Quantitation and localization of laminin A, B1, and B2 chain RNA transcripts in developing kidney

To quantitate expression of laminin A, B1, and B2 chains during kidney development, we prepared riboprobes of similar size (318-365 nucleotides) and carried out scanning densitometry of Northern blots from newborn mouse and rat kidneys. Much higher expression of B1 than A or B2 mRNAs was observed with an A-to-B1-to-B2 ratio of approximately 1:8:4 in mice and approximately 1:12:1 in rats. In addition to the 9.5-kb laminin A chain transcript, we also observed specific hybridization of the A chain probe to approximately 6-kb band in both newborn mice and rats. To localize these mRNAs, newborn rat kidneys were sieved to obtain separate glomerular and tubule fractions. Northern blots from these samples showed that most of the B1, B2, and 9.5-kb laminin A chain mRNAs were in glomerular fractions, whereas tubules contained more of the 6-kb A chain band. Analyses of poly(A)+ mRNA from newborn and adult kidneys showed severalfold decreases in adult RNA encoding the three laminin chains and the alpha 1-chain type IV collagen. However, only a 20% decline from newborn levels was seen for heparan sulfate proteoglycan (HSPG) core protein message. These quantitative results provide additional evidence that different laminin isoforms are present in separate kidney basement membranes and that HSPG core protein synthesis remains at relatively high levels in adult kidneys.