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M. Mailankot, M. Staniszewska, H. Butler, M. Caprara, S. Howell, L. Reneker, R. H. Nagaraj; Indoleamine 2,3-Dioxygenase Over-Expression Causes Kynurenine-Modification of Proteins, Fiber Cell Differentiation Defects and Cataract Formation in the Mouse Lens. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1903.
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Indoleamine 2,3-dioxygenase (IDO) is the first enzyme of the kynurenine pathway, which metabolizes tryptophan. The kynurenines formed in this pathway bind and chemically modify proteins and can induce apoptosis of cells. In the human lens both kynurenines and their protein modifications are present and are causally linked to age-associated pigmentation and crosslinking of proteins. In this study we have investigated the effects of over-expression of IDO on biochemical and morphological changes in the mouse lens
Transgenic animals over-expressing IDO in the lens epithelial and fiber cells was generated using a plasmid construct that had chicken delta1-crystallin enhancer and mouse alphaA-crystallin promoter. IDO expression was determined by measuring enzyme activity, mRNA content and immunohistochemistry. Morphological changes were assessed by slit lamp microscopy, and H & E staining of lens sections. Kynurenines were measured by HPLC and kynurenine-modifications by an ELISA. Apoptosis was detected using an in situ Apoptosis Detection Kit.
The homozygous Tg animals (homTg) displayed higher IDO immunoreactivity, 4-fold higher IDO mRNA content, 8-fold higher IDO activity in the lens when compared to heterozygous Tg lens (hetTg), while in lenses from wild type animals (Wt) neither IDO activity nor IDO-immunoreactivity was detected. The homTg lens showed 91% and 64% reduction in weight and diameter, respectively when compared to the Wt lens, while hetTg lens was normal. The kynurenine content and kynurenine-modifications were 5- and 3-fold higher in homTg in relation to hetTg, but those were not detected in Wt. Organ culture of lenses with L-tryptophan resulted in higher kynurenine content in the homTg and hetTg lenses but not in the Wt lens. In the homTg lenses, epithelial cells and outer cortical fiber cells had large vacuoles in the cytoplasm and a large number of undifferentiated fiber cells. The presence of kynurenine-modification coincided with apoptosis in the secondary fiber cells of homTg. The homTg developed mature bilateral cataract within three months of birth.
These data suggest that IDO-mediated production of kynurenines cause defects in cell differentiation and apoptosis of fiber cells and induce cataract in the lens, and that IDO activity is regulated to prevent such adverse effects in the normal lens.
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