Signaling Mechanisms Regulating Cell to Cell Communication Flashcards
Cells can both sense their environment and also influence the behavior/function of other cells by producing secreted
Signaling Proteins
This form of communication underlies organ/
tissue physiology and homeostasis in adults,
and is fundamental to
Embryonic Development
Include all of the proteins and components that transduce the signal to mediate its effect on the cell
The Signaling Pathway
What are the reversible ways that cells respond to signals?
Become motile or change shape
What are the irreversible was that cells respond to signals?
Divide, differentiate or remain undifferentiated, and die
Allow cells to sense their environment by detecting light, stress or pressure, and chemical signals in the environment
Receptor proteins
Which receptor proteins detect light?
Photoreceptors
Which receptor proteins detect stress or pressure?
Death receptors (TNFR) and mechanoreceptors
A molecule or protein that triggers a signal by binding to a receptor-like protein
Ligand
Typically in an inactive state until they receive a signal from the environment
Receptors
Once stimulated by ligand binding, receptors do not remain active forever. Some receptors are inactivated by specific mechanisms, while others are inactivated by
Self-inactivation
Governed by tertiary (3D) structure and non-covalent bonds between amino acid groups
Specificity of receptor/ligand interaction
Any molecule, protein or drug that occupies ligand binding sites and stimulates receptor activity (either
partially or fully)
Agonists
Any molecule, protein or drug that occupies ligand binding sites and exclude agonists but does not
stimulate receptor activity
Antagonists
Induce receptor activation
Agonists
Block receptor activation
Antagonists
Underlies cancer, neurological disorders, and metabolic disorders
Abnormal cell-cell signaling
About 50% of drugs act on
Receptors
The remaining 50% of drugs act primarily on
Enzymes
What are the four major classes of receptors?
- ) Ion channels
- ) Steroid hormone receptors
- ) Protein kinase receptors
- ) 7-alpha-helix receptors
Pore-forming proteins that allow the flow of ions across
membranes down an electrochemical gradient
Ion channels
Ion channels are present only on
Cell surface and intracellular organelles
Classified by the nature of their gating, the species of ions passing through, and the number of gates (pores)
Ion channels
What are five types of gating seen in ion channels?
- ) ligand
- ) voltage
- ) mechanical or thermal
- ) phospholipid
- ) lipid
Binding of ligand opens a channel to allow flow of a
specific ion across the membrane, or closes a channel to stop the flow
Ligand-gated channel
Ligand gated channels are the basis for
-i.e. where ligands are neurotransmitters
Nerve transmission & muscle contraction
GABA, glycine, serotonin, and nicotinic acetylcholine are examples of
Ligand neurotransmitters
Mutations in ion channel genes may cause either a loss or a gain of
Channel function
Cystic fibrosis is a recessive disease caused by
loss-of-function mutations in the cystic fibrosis
transmembrane conductance regulator (CFTR)
gene which is a
Chloride channel
Mutation in the CFTR gene leads to abnormal salt transport across epithelial cell membranes, resulting in thick mucus build-up in
Respiratory epithelial cells
The ligand for CFTR is
ATP
Typically associated with dominant inheritance of the disease
Gain-of-function mutations
Mutations in voltage-dependent sodium channels can result in defective inactivation and late Na+ currents in
Paramyotonia, cardiac arrhythmia, and epilepsy
A sodium channel blocker that blocks action potentials in nerves
-Found in pufferfish or Fugu
Tetrodoxin
Proteins that function by controlling gene expression
- hydrophobic and can cross the cell membrane
- derived from cholesterol
Steroid hormones
Steroid hormones bind to receptors located in the
Cytosol or nucleus
For estrogen, prior to ligand binding, the steroid hormone receptor is attached to a
Chaperone protein
Induces a conformational change in the
receptor that causes dissociation from the chaperone protein and exposes a nuclear import signal, allowing homodimerization and subsequent nuclear entry
Estrogen binding
Dimerized receptors bind to an estrogen response element (ERE, a DNA promoter) that activates
Activates gene transcription
Overexpressed in cancer cells in 70% of breast cancer cases
Estrogen receptors
Estrogen sensitive (ER+) breast cancer cells require signaling for uncontrolled cell division and
DNA replication
Involves the use of selective estrogen receptor (i.e., competitive) antagonists, such as tamoxifen
Endocrine therapy
In breast tissue, tamoxifen is metabolized into a
protein, hydroxytamoxifen (H-tam) that binds to the
ER and prevents binding of
Estrogen
Functions to repress rather than activate estrogen target genes via recruitment of transcriptional co-repressors
ER/H-tam complexes
A protein or protein domain with enzymatic
activity that transfers phosphate groups from
high energy donor molecules such as ATP to
specific target molecules.
Kinase
Are composed of three domains:
- An extracellular domain that binds the ligand
- A trans-membrane domain
- A cytoplasmic domain that has kinase activity or binds a protein-kinase protein
Protein kinase receptors
Active kinase receptors are typically
Dimers (or other forms of oligomer)
What are the two major subfamilies of protein kinase receptors?
-defined by the amino acid phosphorylated
- ) Tyrosine kinases
2. ) Serine/threonine kinases
One of the most well known receptor tyrosine kinases is the
Insulin receptor
Describe how the receptor kinase works?
- ) Ligand binds to extracellular domain of each receptor subunit causing dimerzation
- ) The cytosolic protein kinase domains phosphorylate and activate each other (often at multiple positions)
- ) Aditional cytosolic proteins are recruited to and bind the phosphorylated receptors
A reversible process that can regulate whether a receptor is “on” or “off”
Phorphorylation
The addition of a phosphate molecule to a polar R
group of an amino acid (like tyrosine) can turn a
hydrophobic portion of a protein into a
Hydrophilic one
This can give rise to a conformational change which can facilitate interaction with other parts of the protein or with other
Molecules
Contains an SH2-domain that recognizes tyr-P and binds to phosphorylated receptors
Grb
Binds to Grb and activates small G-proteins such as Ras
SoS (a GEF)
SoS binds to Grb and activates small G-proteins such as the prototypic monomeric small G-protein
Ras
Activate G-proteins by catalyzing the exchange of GDP (inactive) for GTP (active)
Guanine Nucleotide Exchange Factors (GEFs)
Promote inactivation of G-proteins by stimulating conversion of GTP (active) to GDP (inactive) via hydrolysis
GTPase activating proteins (GAPs)
Bind guanine nucleotides and act as a molecular switch during signaling
G-proteins
Have intrinsic GTPase activity
G-proteins
The GTPase activity of G-proteins is stimulated by
GAPs
The replacement of GDP by GTP is stimulated by
GEFs
Receptor bound SoS is
ras-GEF (ras-guanine exchange factor)
Regulate many aspects of cell function
Small G-proteins
The Ras protein family is actually a small family of proteins made up of
H-, K-, and N-Ras
Functions in receptor signaling and cell division
Ras family
Functions in traffic of membrane vesicles
Rab family
Functions in nucleus/cytoplasmic traffic
Ran family
Functions in the actin cytoskeleton
Rac family
Proteins like the Mitogen Activated Protein Kinae (MAPK) with ras-GTP cause a phosphorylation cascade that activates
-stimulates production of proteins involved in cell division
Transcription factors
Serves as a convergence point for many signals
Ras
Ras is one of the most frequently altered proteins in
Human Tumors
Oncogenic mutations in Ras turn all of its down stream pathways
On
Most oncogenic mutations of Ras are amino acid substitutions at just two positions. What are the two positions?
gly-12/gly-13 or gln-61
These mutations affect the structure of Ras and abolish its ability to hydrolyze GTP, so it is always in the
Active state
What are two ofther Ras-related disease?
Neurofibromatosis Type-1 and Noonan Syndrome
Caused by a mutation in the NF1 gene, which results in overactive Ras. This gene encodes neurofibromin-1, a Ras-GAP.
Neurofibromatosis type 1
Caused by a mutation in the PTPN11 gene which encodes SHP2. This results in a gain of function phenotype which leads to hyperactive Ras
Noonan Syndrome
The most abundant class of receptors
7-α-helix receptors
Most receptor targeted drugs are directed at
7-α-helix receptors
What are four physiological roles for 7-α-helix receptors?
Vision, smell, mood, and autonomic nervous system
Opsins (7-α-helix receptors), such as rhodopsin, convert electromagnetic radiation (light) into
Cellular signals
7-α-helix receptors for neurotransmitters bind serotonin, dopamine, GABA, and glutamate to affect
Mood
Both sympathetic and parasympathetic nervous systems are regulated by G-Protein-coupled
7-α-helix receptors
7-α-helix receptors are coupled to trimeric “large G-proteins” whose α-subunits contain
Ras-like domains
Ligand binding to 7-α-helix receptors causes exchange of GDP by GTP and activation of the G-protein, initiating
Signaling Cascade
The 7-α-helix receptor itself thus acts as a
GEF
Dissociate from ligand-bound receptors and regulate activity of downstream proteins
α and βγ subunits
What are the three main subclasses of large G-proteins α-subunits?
Gs-α, Gi-α, and Gq-α
Activates adenylate cyclase which activates protein kinase A (PKA)
Gs-α
Inhibits adenylate cyclase, which inhibits protein kinase A (PKA)
Gi-α
Activates phospholipase-Cβ (PLC) which affects Protein kinase C (PKC)
Gq-α
Desensitization/dialing back the response, is when the receptor signaling diminishes or terminates, even in the presence of
Ligand
7-α-helix receptor that binds the adrenal-gland hormone epinephrine and the neurotransmitter norepinephrine.
β-adrenergic receptor
The ligand-bound β-adrenergic receptor becomes substrate for
β-adrenergic receptor kinase (BARK)
The phosphorylated β-adrenergic receptor is bound by
-blocks interaction with Gs
β-arrestin
Ligand-bound β-adrenergic receptor is inactivated by a
protein kinase and
-terminates signaling even in the presence of the ligand (desensitization)
β-arrestin
Cytosolic calcium is maintained at a very low
concentration by calcium pumps in the plasma
membrane and the
Smooth ER (SER)
Triggers many events in cells such as muscle contraction, regulated secretion, and cell division
Receptor-mediated calcium influx
Gi-α inhibits adenylyl cyclase, which then prevents PKA from becoming active. This prevents
Calcium influx
Gs-α activates adenylyl cyclase, which in turn activates PKA, which allows for
Calcium influx
Gq-α-GTP activates phospholipase-C (PLC) and triggers
Ca2+ release from the Smooth ER (SER)
Gq-α-GTP activates phospholipase C (PLC), which hydrolyzes membrane phosphatidyl inositol-
4,5-diphosphate (PiP2) into
iP3 and diacyl glycerol (DAG)
Triggers release of Ca2+ from lumen of SER into cytoplasm
iP3
DAG and Ca2+ then activate
Protein kinase C (PKC)
Most kinases are inactive because they are
inhibited
Frees kinases from inhibition
Receptor activation
Inactive because its active site is filled with an N-terminal pseudosubstrate peptide, held in place by its
C1 and C2 domains
PKC
Bind DAG in membranes
C1 domains
In the presence of Ca2+ the C2 domain binds
-found in the inner leaflet of the plasma membrane
Phosphatidyl Serine (PS)
This removes the pseudosubstrate from the active site and PKC becomes active. It can then be cleaved proteolytically and PKC is then
Constitutively (permanently) active
A calcium-binding protein that regulates the activity of many proteins
-binds 4 Ca2+ ions
Calmodulin
Is closely related to the calcium-binding subunit
of the protein troponin which regulates muscle
contraction
Calmodulin
Ca2+ bound calmodulin activates many proteins,
including another self-inhibited kinase known as
Calmodulin-activated protein kinase (CAMK)
An important mediator of learning and
memory and has been implicated in AD
-others are involved in cancer and musculoskeletal diseases
CAMKII
