L5: Proteoglycans and Signalling Flashcards
PG signalling: How do PGs act and why are they so diverse in effect?
- Signalling pathways still function in absence of PGs -> co-receptors
- PGs can influence a vast range of signalling pathways because their side chains (GAGs) are capable of huge diversity in structure
GAGs: Structure, examples
- Glycosaminoglycans
- Repeating disaccharide of around 80 sugar residues
- Each residue, apart from those of HA, may be sulphated
- e.g. chrondroitin / dermatan sulphate, keratan sulphate, hyaluronan, heparan sulphate (HS / heparin
How are PGs structured?
- Core protein
- One or more GAGs attached
Examples of cell-surface processes of PGs (beyond co-receptor activity):
- Dimerisation
- Two-dimensional sliding
- Surface transport
- Transcellular transport
Families of membrane PGs (x2)
- Syndecans (transmembrane with core protein with HS and CS GAGs)
- Glypicans (anchored to PM via GPI, core globular domain with HS GAGs); major part of the extracellular matrix
- 4 mammalian syndecans exist, with differential tissue expression etc
- 6 mammalian glypicans exist
- In both cases, heparan sulfate is able to bind E-C ligands and modulate their interactions with cell surface receptors
What is dally?
- Glypican found in drosophila (discovered as a mutation in which glypican is not being synthesised correctly, producing only a weak signal (signalling still occurs in part)
- Dally is involved in diffusion of Dpp; abnormally delays cell division -> in dally mutant the shape of the morphogen gradient is altered (shortened, doesn’t reach as far from source in short periods)
- Dally-like is needed for Hh signalling in the developing wing
- Dpp: Growth-promoting morphogen in TGFb family
Types of concentration responses in cell signalling (x2):
- Cliff-edge like (defined thresholds)
- Gradient (response observed even in very small concentrations)
Potential defects in PGs: (x3)
- Incorrect synthesis of core or lack of sulphation -> weak signal
- Overactive PG which does not release morphogens once bound -> weak signal
- Overactive notum -> PGs cleaved in excess, unable to localise morphogen properly -> weak signal
Further examples of PG mutants:
- Sugarless (UDP glucose dehydrogenase)
- Sulphateless (sulfotransferase)
Origin of HS diversity:
- Different esters on the disaccharides can be sulfated -> increasing or decreasing negative charge
- e.g. Sulfation or lack thereof at a particular position is essential for binding of certain FGFs
- e.g. specifically, HGF binding necessitates L-iduronic acid to be unsulphated
What are Sulfs?
- e.g. Qsulf
- Qsulf enzymes remove sulphates -> post translational modification of PGs
- Discovered in quail
- Known to have major roles in development and cancer -> explore further?
+ Expression of GPC3 in cancer
- GPC3 is overexpressed in >70% of hepatocellular carcinomas
- Not in normal adult tissues
+ Role of Wnt in healthy adult tissues:
- Wnt signalling promotes tissue renewal and regeneration
- Highly hydrophobic
+ What 4 groups of molecules do glypicans typically interact with?
- Morphogens
- Cytokines
- Chemokines
- Growth factors
+ Disease associated with dysfunction of glypicans (human)
- SGBS (Simpson Golabi Behmel Syndrome)
- X-linked overgrowth syndrome mainly affecting males, also causes susceptibility to certain malignancies
- Associated with loss of function of GPC3
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+ SULF2 in human cancers:
- SULF2 is upregulated in >60% of human cancers
- Wnt is thus released from GPC3 (2-O and 6-O sulfation found to be essential for binding; SULF2 is a 6-O-desulfatase)
+ What tissues are Syndecans 1-3 associated with?
- 1: Epithelial
- 2: Mesenchymal
- 3: Neuronal tissue and cartilage
+ In what context may syndecans be released from the membrane
- They are typically transmembrane proteins
- However, through the action of sheddases they can become soluble active ligands (e.g. MMPs, disintegrin) -> possibly operating using sRNAs (specifically miRNA)
- Termed the shedding process
+ Which 3 glypicans are associated with cancer?
- GPC1
- GPC3
- GPC5
+ How does GPC1 influence carcinogenesis?
- Shows increased expression in human gliomas and glioma-derived cell lines
- Enhances FGF basic signalling and mitogenesis
- Also overproduced in pancreatic and breast cancer cell lines
+ Evidence for role of GPC1 in pancreatic cancer progression:
- Antisense depletion of GPC1 in pancreatic cancer cells reduces their ability to form tumours in vivo
+ Name the cartilage CS proteoglycan and its key GAG chain:
- Aggrecan
- Hyaluronan
+ What are the two components of GAG disaccharide building blocks?
- Amino sugar (which can be N-acetylated or N-sulfated)
- Uronic acid (glucuronic or iduronic acid) or galactose
+ Additional example of cell surface PGs beyond syndecans and glypicans which is found on stem cells:
- NG2: Surface marker expressed on stem cell populations, cartilage chondroblasts, myoblasts etc
+ What are slit proteins? What are their receptor and co-receptor?
- Large secreted glycoproteins characterised by unusual tandem of 4 LRR domains
- Various functions including guiding neural development, vasculature and immune system
- Receptors: Robos via Syndecans
+ What is a ‘part-time’ PG? How is it different from syndecan and glypican?
- Syndecan and glypican bear HS chains at all times
- Part-time PGs have HS chains under certain conditions
- e.g. CD-44, betaglycan
+ What is the function of the protein cores of PGs?
- Proffer HS chains to the E-C environment at defined distances and volumes
- Also influence rate and mechanisms of HS turnover
+ What roles do syndecans 1 and 3 play in HIV infection?
- Syndecan-3: Binding interaction of initial pathogen to host cell -> internalisation to macrophage etc (readily express HSPGs)
- Syndecan-1: Promotes lymphoma migration when HIV expresses Tat protein
