TIMP-3 is expressed in the human retinal pigment epithelium

TIMP-3 is the most recent member of the tissue inhibitor of metalloproteinases (TIMP) family. In the present study, we describe the expression of TIMP-3 messenger RNA (mRNA) by the retinal pigment epithelium of the normal human eye (hRPE). In addition to the three predominant transcripts of approximately 5.1, 2.8, and 2.4 Kbp found in several other human tissues at adult and fetal stages. The hRPE also expresses two RNA species of 1.2 and 1.0 Kbp. Based on the sequence analysis of cDNA clones isolated from a hRPE cDNA library, the use of alternate polyadenylation signals could account for the expression of these smaller transcripts. The possibility of an alternative mechanism of regulation of the expression of TIMP-3 by the RPE is not discarded. The number of RNA transcripts specific for TIMP-3 per nanogram of poly A+ RNA was quantified by RT-PCR. 9.6 x 10(5) transcripts per nanogram of polyA +RNA were found at the adult stage and 1.2 x 10(6) transcripts per nanogram of polyA +RNA were detected at the fetal stage. These findings were supported by the predominant labeling in the RPE layer of retinal tissue sections in in situ hybridization experiments. All of these data support the hypothesis that the production of TIMP-3 by the RPE may be crucial for the maintenance of Bruch's membrane, the complex layer of extracellular matrix that provides a structural substrate for the RPE in the healthy retina and is perturbed during the ageing process and in Sorby's Fundus Dystrophy a inherited disease.