Lecture 7: Signaling Through Lymphocyte Receptors

Questions to Consider After recognition of its cognate MHC:peptide, how does the T cell receptor activate immune response genes? What are the structural motifs used by signal transduction molecules that permit specific interaction and activation of effector proteins? How do genes know what is going on in the outside environment requiring them to make their proteins at the appropriate time?

General Signal Transduction Concepts

Recognition of MHC:Peptide Complex by TCR Initiates Signaling Cascade in the T cell

Some Cellular Receptors Have Their Own Transduction Molecules Activated by Dimerization

Other Cellular Receptors Are Transduction Molecule-challenged

Signal Transduction Pathways Amplify the Initial Signal

Signal Transduction Pathways Must Have the Capacity to be Turned Off

Adapter Proteins Recruit Target Molecules From the Cytoplasm to Membrane-Associated Kinases

Src-family Kinases Are Regulated Tyrosine Kinases and Also Function as Adapter Proteins Adapter Adapter Adapter Kinase Binds to CD4/CD8 or TCR /CD3 Binds to PI3-kinase (prolinerich motif) Binds to P120/130 PI3-kinase (pyeei motif) PTK Phosphorylation of specific tyrosine residues within TCRz/CD3, CD5 P120/130 Regulates PTK activity (dephosphorylation by CD45 upregulates activity:phosphorylation by p50csk downregulates

Scaffolding Proteins Permit Assembly of Signaling Complexes

Overview of T Cell Signal Transduction

The T Cell Receptor Complex Consists of Eight Proteins With Unique Functions Ligand binding heterodimer or binds peptide + MHC CD3 Complex and Nonpolymorphic chains present as or dimers Intracellular - present as dimers present as a homodimer

Interaction Between MHC Class II:Peptide and TCR Recruits the CD4 Molecule

TCR-associated Proteins: src-family Kinases lck binds to CD4 and CD8 fyn binds to non-phosphorylated ITAMs SH3/SH2 domains inactive when bound to phosphorylated C-terminus activated by dephosphorylation by phosphatase such as CD45

Regulation of Src-family Kinases by Phosphorylation and Dephosphorylation of Activation and Inhibitory Tyrosine Sites

MHC:Peptide Interaction Recruits the CD45 Tyrosine Phosphatase Which Dephosphorylates and Activates Src-kinases: Fyn and Lck

Phosphorylation of Receptors Permits Them to Recruit Adapter Proteins That Bind Phosphotyrosine Residues

Role of Immunoreceptor Tyrosine- based Activation Motifs (ITAMs) Tyrosine residues on ITAMs are phosphorylated and thereby bind proteins containing SH2 domains Total of 10 ITAMs/T cell receptor complex 1 ITAM/CD3 or chain = 4 ITAMs 3 ITAMs/CD3 chain = 6 ITAMs

Why So Many ITAMs? Required for amplification of signal transduced by few TCR molecules Different transduction molecules bind to different ITAMs Required for association of TCR complex with actin cytoskeleton

Phosphorylated ITAMs Recruit ZAP70 Permitting Lck to Phosphorylate and Activate ZAP70

ZAP-70 Phosphorylates Scaffold Proteins LAT and SLP-76 to Activate Phospholipase C-

Phospholipase C- Activates Protein Kinase C and Ras-GEF (RasGRP)

ZAP-70 Initiates Cascade Activating Guanine-nucleotide Exchange (GEF) Factors That Activate Ras

Transduction of the Signal Into the Nucleus Activates Genes There are multiple pathways that transduce the signal from the cytoplasm into the nucleus Different transcription factors activate different groups of genes

MAP Kinase Cascade Transduces a Signal Into the Nucleus Ras activates MAP kinase kinase kinase (Raf). Raf phosphorylates serine/threonine on MAP kinase kinase (Mek) and activates it. MAP kinase kinase (Mek) phosphorylates threonine and tyrosine on MAP kinase (Erk) thereby activating it. MAP kinase (Erk) activates the Elk transcription factor which stimulates Fos gene transcription.

Calcium Induces the Translocation of the Transcription Factor NFAT Into the Nucleus

Phosphorylation of I B Releases the Transcription Factor NF B Thereby Enabling It to Translocate Into the Nucleus

T Cell and B Cell Activation Requires Two Signals

Co-stimulatory Signal Activates a Parallel MAPK Path That Activates Complementary Genes Ras activates MAP kinase kinase kinase (Raf). Raf phosphorylates serine/threonine on MAP kinase kinase (Mek) and activates it. MAP kinase kinase (Mek) phosphorylates threonine and tyrosine on MAP kinase (Erk) thereby activating it. MAP kinase (Erk) activates the Elk transcription factor which stimulates Fos gene transcription.

Stimulatory Signal Pathways Integrated by Production of Fos and Jun- Components of the Heterodimer AP-1 Transcription Factor

Broad Range of Nuclear Factors Are Activated in TCR Signal Transduction Cascade Initiation of TCR-mediated signals Biochemical intermediates Active enzymes Transcription factors Abbas- Cellular and Molecular Immunology

Activation of Transcription Factors To Turn on Genes by Activation of Kinases and Phosphatases Abbas- Cellular and Molecular Immunology

Cyclosporin and Tacrolimus Block Calcineurin Signaling and Thereby Suppress T Cell Activation

In 1984, Fujisawa scientists discovered tacrolimus in a soil sample taken from Mt. Tsukuba in Japan. Not long after, scientists realized that some of the properties of tacrolimus may help treat certain conditions and diseases. Eventually, they saw that this drug could be used to safely treat patients with moderate to severe eczema thus, pioneering the use of topical immunomodulators (TIMs). Protopic was the first of this new class of nonsteroid drugs approved for the treatment of moderate to severe eczema giving doctors the first prescription alternative for the treatment of moderate to severe eczema in 40 years.

Cross-linking Surface Immunoglobulin Activates B Cells

Cross-linking Surface Immunoglobulin Activates ITAMs