Identification and spatial analysis of metallothioneins expressed by the adult human lens

Metallothioneins (MTs) are a large family of proteins involved in multiple protective pathways including binding of toxic metals, free radical scavenging, and oxidative stress. Increased expression of the MT IIA gene in age-related cataractous relative to normal human lenses, suggesting a role for MTs in the maintenance of lens transparency, has previously been detected. The MT family consists of many closely related isoforms grouped into four classes (I-IV). As a first step toward defining the function of MTs in the lens, the range and expression patterns of those MT isoforms expressed by the adult human lens were established.

Normal human lenses were microdissected into epithelia and fiber cells. Primers specific for individual MT isoforms were designed. MT transcripts were monitored in whole lenses, epithelia, and fibers by RT-PCR and confirmed to be authentic by sequencing. MT protein levels were evaluated by immunoblotting and by immunostaining.

Transcripts encoding MT classes I and II but not III or IV were detected in adult human lenses. In addition to MT IIA, five other MT transcripts were identified including IE, IF, IG, IH, and IL. MT IIA was detected almost exclusively in the lens epithelium, whereas the class I isoforms were detected at high levels in both lens epithelia and fibers. MT protein was detected almost exclusively in the lens epithelium.

The present report establishes the spectrum of MTs expressed by the adult human lens, defines their spatial expression patterns in lens epithelia and fibers, and demonstrates that MT protein is abundantly present in the lens epithelium.