Localization of Xenobiotic Transporters Expressed at the Human Blood-Testis Barrier

Blood-testis barrier: a review on regulators in maintaining cell junction integrity between Sertoli cells

The blood-testis barrier (BTB) is formed adjacent to the seminiferous basement membrane. It is a distinct ultrastructure, partitioning testicular seminiferous epithelium into apical (adluminal) and basal compartments. It plays a vital role in developing and maturing spermatocytes into spermatozoa via reorganizing its structure. This enables the transportation of preleptotene spermatocytes across the BTB, from basal to adluminal compartments in the seminiferous tubules. Several bioactive peptides and biomolecules secreted by testicular cells regulate the BTB function and support spermatogenesis. These peptides activate various downstream signaling proteins and can also be the target themself, which could improve the diffusion of drugs across the BTB. The gap junction (GJ) and its coexisting junctions at the BTB maintain the immunological barrier integrity and can be the "gateway" during spermatocyte transition. These junctions are the possible route for toxicant entry, causing male reproductive dysfunction. Herein, we summarize the detailed mechanism of all the regulators playing an essential role in the maintenance of the BTB, which will help researchers to understand and find targets for drug delivery inside the testis.

Organic anion transporting polypeptide 3a1 is a novel influx pump for Perfluorooctane sulfonate in Sertoli cells and contributes to its reproductive toxicity

Persistent organic pollutant perfluorooctane sulfonate (PFOS) is strongly associated with male reproductive disorders, but the related mechanisms are still not fully understood. In this study, we used in vivo and in vitro models to explore the role of organic anion transporting polypeptide 3a1 (Oatp3a1) on PFOS-induced male reproductive injury. Thirty male C57BL/6 (B6) mice were orally given PFOS (0-10 mg/kg/bw) for 28 days. Body weight, organ index, sperm count, histology, and blood-testis barrier (BTB) integrity were evaluated. Primary Sertoli cells were used to describe the related molecular mechanisms of male reproductive injury caused by PFOS. Our results showed that PFOS induced a decrease in sperm count, morphological damage to testicular Sertoli cells, and disruption of BTB. In the in vitro model, exposure to PFOS significantly increased Oatp3a1 mRNA and protein levels and decreased miR-23a-3p expression in Sertoli cells, accompanied by reduced trans-epithelial electrical resistance (TEER) value. By performing the 14C-PFOS uptake experiment, we showed that 14C-PFOS uptake in HEK293-Oatp3a1 cells was apparently higher than in HEK293-MOCK cells. Meanwhile, treating Sertoli cells with Oatp3a1 siRNA significantly decreased Oatp3a1 expression and rescued PFOS-induced decreases in TEER value. As such, the present study highlights that Oatp3a1 may play an important role in the toxic effect of PFOS on Sertoli cells, advancing our understanding of molecular mechanisms for PFOS-induced male reproductive disorders.

In Vitro and In Vivo Models for Drug Transport Across the Blood-Testis Barrier

The blood-testis barrier (BTB) is a selectively permeable membrane barrier formed by adjacent Sertoli cells (SCs) in the seminiferous tubules of the testes that develops intercellular junctional complexes to protect developing germ cells from external pressures. However, due to this inherent defense mechanism, the seminiferous tubule lumen can act as a pharmacological sanctuary site for latent viruses (e.g., Ebola, Zika) and cancers (e.g., leukemia). Therefore, it is critical to identify and evaluate BTB carrier-mediated drug delivery pathways to successfully treat these viruses and cancers. Many drugs are unable to effectively cross cell membranes without assistance from carrier proteins like transporters because they are large, polar, and often carry a charge at physiologic pH. SCs express transporters that selectively permit endogenous compounds, such as carnitine or nucleosides, across the BTB to support normal physiologic activity, although reproductive toxicants can also use these pathways, thereby circumventing the BTB. Certain xenobiotics, including select cancer therapeutics, antivirals, contraceptives, and environmental toxicants, are known to accumulate within the male genital tract and cause testicular toxicity; however, the transport pathways by which these compounds circumvent the BTB are largely unknown. Consequently, there is a need to identify the clinically relevant BTB transport pathways in in vitro and in vivo BTB models that recapitulate human pharmacokinetics and pharmacodynamics for these xenobiotics. This review summarizes the various in vitro and in vivo models of the BTB reported in the literature and highlights the strengths and weaknesses of certain models for drug disposition studies. SIGNIFICANCE STATEMENT: Drug disposition to the testes is influenced by the physical, physiological, and immunological components of the blood-testis barrier (BTB). But many compounds are known to cross the BTB by transporters, resulting in pharmacological and/or toxicological effects in the testes. Therefore, models that assess drug transport across the human BTB must adequately account for these confounding factors. This review identifies and discusses the benefits and limitations of various in vitro and in vivo BTB models for preclinical drug disposition studies.

Study of spermatogenic and Sertoli cells in the Hu sheep testes at different developmental stages

Spermatogenesis is a highly organized process by which undifferentiated spermatogonia self-renew and differentiate into spermatocytes and spermatids. The entire developmental process from spermatogonia to sperm occurs within the seminiferous tubules. Spermatogenesis is supported by the close interaction of germ cells with Sertoli cells. In this study, testicular tissues were collected from Hu sheep at 8 timepoints after birth: 0, 30, 90, 180, 270, 360, 540, and 720 days. Immunofluorescence staining and histological analysis were used to explore the development of male germ cells and Sertoli cells in the Hu sheep testes at these timepoints. The changes in seminiferous tubule diameter and male germ cells in the Hu sheep testes at these different developmental stages were analyzed. Then, specific molecular markers were used to study the proliferation and differentiation of spermatogonia, the timepoint of spermatocyte appearance, and the maturation and proliferation of Sertoli cells in the seminiferous tubules. Finally, the formation of the blood-testes barrier was studied using antibodies against the main components of the blood-testes barrier, β-catenin, and ZO-1. These findings not only increased the understanding of the development of the Hu sheep testes, but also laid a solid theoretical foundation for Hu sheep breeding.

The Effect of Statins on Male Reproductive Parameters: A Mechanism Involving Dysregulation of Gonadal Hormone Receptors and TRPV1

Statins have been shown to cause diverse male reproductive function impairment, and in some cases, orchialgia. Therefore, the current study investigated the possible mechanisms through which statins may alter male reproductive parameters. Thirty adult male Wistar rats (200-250 g) were divided into three groups. The animals were orally administered rosuvastatin (50 mg/kg), simvastatin (50 mg/kg), or 0.5% carboxy methyl cellulose (control), for a 30-day period. Spermatozoa were retrieved from the caudal epididymis for sperm analysis. The testis was used for all biochemical assays and immunofluorescent localization of biomarkers of interest. Rosuvastatin-treated animals presented with a significant decrease in sperm concentration when compared to both the control and simvastatin groups (p < 0.005). While no significant difference was observed between the simvastatin and the control group. The Sertoli cells, Leydig cells and whole testicular tissue homogenate expressed transcripts of solute carrier organic anion transporters (SLCO1B1 and SLCO1B3). There was a significant decrease in the testicular protein expression of the luteinizing hormone receptor, follicle stimulating hormone receptor, and transient receptor potential vanilloid 1 in the rosuvastatin and simvastatin-treated animals compared to the control. The expression of SLCO1B1, SLCO1B2, and SLCO1B3 in the different spermatogenic cells portray that un-bio transformed statin can be transported into the testicular microenvironment, which can subsequently alter the regulation of the gonadal hormone receptors, dysregulate pain-inflammatory biomarkers, and consequently impair sperm concentration.

Drug Transporters at the Human Blood-Testis Barrier

Transporters are involved in the movement of many physiologically important molecules across cell membranes and have a substantial impact on the pharmacological and toxicological effect of xenobiotics. Many transporters have been studied in the context of disposition to, or toxicity in, organs such as the kidney and liver; however, transporters in the testes are increasingly gaining recognition for their role in drug transport across the blood-testis barrier (BTB). The BTB is an epithelial membrane barrier formed by adjacent Sertoli cells (SCs) in the seminiferous tubules that form intercellular junctional complexes to protect developing germ cells from the external environment. Consequently, many charged or large polar molecules cannot cross this barrier without assistance from a transporter. SCs express a variety of drug uptake and efflux transporters to control the flux of endogenous and exogenous molecules across the BTB. Recent studies have identified several transport pathways in SCs that allow certain drugs to circumvent the human BTB. These pathways may exist in other species, such as rodents and nonhuman primates; however, there is (1) a lack of information on their expression and/or localization in these species, and (2) conflicting reports on localization of some transporters that have been evaluated in rodents compared with humans. This review outlines the current knowledge on the expression and localization of pharmacologically relevant drug transporters in human testes and calls attention to the insufficient and contradictory understanding of testicular transporters in other species that are commonly used in drug disposition and toxicity studies. SIGNIFICANCE STATEMENT: While the expression, localization, and function of many xenobiotic transporters have been studied in organs such as the kidney and liver, the characterization of transporters in the testes is scarce. This review summarizes the expression and localization of common pharmacologically-relevant transporters in human testes that have significant implications for the development of drugs that can cross the blood-testis barrier. Potential expression differences between humans and rodents highlighted here suggest rodents may be inappropriate for some testicular disposition and toxicity studies.

Transporter Inhibition Profile for the Antivirals Tilorone, Quinacrine and Pyronaridine

Pyronaridine, tilorone and quinacrine are cationic molecules that have in vitro activity against Ebola, SARS-CoV-2 and other viruses. All three molecules have also demonstrated in vivo activity against Ebola in mice, while pyronaridine showed in vivo efficacy against SARS-CoV-2 in mice. We have recently tested these molecules and other antivirals against human organic cation transporters (OCTs) and apical multidrug and toxin extruders (MATEs). Quinacrine was found to be an inhibitor of OCT2, while tilorone and pyronaridine were less potent, and these displayed variability depending on the substrate used. To assess whether any of these three molecules have other potential interactions with additional transporters, we have now screened them at 10 μM against various human efflux and uptake transporters including P-gp, OATP1B3, OAT1, OAT3, MRP1, MRP2, MRP3, BCRP, as well as confirmational testing against OCT1, OCT2, MATE1 and MATE2K. Interestingly, in this study tilorone appears to be a more potent inhibitor of OCT1 and OCT2 than pyronaridine or quinacrine. However, both pyronaridine and quinacrine appear to be more potent inhibitors of MATE1 and MATE2K. None of the three compounds inhibited MRP1, MRP2, MRP3, OAT1, OAT3, P-gp or OATP1B3. Similarly, we previously showed that tilorone and pyronaridine do not inhibit OATP1B1 and have confirmed that quinacrine behaves similarly. In total, these observations suggest that the three compounds only appear to interact with OCTs and MATEs to differing extents, suggesting they may be involved in fewer clinically relevant drug-transporter interactions involving pharmaceutical substrates of the other major transporters tested.

Pharmacokinetic considerations to optimize clinical outcomes for COVID-19 drugs

The development of clinically effective drugs that could complement existing vaccines is urgently needed to reduce the morbidity and mortality associated with COVID-19. Drug-metabolizing enzymes, membrane-associated drug transporters, and inflammatory responses can partly determine the safety and efficacy of COVID-19 drugs by controlling their concentrations in both the systemic circulation and in peripheral tissues. It is still unknown how these factors affect how well COVID-19 drugs work in the clinic. We explore how drug metabolism and transport, as well as SARS-CoV-2-associated inflammatory response at disease target sites, may affect the clinical outcomes of COVID-19 drugs. In addition, we provide expert opinion on potential strategies for overcoming the clinical pharmacology and pathophysiological obstacles to improve COVID-19 drug effectiveness.

Physiological Characterization of the Transporter-Mediated Uptake of the Reversible Male Contraceptive H2-Gamendazole Across the Blood-Testis Barrier

The blood-testis barrier (BTB) is formed by a tight network of Sertoli cells (SCs) to limit the movement of reproductive toxicants from the blood into the male genital tract. Transporters expressed at the basal membranes of SCs also influence the disposition of drugs across the BTB. The reversible, nonhormonal contraceptive, H2-gamendazole (H2-GMZ), is an indazole carboxylic acid analog that accumulates over 10 times more in the testes compared with other organs. However, the mechanism(s) by which H2-GMZ circumvents the BTB are unknown. This study describes the physiologic characteristics of the carrier-mediated process(es) that permit H2-GMZ and other analogs to penetrate SCs. Uptake studies were performed using an immortalized human SC line (hT-SerC) and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Uptake of H2-GMZ and four analogs followed Michaelis-Menten transport kinetics (one analog exhibited poor penetration). H2-GMZ uptake was strongly inhibited by indomethacin, diclofenac, MK-571, and several analogs. Moreover, H2-GMZ uptake was stimulated by an acidic extracellular pH, reduced at basic pHs, and independent of extracellular Na+, K+, or Cl- levels, which are intrinsic characteristics of OATP-mediated transport. Therefore, the characteristics of H2-GMZ transport suggest that one or more OATPs may be involved. However, endogenous transporter expression in wild-type Chinese hamster ovary (CHO), Madin-Darby canine kidney (MDCK), and human embryonic kidney-293 (HEK-293) cells limited the utility of heterologous transporter expression to identify a specific OATP transporter. Altogether, characterization of the transporters involved in the flux of H2-GMZ provides insight into the selectivity of drug disposition across the human BTB to understand and overcome the pharmacokinetic and pharmacodynamic difficulties presented by this barrier. SIGNIFICANCE STATEMENT: Despite major advancements in female contraceptives, male alternatives, including vasectomy, condom usage, and physical withdrawal, are antiquated and the widespread availability of nonhormonal, reversible chemical contraceptives is nonexistent. Indazole carboxylic acid analogs such as H2-GMZ are promising new reversible, antispermatogenic drugs that are highly effective in rodents. This study characterizes the carrier-mediated processes that permit H2-GMZ and other drugs to enter Sertoli cells and the observations made here will guide the development of drugs that effectively circumvent the BTB.