Polyamine Oxidase Is Involved in Spermidine Reduction of Transglutaminase Type 2-Catalyzed <i>β</i>H-Crystallins Polymerization in Calcium-Induced Experimental Cataract

• Main Authors: Carlo Mischiati, Giordana Feriotto, Claudio Tabolacci, Fabio Domenici, Sonia Melino, Ilaria Borromeo, Cinzia Forni, Angelo De Martino, Simone Beninati
• Format: Article
• Language: English
• Published: MDPI AG 2020-07-01
• Series: International Journal of Molecular Sciences
• Subjects: rabbit eye lens; cataract; TG2; spermidine; FAD-PAO; protein post-translation modification
• Online Access: https://www.mdpi.com/1422-0067/21/15/5427

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 <i>N</i>¹-<i>N</i>⁸-<i>bis</i>(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 <i>β</i>H-crystallins and in particular to the <i>β</i>B2- and mostly in <i>β</i>B3-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 <i>N</i>⁸-terminal reactive portion of <i>N</i>¹-<i>mono</i>(γ-glutamyl) SPD, the protein-bound <i>N</i>⁸-<i>mono</i>(γ-glutamyl) SPD was found the mainly available derivative for the potential formation of <i>β</i>B3-crystallins cross-links by protein-bound <i>N</i>¹-<i>N</i>⁸-<i>bis</i>(γ-glutamyl)SPD. In conclusion, FAD-PAO degradation of the <i>N</i>⁸-terminal reactive residue of the crystallins bound <i>N</i>¹-<i>mono</i>(γ-glutamyl)SPD together with the increased concentration of exogenous SPD, leading to saturation of glutamine residues on the substrate proteins, drastically reduces <i>N</i>¹-<i>N</i>⁸-<i>bis</i>(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.