Question

Please explain the following:

Understand the family of TGF beta signaling molecules (ligands).

Understand that the TGF beta superfamily of growth factor ligands are synthesized and secreted in an Inactive form, and need to be activated extracellularly by proteolytic cleavage.

Understand the structure of the bioactive form of TGF beta.

Understand the family of TGF beta receptor structure and function

Answer 1

TGF beta signaling pathway is involved in many cellular processes in both the adult organism and the developing embryo. The TGF beta family includes : TGF beta 1, TGF beta 2 and TGF beta 3. TGF betas are involved in embryogenesis and cell differentiation as well as apoptosis. They bind to TGF beta receptor type II ( TGFBR2).The TGF beta superfamily of ligands include : Bone morphogenetic proteins (BMPs), Growth and differentiation factors (GDFs), Anti- mullerian hormone ( AMH), Activin, Nodal and TGFB's. Signalling begins with the binding of a TGF beta superfamily ligand to a TGF beta type II receptor. The type II receptor is a serine/threonine receptor kinase which catalyzes the phosphorylation of type I receptor. Each class of ligand binds to a specific type II receptor. In mammals, there are 7 known type I receptors and 5 type II receptors.

The TGF beta superfamily ligands are dimers that are held together by 3 disulfide bonds that are part of a characteristic structure called a cysteine knot. Secreted TGF beta is a latent complex that is composed of the TGF beta dimer associated with the latency associated peptide or LAP and the latent TGF beta binding protein or LTBP. The latent complex is proteolytically cleaved by amino acid cleaving enzyme furin in the extracellular space resulting in the release of the TGF beta dimer allowing TGF beta to bind cell surface receptors.

The proteolysis results in creation of a small latent TGF beta complex (SLC). SLC is then connected with the disulfide bond formed between cysteine residue in a 33 location and a cysteine residue in the third of four cysteine-rich domains of the latent TGF beta binding protein and the resultant protein is called as large latent TGF beta 1 complex or LLC. LCC secretes from the cell faster than SLC and after secretion part of LCC interacts with extracellular matrix components which results its anchoring with forming covalent bond , with participation of a transglutaminase enzyme. LCC bond with intigrins allows releasing and activating mature TGF beta form by changing formation of the whole complex. Mature bioactive TGF beta releases from LCC by the help of proteases such as plasmin, matrix metalloproteases etc.

The TGF beta family receptors are single pass serine/ threonine kinase receptors. They exist in several different isoforms that can be homo or heterodimeric. 3 TGF beta family receptors are there which can be distinguished by their structural and functional properties :

TGFBR1( activin A receptor type II like kinase, 53 kDa) : functions - type II transforming growth factor beta receptor binding, kinase activity, I-SMAD and ATP binding, bicellular tight junction and receptor complex formation, protein phosphorylation, response to cholesterol, skeletal system development.

TGFBR2 : mitogen activated protein , vasculogenesis, growth plate cartilage development, 70-80 kDa.

TGFBR3 : cell surface chondroitin sulfate / heparan sulfate proteoglycan greater than 300 kDa, helps in heparan binding and cardiac muscle cell proliferation.

TGFBR1 (ALK 5) and 2 have similar ligand binding affinity and peptide mapping is used to distinguish them. Both TGFBR1 and 2 have high affinity for TGF beta 1and low affinity for RGF beta 2. TGFBR3 ( beta glycan) has a high affinity for both homodimeric TGF beta 1 and 2 and heterodimer TGF beta 1.2. The TGF beta receptors also bind TGF beta 3.