Signaling in immune cells

associates with ITAM+ proteins

TCR expressed with CD3 chain at the surface —>needs association with ITAM proteins

coreceptor associates with starting tyrosine kinase Lck

Lck phosphorylates the ITAMs in the TCR upon co-receptor engagement with antigen: MHC

CD4 receptor binds MHC: peptide & constitutively associates with Lck

Lck inactive when ita terminal tyrosine is phosphorylated & binds the SH2 domain & the linker region binds the SH3 domain (binds proline-rich sequences)

unique phosphorylation leading to binding of own SH2 (inactive) removed by CD45 phosphatase —> SH3 domain releases linker region (CD45 pan leukocyte marker with phosphatase activity)

Lck is fully activated when its activation loop tyrosine in the kinase domain is autophosphorylated

Lck leads to phosphorylation of ITAMs in CD3

ITAM recruited kinase specific for T-cells

ZAP-70 is recruited by tandem SH2 domains to the ITAMs

phosphorylated by Lck

inactive conformation: autoinhibition

activation: by phosphorylation

recruited via SH2 to ITAM

active conformation can transmit signal by phosphorylating substrates

phosphorylation of scaffold proteins

activated ZAP-70 phosphorylates scaffold proteins

complex leads to accumulation of phosphatidylinositol triphosphate PIP3 at the plasma membrane

scaffold complex & PIP3 recruit PLCγ & Itk (IL-2 inducible T-cell kinase)

PLCγ activated by phosphorylation from Itk

PH: pleckstrin homology domains binding to phosphorylated lipids (PH domains in PIP3)

crucial step in T-cell activation

B7.1 & B7.2 are CD28 ligands expressed on specialized APC

B7 binding induces CD28 phosphorylation, activating PI3-kinase to produce PIP3

converts some phospholipids into PIP3

IP-3 & DAG for calcium flux into cells

PLCγ cleaves phosphatidylinositol bisphosphate PIP2 into diacylglycerol DAG & inositol triphosphate IP3

DAG anchored in the membrane —> recruits PKCθ /Ser/Thr kinase NFκB) & RasGRF (guanine exchange factor AP-1)

IP3 opens Ca2+ channels to allow Ca2+ from the ER into the cytosol

depletion of Ca2+ in the ER stimulates STIM1 aggregation

aggregated STIM1 binds & opens ORAI1 channels in the plasma membrane —>allows entering of extracellular Ca2+

—>two sources of Ca2+ (membrane vesicle + plasma membrane Ca2+ channel)

NFAT = nuclear factor of activated T-cells

activation by dephosphorylation

phosphorylation on Ser & Thr residues keeps NFAT in the cytoplasm of unstimulated cells

calcium entry activates the Ser/Thr phosphatase calcineurin which dephosphorylates NFAT

calmodulin forms a complex with calcineurin for its activation (calmodulin binds Ca2+)

dephosphorylated NFAT enters the nucleus & activates gene transcription

cyclosporin: immuno suppressor targeting calcineurin, used against auto-immune diseases

Ras initially inactive —>TCR signaling produces DAS which recruits Ras GRP to the membrane where it activates Ras

RasGRP = guanine exchange factor —>exchange GDP to GTP in Ras

Ras activates Raf (MAPKKK)—> phosphorylates Mek (MAPKK) —> phosphorylates Erk (MAPK)

three kinase cascade to activate MAPK (mitogen-activated protein kinase)

activated Erk enters the nucleus & activates TF Elk-1

Elk-1 stimulates transcription of FOS genes (intermediate TF)

activation of MAPK JNK (via PKCθ) allows it to enter the nucleus & phosphorylate c-Jun

c-Jun/c-FOS dimerization —> AP-1 TF

resting state: small G protein bound to GDP

signaling activates guanine-nucleotide exchange factors —> increased rate of GDP & GTP exchange

GTP-bound small G protein is the active effector molecule

over time G-protein hydrolyzes GTP to GDP —>becomes inactive —>accelerated by GAPs

PKCθ dependent

DAG recruits PKCθ to the membrane where it phosphorylates intermediate proteins

scaffold complex recruits TRAF-6 (E3 ubiquitin ligase)

makes polyubiquitin scaffold on itself & NEMO

TAK1 recruited by TAB1/2 phosphorylates IκκB —> phosphorylates IκB inducing its ubiquitination & degradation

degradation of IκB releases NFκB —>activates transcription in the nucleus

ubiquitination: ubiquitin residue coupled by C-ter glycine to lysine residues of proteins by ubiquitin ligases

—>TRAF-6 K63 poly-ubiquitination for signal transmission

—>K48 poly-ubiquitination protein degradation (IκB)—> proteasome / lysosome mediated

3 TF in total involved

other effect than changes in gene expression

—> cell survival

PIP3 recruits PDK1 & Akt —> PDK1 activates Akt

Akt phosphorylates Bad which releases Bcl-2 —> free Bcl-2 promotes cell survival

active Bcl-2 at the mitochondrial membrane prevents activation-induced cell death

—>biosynthesis

Akt phosphorylates TSC1/2 complex (GAP for the small GTPase Rheb)

release of Rheb—>binds mTOR (mammalian target of rapamycin)

increases lipid production, ribosome biosynthesis, mRNA synthesis & protein translation

inside/outside signaling

activation of integrin adhesion & aggregation

after TCR stimulation, ADAP recruited to the LAT: Gads: SLP-76 complex

ADAP recruits SKAP & RIAM activating the small GTPase Rap1

active Rap1 induces LFA-1 aggregation & conversion to the high-affinity binding state

no CD3 but CD3-like Igα/Igß (no equivalent of coreceptor carrying a kinase)

phosphorylation of ITAMs on BCR tails by Src-family kinases

Blk/ Fy/ Lyn —> Lck (not attached to coreceptor but signaling associated with BCR)

associated to the ITAM+ associated proteins are all tyrosine kinases from the Src family

Syk —> ZAP-70 (binds to doubly phosphorylated ITAMS & activated upon binding)

B-cell & coreceptor stimulation induces tyrosine phosphorylation of Igα/Igß & cytoplasmic tail of CD19

BCR multimeric: signaling needs a platform to bring together several receptors (cSMAC in T-cells)

—>several antigens at the cell surface

CD19 (marker B-cells)/ CD21 / CD81 —> CD4/CD8 (B-cell coreceptors)

CD21: complement receptor 2 —> binds proteins tagged with a cleaved product from C3

cross-linking & clustering of the co-receptor with the antigen-receptor results in the phosphorylation of tyrosine residues in the ITAM sequence of cytoplasmic domains of BCR signaling subunits Igα/Igß

Phospholipase γ activation —> Btk phosphorylates PLCγ

BTK = ITK

calcium flux due to other kinase & induction NFAT