow Na+ to move across the membrane.
- Acetylcholine is rapidly degraded in the synaptic junction and the channel closes within ~1 millisecond.
50
Q
Three Conformations of the Acetylcholine Receptor
A
- With acetylcholine bound, the receptor still flickers between open and closed states but has a 90% chance of being open.
- This continues until acetylcholine is degraded by acetylcholinesterase and lowers its concentration at the membrane
51
Q
Termination of Synaptic Signaling Requires Rapid Removal of Neurotransmitter
A
- Most commonly, neurotransmitters are taken up by specific transporters (reuptake transporters) in the presynaptic cell and can be reused.
- Reuptake transporters are often targets of drugs - e.g., the antidepressant Prozac inhibits serotonin uptake (hence, prolonging signaling).
- Many drugs act by binding to transmitter-gated channels: ex. curare blocks acetylcholine nicotinic receptors. Barbituates, tranquilizers (Valium), and sleeping pills such as Ambien bind GABA receptors and help open Cl- channels.
