Steady-state Levels of MRNAs Coding for the Type IV Collagen and Laminin Polypeptide Chains of Basement Membranes Exhibit Marked Tissue-specific Stoichiometric Variations in the Rat

Rat retina, lens, and kidney from 8-week-old animals were assayed for the steady-state levels of mRNAs for four basement membrane components: The alpha 1 chain of type IV collagen, the alpha 2 chain of type IV collagen, the B1 chain of laminin, and the B2 chain of laminin. Each tissue exhibited markedly different ratios of the four mRNAs. The mRNA ratio for the alpha 1 chain of type IV collagen to the B1 chain of laminin varied from a value of 0.7 in retina to a value of 17 in lens. Also, the mRNA ratio for the alpha 1 chain to the alpha 2 chain of type IV collagen varied from 1.6 in retina to 17 in lens, and the mRNA ratio for the B1 chain to the B2 chain of laminin varied from 0.6 in lens to 2.9 in kidney. The mRNA coding for the alpha 1 chain of type IV collagen decreased in all three tissues as the animals increased in age from 8 to 16 weeks, with the rate of decline being greater in retina than in lens of kidney. The levels of mRNA coding for the B1 and the B2 chains of laminin decreased in the kidney between 8 and 16 weeks but at different rates. Comparison of mRNAs from kidney of rats over this time period showed that the ratio of alpha 1 to B1 remained relatively constant with age, whereas the ratio of B1 to B2 increased. One possible explanation for the results is that each tissue has elaborate, tissue-specific controls for translation that provide synthesis of basement membrane components in the same proportion, in spite of the varying steady-state levels of the mRNAs. A more likely explanation is that different tissues synthesize type IV collagen and laminin at different rates, and that even the subunit compositions of the type IV collagen and laminin molecules vary from tissue to tissue and in an age-dependent manner.