Flashcards in L21 – Molecular Composition and Functions of Glomerular barrier Deck (52):
What are basement membranes?
Specialized ECM that underlie all epithelial cell sheets and tubes
What does basement membrane Provide strong connection between?
epithelia and underlying connective tissue
What three core components make up basement membrane?
Type IV collagen
What does BM act as in glomerular?
Filtration barriers between parenchymal cells and connective tissue space
What is the normal GFR range?
What is normal Plasma / blood flow rate from renal arteries (Qp) ?
What are the 3 Structures that govern transport processes at the glomerular barrier?
Fenetrated glomerular endothelial surface
Glomerular Basement membrane
Podocyte foot processes
What is name of space between podocyte foot processes?
Fenestrated glomerular endothelial surface (ESL) other name?
What is ESL formed by?
Adsorbed plasma proteins
What is the function of Chondroitin sulfate in ESL?
Negative charge on surface = barrier to prevent passage of charged molecules
What makes up most Proteoglycans found on ESL?
Syndecan core protein + Chondroitin sulfate/ Heparan Sulphate side chains
Glypican core protein + Heparan sulphate side chain + GPI anchor
How can podocytes produce chemicals that affect endothelial glycocalyx?
Podocyte produces :
VEGF > VEGFR2
Chemicals that allow Endothelial glycocalyx to interact with GBM?
How are collagen molecules arranged?
3 polypeptide chains (α-chains) form triple helix
What is aa sequence of collagen?
repeating triplet sequence “Gly-X-Y” (X, Y = amino acids other than glycine)
Why is glycine in every third position on collagen chain?
small enough to fit into restricted space in center of triple helix, allow tight twisting
What are the 4 steps to intracellular biosynthesis of Type IV collagen?
Pre-procollagen chains are translated
Cleavage of signal peptides to form procollagen α chains
Association of nascent carboxy-terminal propeptide chain (coil around each other)
How is pre-procollagen translated and translocated into ER?
guided by signal peptides to
translocate into ER lumen for intracellular modification
How happens to pre-procollagen to turn into procollagen?
Cleavage of signal peptides (so that they do not go elsewhere after entering lumen of ER) > form procollagen α chains
What intracellular modifications are made before the procollagen form triple helix?
Hydroxylation of certain proline, lysine residues
Glycosylation (add galactose, glucose) of: hydroxylysine, asparagine residues
How is triple helix of collagen stabilized?
Association of nascent carboxy-terminal
propeptide chain (coil around each other)
= stabilize with S-S bond > triple helical procollagen
What supramolecular assembly of Type VI collagen takes place after being exported from cell? (tropocollagen to triple helical domains)
tropocollagens dimerize at Carboxy terminal terminals (NC1) > form tropocollagen dimers
4 tropocollagen dimers aggregate at N-terminal > form type IV collagen tetramer (7S domain)
Lateral association of triple helical domains
What does the structure of collagen do for filtration?
Has big spaces in
between structures for filtration
What is the structure of laminin?
Contains A, B1 and B2 chains > join to form a 4-
armed structure (= one laminin molecule)
What causes laminin molecules to join up?
In the presence of Ca2+, free ends join respective
partners at C-terminal, form polymers/network
What is Laminin always associated with?
How does proteoglycan associate with Laminin and type IV collagen?
Fill spaces between laminin and collagen
What is the glyco- part of Basement membrane (BM)-type Proteoglycans?
heparan sulphates (HS)
How many Heparan Sulphates link to polypeptides?
Linked to serine residues of
2 HS per agrin (spread out)
3 HS per perlecan (clustered at one end)
What other protein can HS form a preteoglycan with?
How does HS on Agrin contribute to the structure of the proteoglycan?
HS has negative charge
-ve charges cannot cluster, so proteoglycan extends
Type IV collagen, proteoglycans and Laminin form what?
What is the first layer of ECM?
laminin polymers anchored to membrane associated receptor proteins
How is Laminin polymer in ECM anchored to the membrane?
Associated receptor proteins
How does Type IV collagen add to Laminin polymer in ECM?
Bridged by nidogen/entactin (associated with Laminin)
How does proteoglycans interact with Collagen and Laminin in ECM? (think about the two types of proteoglycan and their attachments)
1) HS + Agrin = bind to cell surface receptor proteins (e.g. sulfatides) and Laminin
2)HS + Perlecan bind to cell surface receptor proteins and interact with NC1 domain of tropocollagen
Interact with 7s domain of type IV collagen tetramer
How does Laminin + collagen filter molecules passing by?
Overall negative charge
Filter by size, charge:
Positive = trapped
Negative = repelled
If Protein is big and negatively charged = hard to get through
What is the key component of Slit diaphragm, the space between the podocte foot processes? How can it be visualized?
Key component = nephrin molecules
visualize by immunostaining
technique: gold-linked antibodies bind to nephrin
What composes nephrin in slit diaphragm?
From inside cell to further outside cell:
Intracellular domain (C-terminal)
Fibronectin type III domain
8 immunoglobulin-like domains (N-terminal) containing 1 Spacer domain
What allows the immunoglobins in nephrin to be stabilized (except spacer domain)?
stable S-S covalent bonds > form loops (except
in spacer domain: single cysteine = no loop)
How does nephrin network change shape?
Receive signal: cytoskeletal proteins (actin) pull in / out the interdigitating nephrin molecules from 2 opposite foot processes
Mutation in what results in proteinuria/ nephrotic syndorme?
Mutations in nephrin / its intracellular interacting
partners (CD2AP, podocin)
Is the change in concentration of small solutes significant after filtration?
Filtration produces no significant changes in concentration of small solute
How does nephritis change ultrafiltration?
loss of charge selectivity
Affect nephrin, GBM components, only sort by size of molecule, not charge
What charge molecules are filtered more (more go through to Bowman's capsule)?
Positive charge molecules filter through more than negative charge
What is used to test nephritis effect on charged molecule filtration?
Dextran - neutral
Dextran sulphate - -ve
DEAE Dextran - +ve
What is Alport syndrome a deficiency in ?
Alport syndrome: deficient
assembly of the a3-,4-,5-
collagen type IV network
What does diabetic glomerulopathy affect?
Decreased HS proteoglycans
What does minimal change nephrotic syndrome affect ( laminin β2-deficient) ?
Reduction of polyanionic sites, HS
What causes urinary excretion of GBM HS proteoglycan?
> Temporary decrease in charge of the GBM > Urinary excretion of GBM HSPG