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Grarham and Karnovsky first
identified Slit diaphragm as a filtration barrier by ultrastructural
cytochemistry. They proved that myeloperoxidase passed quickly across
endothelial fenestrate and through the GBM, but was obstructed at the close of
the SD.  From these interpretations, they
established that SD was the crucial filtration barrier to macromolecule proteins.
Succeeding observations using other tracers supported evidenced SD as a primary
filtration barrier. Rodewalt and Karnovsky observed that SD as a highly ordered
zippered like substructure with periodic alternating cross bridges extending
from the plasma membrane of podocyte to central filament(8,9).
They highlighted that the similarity between the sizes of the pores in the SD
and size, shape of serum albumin supported the speculation that the SD may
function as important filtration barrier to plasma proteins. SD is a modified
adherens junction, which is a critical component of the glomerular filtration
apparatus.SD consists of specialized membrane proteins such as nephrin, neph1,
podocin and adherens proteins include P-cadherin, catenin, FAT etc.(10).
Previous reports suggests that SD also consists of tight junction proteins such
as cingulin, occludin, JAM-A(11).
ZO-1 is the first protein to be recognized as a constituent of SD, which is
component of the tight junction. Schnabel et
al revealed that ZO-1 positioned in the podocyte foot processes at the point of insertion of SD(12).
Nephrin is a core protein constituent of SD as reported by Kestila and
Ruotsalainen et.al (13,14).
P-cadherin function as an essential protein confined with ZO-1 at the site of SD
(10)
and FAT positioned at the SD (15).
Shih et.al reported CD2AP as
functional molecule of SD and mice lacking CD2AP results in congenital
nephrotic syndrome (16).
Podocin component of SD, mutation in NPHS2 leads to autosomal recessive steroid
resistant nephrotic syndrome (17,18).

ZO-1

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ZO-1 is component of tight junction
protein of SD, which is a 225kD peripheral membrane protein localized to
cytoplasmic surface of the tight junction (19).
ZO-1 concentrated along the cytoplasmic surface of SD of podocyte as reported
by Schnabel et.al (20).
ZO-1 is a multidomain protein also known as membrane associated guanylate
kinase homologues (MAGUK) (21).
ZO-1 forms assembly between transmembrane protein occludin and the actin
cytoskeleton of podocyte(21).

Nephrin

Nephrin is a type-1 transmembrane
protein belongs to immunoglobulin super family, component of SD of podocyte,
the ultimate glomerular plasma filter. Acquired or genetic   mutations disturbing  function of nephrin or expression results in
absence or distortion in SD(22)
and Finish type congenital nephrotic syndrome(13).
It has eight immunoglobulin (Ig)-like modules, 10
putative N-glycosylation sites at the extracellular domain and fibronectin type
III module, single transmembrane domain(23).
SPR studies established that nephrin promotes high affinity cell-cell adhesion
through homophilic interaction. These interaction is augmented by the presence
of Ca+2 designates that nephrin is a calcium binding protein and regulates
nephrin homophilic binding properties(22).

Podocin

Podocin (NPHS2) is a hair-pin like
protein positioned at the SD of podocyte, it is 42kD protein belongs to the
raft-associated stomatin family and mutation in NPHS2 gene results in autosomal
recessive steroid-resistant nephrotic syndrome(17).
Podocin mRNA identified in solely in podocytes by in situ hybridization whereas podocin confined at the vicinity of
podocyte SD with N- and C-termini are situated in the cytoplasm confirming
predicted hairpin structure(24).
NPHS2 is entirely expressed in podocytes of mature glomeruli of kidney. Roselli
et.al reported that podocin expression identified in early capillary stage of
podocyte in the developing nephron, and in mature glomeruli alongside the GBM,
at the basal pole. They also reported that Podocin localized to SD of podocyte
with its N- and C-terminal ends in the cytoplasm of foot processes. Podocin
could assists to function as anchor indirectly or directly constituents of SD
to the cytoskeleton of podocyte(24).
The homology similarity of podocin between human and rat, human and mouse, rat
and mouse are 84.3%, 86%, and 92.7% respectively (18).
The raft associated component of podocin interacts with CD2AP, nephrin of SD of
podocyte(25).
Podocin may act as skeleton and also assists in the structural organization of
SD. Saleem et.al reported that
podocin/nephrin complex has conversant relationship with F-actin (26).

P-cadherin

Reiser et.al reported that P-cadherin confined to slit diaphragm of podocyte
(10).
Cadherins belongs to glycoprotein family play important role in Ca+2 dependent
homophilic cell-cell adhesion and are also associated with the actin
cytoskeleton through ?, ?, and ?-catenins. They are identified to play
significant role in maintaining structural integrity of epithelial tissue and morphogenesis
(27).
The SD is a modified adherens junction rather than a modified tight junction,
this junction consists of zipper- like structure organized by cadherins.
Immuno-electron microscopy studies by Reiser et.al reported that P- cadherin localized in the SD joining between
two neighboring foot processes of podocyte. The mRNA expression of P- cadherin
is exclusively detected in glomeruli of kidney. They also reported that all the
catenins were coexpressed with P-cadherin and ZO-1 in podocytes. The homophilic
interactions of P-cadherin have been proposed to form bridge between podocytes.

 

CD2AP

CD2AP is CD2 associated protein
around 80kDa is an adaptor molecule that recognized as an SH3-containing
protein, which binds to cytoplasmic CD2 domain, a T cell and natural killer
cell membrane protein (28).
It is actin associate protein, it consists of actin binding site at the
N-terminus. CD2AP expression limited to podocytes within glomeruli, but its
expression also detected in some proximal tubular cells and also in collecting
duct cells (29).
CD2AP interacts with nephrin and may provide support
to  nephrin to the cytoskeleton.
Mice deficient or lacking  CD2AP
manifests nephrotic syndrome and leading to renal failure (16).
CD2AP knock out mice exhibit to show loss of integrity of foot processes.
Recent evidence from kim et.al
suggests that human patients associated with FSGS had a mutation expected to
ablate the expression of one allele of CD2AP (30).
These studies suggest that CD2AP plays important role in maintaining the
integrity and architecture of the SD.

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