Immune Receptors and Signal Transduction Flashcards
TCR signaling deficiency can be cause by what?
Deficiencies in : CD40 ligand CD40 CD3γ CD8 ZAP - 70 Ca ++ channel CD25 STAT5b Itk DOCK8
Generally, receptors are what kind of protein?
Integral membrane proteins present on the plasma membrane
What do extracellular domains of receptors recognize?
Soluble ligands or membrane structures of neighboring cells
What are nuclear receptors?
Intracellular transcription factors that are activated by lipid-soluble ligands that can cross the plasma membrane
What does ligand-binding involve?
A conformational alteration of the receptor
What does signaling require?
A ligand-induced clustering of receptors
What is cross-linking?
Clustering of receptors that is ligand-induced
What does clustering and conformational alterations result in?
Changes in the cytosolic portion of the receptor that promotes interactions with other signaling molecules
What initiates the signal transduction?
The enzymatic phosphorylation of tyrosine, serine, or threonine in the cytosolic portion
The enzymes that phosphorylate tyrosine, serine, threonine, or lipid substrates are called what?
Protein kinases:
Tyrosine kinases
Serine/threonine kinases
Lipid kinases
What removes the phosphate residue and thus modulate signaling?
Specific phosphatases for all types of protein kinases
(Kinases/phosphatases) usually play inhibitory roles in signal transduction
Phosphatases
What either targets proteins for degradation or drive signal transduction in many cells?
The covalent addition of ubiquitin molecules
What may promote plasma membrane localization signaling molecules?
Protein signaling molecules modified by the addition of lipids
What modulates gene expression, DNA replication, and DNA recombination events?
Acetylation and methylation of the N-terminal tails of histones
What are the major categories of receptors?
Non-receptor tyrosine kinase Receptor tyrosine kinases Nuclear receptors G protein-coupled receptors Notch
What are the tyrosine kinase families?
The Src family, Syk family, and the Tec family
What are tyrosine kinases of the Src family?
c-Src, Lyn, Fyn, and Lck
What are tyrosine kinases of the Syk family?
Syk and ZAP-70
What are tyrosine kinases of the Tec family?
Tec, Btk, and Itk
What are signaling molecules composed of?
Distinct modules, each with a specific binding or catalytic function
What are the unique domains of tyrosine kinases?
SH2 domain
SH3 domain
PH domain
What is the function of the SH2 domain?
Bind to specific phosphotyrosine-containing polypeptides of certain proteins
What is the function of SH3 domains?
Recognize and bind to proline-rich stretches in certain polypeptides
What is the function of PH domains?
Recognize PIP3 or other phosphatidylinositol-derived lipids
What are SH2 domains composed of?
100 amino acids folded into a particular formation
What serves as binding sites for SH2 domains present in Syk and ZAP-70 tyrosine kinases?
The phosphotyrosine motifs in the Ag receptor complex
What are SH3 domains composed of?
100 amino acids in length
What do pleckstrin homology (PH) domains recognize?
recognize specific phospholipids
What does the TEC family tyrosine kinase Btk recongize?
Recognize phophatidylinositol trisphosphate (PIP3)
What is PIP3?
A lipid moiety on the inner leaflet of the plasma membrane
What are adaptor proteins?
Molecular hubs that physically link different enzymes and promote the assembly of complexes of signaling molecules
What are examples of adaptor proteins?
LAT and BLNK
What is the linker for the activation of T cells?
LAT
What is the B cell linker (Adaptor)?
BLNK
What may adaptor proteins contain a few of?
Few SH2 and SH3 domains
What do adaptor proteins often contain?
Often contain some proline-rich stretches that can bind other proteins that contain SH3 domains
Adaptor proteins contain tyrosine residues that may be phosphorylated by tyrosine kinases and serve as what?
Docking sites for other signaling molecules with SH2 domains
What is an integral membrane protein that functions as an adaptor in T cell activation?
LAT
What are two cytosolic adaptors involved in T cell activation?
GADS and SLP-76
Upon T cell activation what cascades happen with the adaptors of the T cell?
Lat is phosphorylated and recruits PLCγ (phosphoinositide phospholipase c) and the GADS adaptor (both contain SH2 domains)
Proline-rich amino acid stretch in SLP-76 associates with an SH3 domain of GADS - this then recruits Vav after being trysoine-phosphorylated
What are the four examples of the immune receptor family?
The B cell receptor (BCR)
T cell receptor (TCR)
High-affinity receptor for IgE (FceRI)
And inhibitory receptor found on B cells and myeloid cells (FCγRIIB)
What do activating immune receptors have?
Separate polypeptide chains for recognition and associated signaling polypeptide chains that contain cytosolic ITAMs
What does ITAMs stand for?
Immunoreceptor tyrosine-based activating motifs
What to inhibitory receptors in the immune system typically have?
They have ITIMs on the cytosolic portion of the same chain that uses its extracellular domain for ligand recognition
What inhibitory receptor is found on B cells and myeloid cells?
FcγRIIB
TCR and BCR signaling is (similar/different)
Similar
What does Ag binding and clustering results in?
Activation of an associated Src family kinase
What may expose tyrosine residues of a ITAM motif?
Conformational change or unfolding of the cytoplasmic tail of the receptor
What do Src family kinase phosphorylate?
Available tyrosines in the ITAMs
What are recognized by a Syk family tyrosine kinase that has tandem SHA2 domains
Two phosphorylated tyrosines in a single ITAM
What does each SHD domain of Syk family tyrosine kinase bind to?
An ITAM phosphotyrosine
What does recruited and activated Syk family kinase phosphorylate?
Adaptor proteins and enzymes that activate distinct signaling pathways downstream of the immune receptor
How many signaling chains and ITAMS does the TCR complex have?
6x signaling chains and 10x ITAMS
What does stronger or prolonged binding of Ag to the TCR results in?
Increasing numbers of phosphorylated ITAMs
What is the cytosolic interpretation of Ag affinity to the TCR?
The number of ITAMs phosphorylated
What can influence the nature of the cellular response at different stages of differentiation and activation?
Ag affinity of TCR
What are required for positive selection of T cells in the thymus?
Weak TCR signals
What results in thymocyte death by apoptosis?
Strong TCR signals in the thymus
What is a corereceptor?
A transmembrane signaling protein on a lymphocyte that can facilitate Ag receptor activation
What are the three mechanisms that fine-tune and modulate antigen receptor signaling?
Progressive ITAM use
Increased cellular activation by coreceptors
Modulation of signaling by inhibitory receptors
The coreceptor with its signaling enzymes can do what?
Increase ITAM phosphorylation and activation of the antigen receptor
What are coreceptors on T helper and cytotoxic T cells, respectively?
CD4 and CD8 proteins
What is the coreceptor on B cells?
Complement receptor type 2 (CR2/CD21)
What is the inhibitory T cell receptor?
CTLA-4
What are the inhibitory receptors in B cells?
CD22 and FCγRIIB
What are the costimulatory molecules and prototypic costimulatory receptors on T cells?
CD28 (receptor)
CD80/86 (costimulatory molecules)
What do costimulatory receptors add?
Add another level of control to the process of lymphocyte activation
What do costimulatory receptors provide?
So-called second signals for lymphocytes and ensure that immune responses are optimally triggered by infectious pathogens and substances that mimic microbes
Cognate peptide recognition in the presence of co-stimulation results in:
Th-cell activation
Cognate peptide recognition in the absence of co-stimulation results in:
Activation-induced cell death or anergy
What are the components of TCR?
α and β chains
What are the componets of Ig?
heavy and light chains
What is the number of Ig domains in TCR?
One V domain and one C domain in each chain
What is the number of Ig domains in Igs?
Heavy chain: one V domain, three or four C domains
Light chain: One V domain and one C domain
What is the number of CDRs involved in Ag binding in TCR?
Six (three in each chain)
What is the number of CDRs involved in Ag binding in Ig?
Six (three in each chain)
What are the associated signaling molecules of TCR?
CD3 an ζ
What are the associated signaling molecules of Ig?
Igα and Igβ
What is the affinity for Ag (Kd) with TCR?
10^-5 - 10^-7 M
What is the affinity for Ag (Kd) with Ig?
10^-7 - 10^-11 M (secreted Ig)
What does the TCR complex consist of?
The αβ TCR non-covalently linked to the CD3 and ζ proteins
The association of proteins, of the TCR complex, with one another is mediated by what?
Their charged residues in their transmembrane regions
What do both of the TCR chains, α and β, have in common?
Both have carboxyl-terminal cytoplasmic tails that are 5 to 12 aa long
What are the cytoplasmic regions of the TCR chains too small to do?
Transduce signals.
What serves as signal-transducing subunits of the TCR complex?
CD3 and ζ proteins
What is CD3 composed of?
Three polypeptide chains, named γ, δ, and ε, arranged in two pairs (γε and δε) - These proteins are homologous to each other and are members of the Ig superfamily
What are the major surface molecules of CD4 T cells and APCs involved in T cell activation?
CD4 TCR---Class II MHC CD3 CD28---B7-1/2 LFA-1---ICAM-1
What are the regions of signaling proteins that are phosphorylated and dock other signaling molecules?
ITAMs
What is the function of CD3 and ζ ?
Signal transduction by TCR complex
What is the function of CD4 and what is its ligand?
Function: signal transduction Ligand: class II MHC expressed in APCs
What is the function of CD8 and what is its ligand?
Function: signal transduction
Ligand: Class I MHC expressed on all nucleated cells
What is the function of CD28 and what is its ligand?
Signal transduction as a costimulator
B7-1/B7-2 on APCs
What is the function of CTLA-4 (CTL antigen 4)? What is its ligand?
Signal transduction (negative regulation) Ligand: B7-1/B7-2 on APCs
What is the function of PD-1 (program death-1) and its ligand?
Signal tranduction (negative regulation) Ligand: PD-L1/PD-L2 on APCs, tissue cells and tumor cells
What is the function of LFA-1 and its ligand?
Adhesion
Ligand: ICAM-1 on APCs and endothelium
CD4 and CD8: which one exists as a monomer and the other as a heterodimer?
CD4 is a monomer and CD8 is a disulfide-linked heterodimer
CD4 is composed of what?
Four extracellular Ig-like domains
A hydrophobic transmembrane region
And a highly basic cytoplasmic tail 38 aa long
What is CD8 composed of?
Two related chains called CD8α and CD8β
Each chain has a single extracellular Ig domain
A hydrophobic transmembrane region
and a highly basic cytoplasmic tail that is about 25 aa long
What does CD4 mainly binds to?
To class I MHC molecules and also interacts with β2 microglobulin
What does the two N-terminal Ig like domains of CD4 protein bind to?
The non-polymorphic α2 and β2 domains of the class II MHC
What does the cytoplasmic tails of both CD4 and CD8 bind to? What is the purpose?
The Src family kinase Lck: helps these proteins to be drawn adjacent to the TCR that contacts the same MHC-peptide complex on the APC
What does Lck do?
Phosphorylates the tyrosine residues on ITAMs
On Ag recognition, what does the TCR complexes cluster with?
CD4 or CD8
Once the CD4-associated Lck becomes active what does it do?
Phosphorylates ITAMs of CD3 and ζ chains
Once ζ chains are phosphorlyated, what bind to the phosphotyrosines of the ζ chains?
ZAP-70 binds and is itself phosphorylated and activated
What does active ZAP-70 then activate?
Various adaptors such as LAT
What do the activated adaptors become?
Docking sites for PLCγ1 and exchange factors that activate Ras and MAP kinases
Involving cell survival, where is activated PI3-kinase recruited to and what does it generate?
The TCR complex and generates phosphatidylinositol trisphosphate (PIP3) from phosphatidylinositol biphosphate (PIP2)
What is PDK1?
A PIP3-dependent kinase which phosphorylates and activates a downstream Akt kinase
What does activated Akt contribute to?
Cell survival by inactivation pro-apoptotic proteins of the Bcl-2 family of apoptotic proteins
Where is PKC-0 located in reference to talin in the synapse?
PKC-0 is central while talin has a peripheral location
What is talin associated with?
It is a protein that associates with the cytoplasmic tail of the LFA1 integrin
What does PKC0 associate with?
The TCR complex
What does the immunologic synapse form?
A stable contact between an antigen-specific T cell and an APC displaying that antigen and becomes the site for assembly of the signaling machinery of the T cell
What does Lck activate upon Ag recognition in the Ras-MAP kinase pathway?
ZAP-70
What does ZAP-70 phosphorylate after it is activated in the Ras-MAP kinase pathway?
membrane-associated adaptor protein LAT
What does Lat serve as in the Ras-MAP kinase pathway?
The docking site for SH2 domain of Grb-2
Once Grb-2 is bound to activated LAT in the Ras-MAP kinase pathway, what does it do?
Recruits the Ras GTP/GDP exchange factor called SOS
Once the SOS is docked on Grb-2 in the Ras pathway, what does SOS do?
Converts Ras-GDP to Ras GTP
What does Ras-GTP activates?
Directly activates MAP kinase and Raf
Raf then goes to phosphorylate and activate MED-1, which then activates ERK
What does ERK do in the Ras pathway?
Translocates to the nucleus and phosphorylates Elk (protein)
What does Elk in the Ras pathway do?
Elk stimulates transcription of c-FOS which is a component of the AP-1
What does the Ras cascade activate?
AP-1
Describe the steps that involve PLCγ1 in T cell signaling
- activated LAT binds PLCγ1 and it is activated by ZAP-70
- Activated PLCγ1 hydrolyzes membrane PIP2 to generate inositol trisphosphate (IP3) and diacyl-glycerol(DAG)
- IP3 stimulates an increase in cytosolic Ca ions released from ER
- DAG activates enzyme PKC
- PKC and cytosolic Ca ions induce numberous cellular responses
Multiple signaling pathways converge in Ag-stimulated T cells all leading to what?
Transactivation of IL-2 gene
What does calcium-calmodulin activate?
Nuclear factor of activated T cells (NFAT) and the Ras and Rac pathways which generate AP-1
what is important in activating NF-kB?
PKC
What is NF-kB?
Is a p50p60 heterodimer inactivated by IkB
How is NF-kB activated?
PKC phosphorylates IkB(inhibitor of NF-kB)
Where is activated NF-kB translocated?
To the nucleus
Together, AP-1 and NF-kB do what?
regulate gene expression
During the classical pathwya for NF-kB induction, what have been demonstrated to be targets of TCR or CD28 signaling?
IKKα, IKKβ, IkBα, IkBβ, NF-kB1 p50, RelA p65, c-Rel
TCR (signal 1) and CD28 (signal 2) induce different signaling pathways that results in what?
The activation of several transcription factors
In the nucleus, what does NFAT cooperate with?
AP1 and NF-kB to induce gene expression of a productive immune response
When TCR engagement occurs in the absence of co-stimulation, what happens?
Calcium-mediated signals induce the activation of NFAT only
What doe NFAT alone elicit?
The expression of a distinct set of anergy-inducing genes
What does the products of anergy-inducing genes do?
inhibit T - cell function at different levels and induce a status of T-cell unresponsivemenss
