Abstract
Taurine transport was investigated at the blood–testis barrier (BTB) formed by Sertoli cells. An integration plot analysis of mice showed the apparent influx permeability clearance of [3H]taurine (27.7 μL/(min·g testis)), which was much higher than that of a non-permeable paracellular marker, suggesting blood-to-testis transport of taurine, which may involve a facilitative taurine transport system at the BTB. A mouse Sertoli cell line, TM4 cells, showed temperature- and concentration-dependent [3H]taurine uptake with a Km of 13.5 μM, suggesting that the influx transport of taurine at the BTB involves a carrier-mediated process. [3H]Taurine uptake by TM4 cells was significantly reduced by the substrates of taurine transporter (TauT/SLC6A6), such as β-alanine, hypotaurine, γ-aminobutyric acid (GABA), and guanidinoacetic acid (GAA), with no significant effect shown by L-alanine, probenecid, and L-leucine. In addition, the concentration-dependent inhibition of [3H]taurine uptake revealed an IC50 of 378 μM for GABA. Protein expression of TauT in the testis, seminiferous tubules, and TM4 cells was confirmed by Western blot analysis and immunohistochemistry by means of anti-TauT antibodies, and knockdown of TauT showed significantly decreased [3H]taurine uptake by TM4 cells. These results suggest the involvement of TauT in the transport of taurine at the BTB.
Highlights
Taurine (2-aminoethanesulfonic acid) is known as a β-amino acid that abundantly exists in the human body, and its involvement in various physiological events, such as neuroprotection and osmotic regulation, has been suggested by cumulative studies [1,2,3,4,5,6]
TM4 cells showed a significant reduction of [3H]taurine uptake at 4 ◦C, and the uptake was significantly decreased in the assay with Na+-free, Cl−-free, and K+-replacement buffers, with no change shown by extracellular pH (Figure 2B)
The inhibition of [3H]taurine uptake was examined in TM4 cells, and uptake was significantly decreased in the presence of taurine, β-alanine, hypotaurine, GABA, and guanidinoacetic acid (GAA)
Summary
Taurine (2-aminoethanesulfonic acid) is known as a β-amino acid that abundantly exists in the human body, and its involvement in various physiological events, such as neuroprotection and osmotic regulation, has been suggested by cumulative studies [1,2,3,4,5,6]. Low expression of antioxidant enzymes such as superoxide dismutase (SOD) in the testis is known, and abnormalities in the motility and form of sperm are caused by oxidative stress [13]. The oral administration of taurine showed a protective effect against arsenic-induced oxidative stress in rats, supporting the significant role of taurine in the testis [14], and the administration of taurine in streptozotocin-induced diabetic rats suggested that taurine acts as an antioxidant in the seminiferous tubules harboring germ cells [15]. The study in diabetic rats supported the contribution of taurine to the seminiferous tubules, since its dietary intake improved the sperm characteristics, including sperm count and motility, that are closely related to male infertility [16,17,18]
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