Polyamine Oxidase Is Involved in Spermidine Reduction of Transglutaminase Type 2-Catalyzed βH-Crystallins Polymerization in Calcium-Induced Experimental Cataract

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.

Research Progress and Potential Applications of Spermidine in Ocular Diseases

“…In the Ca 2+ -induced cataract model, simulating age-related cataracts, endogenous spermidine decreased rapidly as lens opacification progressed, in parallel with the increase in crystallin cross-linking by N1–N8-bis(c-glutamyl)SPD [ 19 ]. When incubating a rabbit lens in a high concentration of free (unbound) Ca 2+ with exogenous spermidine in vitro, delayed lens opacification was observed in two studies [ 19 , 84 ], indicating that spermidine may become a promising agent for preventing cataracts, especially age-related, the most common subtype.…”

“…In cataracts, an increase in the intracellular levels of free (unbound) Ca 2+ has been seen in the lens during aging, which may trigger enzymes, such as Ca 2+ -dependent transglutaminase2 (TG2) and decrease the activity of flavin adenine dinucleotide-dependent polyamine oxidases (FAD-PAO) [ 84 ]. In that circumstance, activated TG2 catalyzes lens crystallins cross-linked by protein-bound N1–N8-bis(γ-glutamyl)SPD.…”

“…Meanwhile, the suppressed FAD-PAO does not degrade the reactive N8-terminal residue of the crystallin-bound N1-mono(γ-glutamyl)SPD, which reduces the crosslink formation, thus leading to crystallin polymerization and lens opacification [ 19 , 85 , 86 ]. Supplementation of spermidine can decrease TG2 activity by saturating its binding sites and enhancing the activity of FAD-PAO by more than double in vitro [ 84 ]. Administration of exogenous spermidine may restore the endogenous spermidine and reduce the N1–N8-bis(γ-glutamyl)SPD cross-linking while increasing mono(γ-glutamyl)SPD, which may be responsible for inhibiting the cross-linking of lens crystallins and promoting lens transparency [ 19 ].…”

Solid-Phase Synthesis of Selectively Mono-Fluorobenz(o)ylated Polyamines as a Basis for the Development of 18F-Labeled Radiotracers

“…In contrast, the monobasic oxopolyamines 6, 7 and 8 should not be recognized by the polyamine transport system but act as substrates of transglutaminases, which should thus allow for the dissection of polyamine transport and transglutaminase-catalyzed conversion. The terminally fluorobenzoylated spermidines 9 and 11 should be capable of dual targeting as they represent both potential transport and transglutaminase substrates [85,86].…”