Metallothionein-III deficiency exacerbates light-induced retinal degeneration

Retinal photoreceptor damage is a common feature of ophthalmic disorders, such as age-related macular degeneration and retinitis pigmentosa. Oxidative stress has a key role in these diseases. Metallothioneins (MTs) are a family of cysteine-rich proteins, and various physiologic functions have been reported, including protection against metal toxicity and antioxidative potency. We investigated the functional role of MT-III in light-induced retinal damage.

The expression of retinal MT-I, -II, and -III mRNA was evaluated by real-time reverse-transcription PCR in retina exposed to light. Retinal damage in MT-deficient mice was induced by exposure to white light at 16,000 lux for 3 hours after dark adaptation. Photoreceptor damage was evaluated histologically by measuring the thickness of the outer nuclear layer (ONL) 5 days after light exposure and by electroretinogram recording. In an in vitro experiment, the MT-III siRNAs were tested for their effects on light-induced mouse photoreceptor cell (661W) damage.

The mRNAs of the MTs were increased significantly in murine retina after light exposure. The ONL in the MT-III-deficient mice was remarkably thinner compared to light-exposed wild-type (WT) mice, and a- and b-wave amplitudes were decreased; the damage induced in MT-I/-II-deficient mice was comparable to that observed in WT mice. MT-III knockdown by siRNA in 661W exacerbated the cell damage and increased the production of reactive oxygen species in response to light exposure.

These findings suggested that MT-III can help protect against light-induced retinal damage compared to MT-I/II. Some of these effects may be exerted by its antioxidative potency.