Androgenic regulation of beta-defensins in the mouse epididymis

Beta-defensins are small cationic peptides that exhibit broad-spectrum antimicrobial properties and contribute to mucosal immune responses at epithelial sites. Recently, the complete genome sequences of different species and computational prediction and experimental verification have identified 30–50 novel beta-defensin genes in humans, rats and mice that are organized into gene clusters localized at specific chromosomes [1, 2]. The mRNAs encoding the majority of beta-defensins are exclusively expressed in the epididymis [3, 4]. Epididymal beta-defensins with antimicrobial activity [5‐7] have been shown to be involved in epididymal innate immune protection; however, recent studies have indicated that several of these peptides could bind to the sperm surface and play novel roles in male reproductive physiology. Rat Bin1b, which binds to the sperm head, initiates sperm motility in immature sperm from the caput epididymidis by a mechanism dependent on calcium uptake [8]. Likewise, immunization with the Bin1b peptide induced the production of anti-Bin1b antibodies and resulted in reduced fertility in rats [9]. Defb15, which binds to the sperm acrosomal region and forms part of the sperm glycocalyx, is required for sperm motility and male fertility. The in vivo knockdown of the rat Defb15 gene by RNAi led to a considerable attenuation of sperm motility and fertility [7]. Studies that include target deletion of Defb15 have also demonstrated homozygous males with low motility sperm and a reduced fertility phenotype [10]. Recent studies have further enhanced our understanding of the role of beta-defensins in fertility and sterility. Zhou et al. reported that the homozygous deletion of a cluster of nine β-defensin genes (DefbΔ9) in mice resulted in male sterility [11]. Tollner et al. reported that a common mutation in the human defensin Defb126 causes reduced sperm penetration ability and is associated with subfertility [12].

Because of their important functions in sperm maturation and fertility, beta-defensins are receiving more attention and are hypothesized to be potential targets for diagnosing and treating infertility. Therefore, clarifying the regulation mechanisms on their expression is greatly required. However, up to now, very little is known about the regulation of the production and secretion of epididymal beta-defensins.

Androgen signaling plays an important regulatory role in epididymal structure and function. Many epididymal secretory proteins involved in sperm maturation have been identified as androgen-regulated proteins. The effects of androgen are mediated through the androgen receptor (AR), a ligand-inducible nuclear receptor that regulates the expression of target genes by binding to androgen response element (ARE) DNA [13]. The identification of the beta-defensin transcripts that are regulated by androgens might be important to help elucidate the process of sperm maturation. Several beta-defensins have been reported to be regulated by androgen in different species, including rats, monkeys, humans and mice. In mice, only five beta-defensins, including Spag11a (Bin1b), Defb20, 22, 41, and 42 [14‐17], have been shown to be regulated by androgen. However, the previous study was unable to determine whether these beta-defensins are direct or indirect AR targets. Additionally, these previous studies were focused on the individual beta-defensin genes; a systematic profile of the androgenic regulation of the beta-defensin family is lacking.

In the epididymis, the majority of beta-defensins are abundantly expressed in the caput region, which expresses high levels of AR and proteins required for sperm maturation. In this study, we systematically investigated androgen regulation on the expression of beta-defensins in the mouse caput epididymidis. Additionally, we screened and identified the AR binding sites associated with some androgen-regulated beta-defensins, indicating direct AR regulation of these defensins.

The epididymis is an androgen-responsive organ responsible for sperm maturation and storage. Compared with other hormones and testicular factors, androgens play important regulatory roles in epididymal structure and function [20]. There is a high level of androgen receptors in the murine epididymis, and because of the importance of the epididymidis in the regulation of sperm maturation, the identification of the transcripts regulated by androgens might facilitate the elucidation of the process of sperm maturation [21].

Many members of the beta-defensin family have shown exclusive expression in the epididymis. Except for the 6-cysteine domain, which is rich in positively charged amino acids, beta-defensins differ considerably in their amino acid sequences and target pathogen specificity. They form an important component of the epididymal innate immune system and provide protection against a broad range of pathogens that could invade the male reproductive tract, including E. coli, S. aureus and C. albicans [5‐7]. Recent research has identified additional functions for beta-defensins that link the innate and adaptive immune responses. Beta-defensins cross-talk with the adaptive immune system by interacting with specific chemokine and toll-like receptors on immature dendritic cells and memory T cells resulting in the modulation of immunocompetent cell responses of the host [22].

Epididymal beta-defensins have evolved to perform functions other than immunoprotection, such as promoting sperm maturation, as suggested by the presence of several defensins on the sperm surface. However, little is known about the hormonal regulation, especially androgenic regulation, of the expression of beta-defensins.

In this study, androgen and AR regulation on caput epididymal beta-defensins was investigated. Beta-defensins exhibited different responses to androgen manipulation; most were regulated by androgen, some were not regulated by androgen or testicular factors, and one, Defb25, was regulated by testicular factors and not by androgen.

Among the androgen regulated beta-defensin genes, Defb20, 41, 42 and Spag11a were previously reported to be regulated by androgen [14‐16]; our study further verified that these genes were direct targets of AR and had AR binding sites in the proximal promoter regions. Defb30 was previously shown to be regulated by testicular factors and not by androgens [16]. In our study, Defb30 was partially regulated by androgen, and it could be a direct target of AR. This discordance might be attributed to the sensitivity of different methods. The previous study used Northern blot to detect the expression of Defb30, whereas the current study used quantitative RT-PCR. Compared with quantitative RT-PCR, Northern blot has a low sensitivity. We reanalyzed the previous data and found a weak staining signal in the androgen replacement mouse epididymis compared with the castrated mouse epididymis, although the authors ignored it.

Jalkanen et al. reported that Defb42 was partially regulated by androgen [15], whereas Oh et al. reported that Defb42 was regulated by testicular factors and not by androgen [16]. Our results demonstrated that Defb42 was regulated by androgens. To investigate this discrepancy, we reanalyzed the data from Oh et al. and determined that the Defb42 in their paper should be classified as Defb48 based on sequence alignment. Oh et al. also reported that Defb44 was regulated by androgen; however, in that paper, Defb44 should be classified as Defb42 based on the sequence alignment.

AR regulates the expression of target genes by binding to AREs in the genome. Our previous work has identified genome-wide AR binding sites in the mouse epididymis, and in this study, we screened and identified ARBSs associated with androgen-regulated beta-defensins. These sites were located in the proximal promoters or intronic regions of target genes, indicating that their expression is directly regulated by AR. Consistent with our expectations, potential ARE motifs were identified in the majority of ARBSs, and these motifs conformed to consensus sequences that were highly similar to the established ARE in the MatBase database. However, no potential AREs were identified in the two ARBSs associated with Defb20 and Defb30/42, suggesting AR regulation by a tethering mechanism. AR regulation occurs without the direct binding of AR to AREs and likely through tethered associations of the receptor with its target genes [23]. For the ARBS associated with Defb30/42, an AP-2 motif was found in the binding summit. AP-2 has been reported to be an AR coregulator in the mouse caput epididymidis [14, 24]. Thus, AR might be recruited to the ARBS by interacting with AP-2. Another possibility that could not be excluded is that there could be AREs that do not conform to the consensus sequence in the ARBSs associated with Defb20 and Defb30/42. For example, our previous data showed that noncanonical or half-site-like AREs were enriched in 38.3% of ARBSs without canonical AREs in the mouse epididymis. Those genes that changed expression, although were not bound by an AR, might be indirect AR target genes or might be regulated via long-range regulation (binding outside our defined window).

We tested the androgen responsiveness of Defb22, which is the mouse orthologue of human Defb126, and determined that it was partially regulated by androgen. Our ChIP-qPCR data identified an AR binding site in the distal promoter region, suggesting that Defb22 was a direct target of AR. Among the 9 genes deleted by Zhou et al. [11], four (Defb1, 13, 15, and 35) were tested in our study. Defb15 was partially regulated by androgen, whereas the three other defensins (Defb1, 13, 35) were not regulated by androgen.

The present study provides novel insights into the mechanisms of androgen regulation on epididymal beta-defensins. Sperm motility and fertilization capacity have been reported to be regulated by androgen [20]. Some AR-target beta-defensins, such as Spag11a, Defb22 and Defb15, might be involved in this regulatory process. Further studies are needed to clarify the functions of all of the AR target beta-defensins to help elucidate the molecular mechanism of sperm maturation and fertility.

The current study demonstrated that 23 beta-defensins from caput epididymis showed different responsiveness to androgen, and twelve of them were identified to be directly regulated by androgen receptor. These results shed new light on the mechanisms of androgen/AR regulation on epididymal gene expression.