Laminin (LM) is a large heterotrimeric extracellular matrix glycoprotein that plays important roles in cell adhesion, proliferation, migration, contraction, and differentiation. Each glomerular cell type expresses a unique subset of LM isoforms. Temporal switches in their expression correlate with morphologic transitions during glomerulogenesis. In some cases, initiation of cellular differentiation or maintenance of cellular pheontype has been linked to a particular LM isoform. Glomerular mesangial cells (MC) express LM-8 (a1B1g1), which binds entactin, cross-links into the fibrillar extracellular matrix and facilitates attachment and spreading. MC also express LM-9 (a1B1g1), which remains localized to discrete regions on the cell surface and initiates filopodia formation. This proposal will further characterize the mechanisms whereby LM isoforms direct MC function. In the first specific aim, we propose that the LMa4 chain forms a complex with filamin-1 (actin binding protein 280) that functions to generate force and mechano-protection of MC during contraction. Because this complex is shared with vascular smooth muscle cells and smooth muscle in the intestine, we propose that it is the smooth muscle equivalent of the LMa2-dystroglycan-dystrophin complex that is required for skeletal muscle contractility. The two LM isoforms expressed by MC differ in their use of B chains; thus it is likely that they also engage proteins on the MC surface that convey additional specificity of function. Proposed experiments will identify the molecular partners in these complexes. The goal of the second specific aim is to define the LM-receptors complexes that mediate MC contraction and migration. During biologic responses to a variety of stimuli, proteases are released that generate specific fragments of LM. In some cases these fragments acquire new biologic activity. Preliminary data show that a fragment of the LM B1 chain is translocated to the nucleus where it regulates gene expression. It is the goal of the third specific aim to confirm these observations and elucidate mechanisms whereby LMB1 acts in the nucleus. Progressive glomerular diseases such as glomerulosclerosis of aging and diabetic nephropathy are characterized by changes in the expression of LM isoforms. Further elucidation of the mechanisms whereby LM isoforms modulate the MC capacity to contract, protect the capillary wall from over distention, migrate during wound healing, and maintain normal cellular phentype and function should provide important insights into disease pathogenesis.
