Comparison of age-dependent alterations in thioredoxin 2 and thioredoxin reductase 2 expressions in hippocampi between mice and rats

Abstract

Background: Aging is one of major causes triggering neurophysiological changes in many brain substructures, including the hippocampus, which has a major role in learning and memory. Thioredoxin (Trx) is a class of small redox proteins. Among the Trx family, Trx2 plays an important role in the regulation of mitochondrial membrane potential and is controlled by TrxR2. Hitherto, age-dependent alterations in Trx2 and TrxR2 in aged hippocampi have been poorly investigated. Therefore, the aim of this study was to examine changes in Trx2 and TrxR2 in mouse and rat hippocampi by age and to compare their differences between mice and rats.

Results: Trx2 and TrxR2 levels using Western blots in mice were the highest at young age and gradually reduced with time, showing that no significant differences in the levels were found between the two subfields. In rats, however, their expression levels were the lowest at young age and gradually increased with time. Nevertheless, there were no differences in cellular distribution and morphology in their hippocampi when it was observed by cresyl violet staining. In addition, both Trx2 and TrxR2 immunoreactivities in the CA1-3 fields were mainly shown in pyramidal cells (principal cells), showing that their immunoreactivities were altered like changes in their protein levels.

Conclusions: Our current findings suggest that Trx2 and TrxR2 expressions in the brain may be different according to brain regions, age and species. Therefore, further studies are needed to examine the reasons of the differences of Trx2 and TrxR2 expressions in the hippocampus between mice and rats.

Keywords: Aging; Hippocampus; Rodent; Trx2; TrxR2.

Fig. 1

Representative immunoblots of Trx2 and TrxR2 (a) and normalized protein levels of Trx2 (b) and TrxR2 (b) versus each level of β-actin in the hippocampi of mice and rats. Expression levels of Trx2 and TrxR2 are decreased in the mice by age; contrastively, age-dependent increases of their expression levels are shown in the rats. The bars indicate the means ± SEM (n = 7/group; ^*P < 0.05 versus mice at the same age or rats at early age; ^#P < 0.05 versus mice at the same age)

Fig. 2

CV staining in mouse hippocampi (a; a1-c1) and rat hippocampi (b; a1-c1) containing the CA1 field (a2-c2) and CA2/3 field (a3-c3). Regardless of age and species, there are no significant differences in distribution pattern and morphology of CV-stained cells. Note that numerous CV-stained cells consist of stratum pyramidale (SP, arrows). DG, dentate gyrus; SO, stratum oriens; SR, stratum radiatum Scale bars = 400 µm (a1-c1) and 100 µm (a2-c2 and a3-c3) (n = 7/group)

Fig. 3

Immunohistochemistry for Trx2 in the CA1 (a) and CA2/3 (c) field of the mice (a-c) and rats (d-f). Trx2 immunoreactivity is principally shown in the SP of the CA1-3 fields. In the mice, Trx2 immunoreactivity is reduced with age, however, Trx2 immunoreactivity in the rats is increased age-dependently. b and c: ROD of Trx2 immunoreactivity in the CA1 (b) and CA2/3 (c) field. Scale bar = 100 µm. The bars indicate the means ± SEM (n = 7/group; ^*P < 0.05 versus mice at the same age or rats at early age; ^#P < 0.05 versus mice at the same age)

Fig. 4

Immunohistochemistry for TrxR2 in the CA1 (a) and CA2/3 (c) field of the mice (a-c) and rats (d-f). TrxR2 immunoreactivity is typically shown in the SP. In the mice, age-dependent decrease of TrxR2 immunoreactivity is shown in the SP. Contrastively, TrxR2 immunoreactivity in the rats is gradually increased with age. b and c: ROD of TrxR2 immunoreactivity in the CA1(b) and CA2/3 (c) field. Scale bar = 100 µm. The bars indicate the means ± SEM (n = 7/group; ^*P < 0.05 versus mice at the same age or rats at early age; ^#P < 0.05 versus mice at the same age)