Spatial and temporal expression of cell surface molecules during nephrogenesis

LRP: a new adhesion molecule for endothelial and smooth muscle cells

We recently generated a monoclonal antibody that disrupted the association of endothelial cells with their target location during kidney development. Here, we purified the antigen of this monoclonal antibody to homogeneity using rat mesangial cell cytosol. Sequence revealed that it is a previously identified protein, termed the "laminin receptor precursor" (LRP). We found that this protein is expressed in most tissues, but immunocytochemistry revealed that it is present largely or entirely in blood vessels where it is located underneath endothelial cells and in between smooth muscle cells of the vascular wall. Vascular smooth muscle cells such as mesangial cells produce and secrete LRP into their extracellular matrix where it is present in several molecular weight forms. Endothelial cells produce very little if any of the protein, but they bind avidly to LRP-coated dishes. Anti-LRP antibodies prevent the binding of smooth muscle cells to uncoated plates, implying that cells that secrete it use it for attachment. In an assay for heterologous cell-to-cell interaction, antibodies to LRP inhibited the binding of smooth muscle cells to endothelial cells. Maturation and differentiation of blood vessels require interaction between endothelial and smooth muscle cells. LRP is a new component of the mesangial matrix, and we propose that it is an adhesion molecule that mediates an interaction between smooth muscle cells and endothelia.

Architectural patterns in branching morphogenesis in the kidney

During kidney development, several discrete steps generate its three-dimensional pattern including specific branch types, regional differential growth of stems, the specific axes of growth and temporal progression of the pattern. The ureteric bud undergoes three different types of branching. In the first, terminal bifid type, a lateral branch arises and immediately bifurcates to form two terminal branches whose tips induce the formation of nephrons. After 15 such divisions (in humans) of this specifically renal type of branching, several nephrons are induced whose connecting tubules fuse and elongate to form the arcades. Finally, the last generations undergo strictly lateral branching to form the cortical system. The stems of these branches elongate in a highly regulated pattern. The molecular basis of these processes is unknown and we briefly review their potential mediators. Differential growth in three different axes of the kidney (cortico-medullary, dorsoventral and rostro-caudal) generate the characteristic shape of the kidney. Rapid advances in molecular genetics highlight the need for development of specific assays for each of these discrete steps, a prerequisite for identification of the involved pathways. The identification of molecules that control branching (the ultimate determinant of the number of nephrons) has acquired new urgency with the recent suggestion that a reduced nephron number predisposes humans to hypertension and to progression of renal failure.