Cell signaling Lecture 8 Transforming growth factor TGF

Cell signaling Lecture 8

Transforming growth factor (TGFβ) Receptors/Smad pathway BMP 7 TGFβ 1, TGFβ 2, TGFβ 3 Dpp Inhibins Activins TGFβ receptors Autocrine/Paracrin e signaling Smad proteins located in the cytotsol which moves into the nucleus to regulate transcription

Ligand Receptor Function Activins (A, B, AB) ACVR 2 A Embryogenesis , Osteogenesis, hormone regulation BMPs BMPR 2 Osteogenesis, formation of mesoderm, and earliest blood-forming cells TGFβ 1 -3 TGFβR 2 Cell cycle regulation

Formation and activation of TGFβ

RIII: 280 k. Da (Monomer, most abundant) RII: 85 k. Da (Diamer) RI: 55 k. Da (Diamer)

Ligand binding � Signaling begins with the binding of a ligand to a TGF beta type II receptor. Ligand binding induces formation of complexes containing 2 copies of RI and RII. � The RII is a serine/threonine receptor kinase. � It catalyzes the phosphorylati on of the RI. � Each class of ligand binds to a specific RII. � In mammals there are seven known type I receptors and five type II receptors. ATP + [receptor-protein] ADP + [receptor-protein] phosphate

SMAD Phosphorylation � There are five receptor regulated SMADs: SMAD 1, SMAD 2, SMAD 3, S MAD 5, and SMAD 9. � TGF beta's, Activins, and some GDFs are mediated by SMAD 2 and SMAD 3, while BMPs, and a few GDFs are mediated by SMAD 1, SMAD 5 and SMAD 9. � The binding of the R-SMAD to the type I receptor is mediated by a zinc double finger FYVE domain containing protein. � Two such proteins that mediate the TGF beta pathway include SARA (The SMAD anchor for receptor activation) and HGS (Hepatocyte growth factorregulated tyrosine kinase substrate).

�SARA recruits an R-SMAD. �SARA permits the binding of the R-SMAD to RI. �SARA orients the R-SMAD such that serine residue on its C-terminus faces the catalytic region of the RI. �The RI phosphorylates the serine residue of the R-SMAD. �Phosphorylation induces a conformational change in the MH 2 domain of the RSMAD and its subsequent dissociation from the receptor complex and SARA.

Co. SMAD binding �The phosphorylated RSMAD has a high affinity for a co. SMAD (e. g. SMAD 4) and forms a complex with one. �The phosphate group does not act as a docking site for co. SMAD, rather the phosphorylation opens up an amino acid stretch allowing interaction.

Transcription � The phosphorylated RSMAD/co. SMAD complex enters the nucleus where it binds transcription promoters/cofactors and causes the transcription of DNA. � Bone morphogenetic proteins cause the transcription of m. RNAs involved inosteogenesis, neurogenesis, and ventral mesoderm specification. � TGF betas cause the transcription of m. RNAs involved in apoptosis, extracellular matrix neogenesis and immunosuppression. It is also involved in G 1 arrest in thecell cycle. � Activin causes the transcription of m. RNAs involved in gonadal growth, embryo differentiation and placenta formation.

Pathway regulation �The TGF beta signaling pathway is involved in a wide range of cellular process and subsequently is very heavily regulated. �There a variety of mechanisms where the pathway is modulated either positively or negatively

Role of inhibitory SMADs � There are two other SMADs which complete the SMAD family, the inhibitory SMADs (ISMADS), SMAD 6 and SMAD 7 (Sno. N and Ski). � They play a key role in the regulation of TGF beta signaling and are involved in negative feedback. � Like other SMADs they have an MH 1 and an MH 2 domain. � SMAD 7 competes with other R-SMADs with RI and prevents their phosphorylation. � It resides in the nucleus and upon TGF beta receptor activation translocates to the cytoplasm where it binds the RI. � SMAD 6 binds SMAD 4 preventing the binding of RSMADs with the co. SMAD. The levels of I-SMAD increase with TGF beta signaling suggesting that they are downstream targets of TGF-beta signaling.