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Abstract
In an in vitro Ca2+-induced cataract model, the progression of opacification is paralleled by a rapid decrease of the endogenous levels of spermidine (SPD) and an increase of transglutaminase type 2 (TG2, EC 2.3.2.13)-catalyzed lens crystallins cross-linking by protein-bound N1-N8-bis(γ-glutamyl) SPD. This pattern was reversed adding exogenous SPD to the incubation resulting in a delayed loss of transparency of the rabbit lens. The present report shows evidence on the main incorporation of SPD by the catalytic activity of TG2, toward βH-crystallins and in particular to the βB2- and mostly in βB3-crystallins. The increase of endogenous SPD in the cultured rabbit lens showed the activation of a flavin adenine dinucleotide (FAD)-dependent polyamine oxidases (PAO EC 1.5.3.11). As it is known that FAD-PAO degrades the N8-terminal reactive portion of N1-mono(γ-glutamyl) SPD, the protein-bound N8-mono(γ-glutamyl) SPD was found the mainly available derivative for the potential formation of βB3-crystallins cross-links by protein-bound N1-N8-bis(γ-glutamyl)SPD. In conclusion, FAD-PAO degradation of the N8-terminal reactive residue of the crystallins bound N1-mono(γ-glutamyl)SPD together with the increased concentration of exogenous SPD, leading to saturation of glutamine residues on the substrate proteins, drastically reduces N1-N8-bis(γ-glutamyl)SPD crosslinks formation, preventing crystallins polymerization and avoiding rabbit lens opacification. The ability of SPD and MDL 72527 to modulate the activities of TG2 and FAD-PAO involved in the mechanism of lens opacification suggests a potential strategy for the prevention of senile cataract.
Highlights
The majority of the dry weight of the eye lens is composed of soluble and long-lived proteins called crystallins [1]
Changes in the crystalline structure, degradation, and other chemical modifications lead to the insolubilization and aggregation of these proteins resulting in cataract [5], which represents the major cause of blindness worldwide
We have previously reported the modulatory effect of SPD, on transglutaminase 2 (TG2) activity, during in vitro rabbit eye lens opacification
Summary
The majority of the dry weight of the eye lens is composed of soluble and long-lived proteins called crystallins [1]. Lens crystallins have the remarkable ability to exist in high concentrations in the transparent lens, without causing light scatter. Crystallins possess a high degree of packing regularity, leading to optical properties responsible for lens transparency. Changes in the crystalline structure, degradation, and other chemical modifications lead to the insolubilization and aggregation of these proteins resulting in cataract [5], which represents the major cause of blindness worldwide. It is well established that in some cases the lens opacification process may depend on post-translational modification in various proteins, including crystallins [6]
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