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1 Department of Anatomy and Cell Biology, University of Kansas Medical Center, Kansas City, Kansas 66160-7400; and 2 Department of Anatomy and Neurobiology and 3 Renal Division, Department of Internal Medicine, Washington University School of Medicine, St. Louis, Missouri 63110
Glomerular basement membrane (GBM)
assembly and maturation are marked by the replacement of laminin-1
(containing
1-,
1-, and
1-chains) with laminin-11 (consisting
of
5-,
2-, and
1-chains). Similarly, the
1- and
2-chains
of type IV collagen are replaced by collagen
3-,
4-, and
5(IV)-chains. The cellular origins of these molecules and mechanisms
for isoform removal and substitution are unknown. To explore glomerular
laminin isoform transitions in vitro, we assessed metanephric organ
cultures. Standard culture conditions do not support endothelial cell
differentiation, and glomerular structures that form in vitro are
avascular. Nevertheless, extensive podocyte development occurs in these
cultures, including the formation of foot processes and assembly of a
GBM-like matrix. Here, we show that the podocyte-specific markers,
glomerular epithelial protein 1 and nephrin, which are normally
expressed in capillary loop stage glomeruli in vivo, are also expressed
by glomerular figures that form in organ culture. However, the GBM-like
segments that form in vitro do not undergo normal laminin isoform
switching. Instead, both laminin
1- and
5-chains are present, as
is the
1-chain, but not
2. When avascular organ-cultured kidneys
are grafted into anterior eye chambers, however, kidney-derived
angioblasts establish extensive vasculature by 6 days, and glomeruli
are lined by endothelial cells. We evaluated embryonic day
12 (E12) vascular endothelial growth factor receptor
(Flk1)-lacZ kidneys that had first been grown in
organ culture for 6-7 days and then grafted into wild-type mice.
Correct laminin isoform substitution occurred and correlated with the
appearance of endothelial cells expressing Flk1. Our
findings indicate that endothelial cells, and/or factors present in the
circulation, mediate normal GBM laminin isoform transitions in vivo.
endothelial cells; glomerular basement membrane; podocytes
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