Histones appear most abundantly in human uterine secretions at the time the uterus is receptive to blastocyst implantation (Beier and Beier-Hellwig, 1998). Similarly, histones are synthesized at increased rates in the uterine epithelium and stroma of ovariectomized mice upon administration of a hormonal protocol known to result in blastocyst implantation about 25 h later (Smith et al., 1970). Assuming histones appear in mouse uterine fluid when the uterus becomes receptive to blastocyst implantation, what other functions might histones serve there? One good possibility involves amino acid transport system B^{0, +} in mouse and probably human blastocysts (Van Winkle et al., 2006). In order to consider this possibility in context, we first review system B^{0, +} involvement in early embryo development and blastocyst implantation in the uterus.

The process of blastocyst implantation in the mouse is especially amenable to study owing to experimentally-controlled delay of implantation. While delay of implantation (diapause) occurs naturally in mice when blastocysts develop in nursing mothers, it can be produced experimentally in mice by removing their ovaries about 76 h after their eggs have been fertilized (Van Winkle and Campione, 1987; Van Winkle et al., 2006). Daily administration of progesterone followed by estrogen on day 7 of pregnancy then leads to blastocyst implantation 25 h later.

During this activation from delay of implantation, signaling owing to leucine uptake via amino acid transport system B^{0, +} results specifically in development of trophoblast motility and penetration of the uterine epithelium by blastocysts (Van Winkle et al., 2006). This signaling occurs because increases in the Na⁺ and K⁺ concentrations in uterine secretions about 6 h after estrogen administration to ovariectomized, progesterone-treated rodents (Van Winkle et al., 1983; Nilsson and Ljung, 1985) drive net Na⁺-dependent system B^{0, +} leucine uptake by the blastocyst trophectoderm. Leucine then stimulates the mTOR signaling that is needed for development of trophoblast motility and penetration of the uterine epithelium, which occur about 19 h later (Van Winkle et al., 2006). Meanwhile the uterine environment somehow suppresses system B^{0, +} activity beginning about 10 h after estrogen administration (Lindqvist et al., 1978; Van Winkle and Campione, 1987). For example, blastocysts take up a radiolabeled, nonmetabolizable amino acid in utero, when it is administered to their mothers about 6 h after estrogen administration, but little or no uptake occurs when the amino acid is administered 4 h before or after this time (Lindqvist et al., 1978). We calculated that the decrease, in the rate of amino acid uptake between about 6 and 10 h after estrogen administration, is statistically equivalent to finding a drug that lowers the death rate from 94 to 6% (based on the effect size that can be calculated from the data of Lindqvist et al., 1978). At the time of the latter report, however, the full physiological significance of these changes in amino acid uptake by blastocysts in utero were not understood.

Although system B^{0, +} is relatively inactive in utero during the 15 h prior to blastocyst implantation, it can be reactivated to even greater levels of transport activity simply by removing blastocysts from the uterus near the time of blastocyst implantation (Van Winkle and Campione, 1987). This ability to reactivate system B^{0, +} activity also likely serves an important physiological function (Van Winkle et al., 2006). After reactivation, system B^{0, +} would help to remove tryptophan from the implantation chamber during the initial penetration of the uterus by motile trophoblasts and, thus, help to suppress T-cell proliferation and immunologic rejection of the blastocyst (Munn et al., 1998; Baban et al., 2004).

But what reversibly suppresses system B^{0, +} activity beginning about 15 h before blastocyst implantation? Good candidates include histones that are likely secreted by uterine epithelial and possibly stromal cells when the uterus becomes receptive to blastocyst attachment and penetration (Smith et al., 1970). At near the histone concentrations detected in uterine fluid (Beier and Beier-Hellwig, 1998; Dráb et al., 2014), we found these macromolecules to inhibit amino acid uptake by mouse blastocysts. System B^{0, +} activity, in particular, was inhibited much more than the activities of several other amino acid transport systems in blastocysts (Table IV in Van Winkle, 1993). In fact, the extent to which histones inhibited each of four different amino acid transport systems in blasocysts, differed from each other (p < 0.02), and ranged from near 90% inhibition of system B^{0, +} to no inhibition of system L. Hence, it seems unlikely that histone cytotoxicity alone caused histone inhibition of amino acid transport system B^{0, +} activity in blastocysts.

Perhaps not coincidentally, the effect size of this system B^{0, +} inhibition by histones equals the effect size reported above, for reduction of the rate of amino acid transport into blastocysts in utero between about 6 and 10 h after estrogen administration to progesterone-maintained ovariectomized mice (Lindqvist et al., 1978). In addition, histone H2A (one of the more conspicuous histones in secretions of the receptive human uterus; Beier and Beier-Hellwig, 1998) likely is more effective at inhibiting amino acid uptake by blastocysts than other histones (Doman and Van Winkle, 1979). Reactivation of system B^{0, +} in blastocysts, at the time of blastocyst penetration of the uterine epithelium, could be accomplished simply by removing histones from the relatively small amount of uterine fluid in implantation chambers. In this regard, proteases, needed to hydrolyze histones to inactive products, appear to abound in these chambers (e.g., Afonso et al., 1997).

While it is unclear why system B^{0, +} activity needs to be suppressed after mTOR signaling, we observed one tantalizing possibility. When we incubated delayed-implantation blastocysts for 25 h in vitro in medium containing a relatively high Na⁺ concentration, they irreversibly lost their Na⁺-dependent component of amino acid uptake (Van Winkle, 1977). This apparent loss of Na⁺-dependent system B^{0, +} activity would likely mean that the ability of blastocysts to activate net Na⁺-dependent tryptophan uptake would also be lost. If such loss were to occur in the implantation chamber in utero, then implanting blastocysts could face immunological rejection. Hence, suppression of system B^{0, +} in blastocysts by histones after initiation of mTOR signaling, could preserve this activity for activation and concentration of tryptophan into trophoblast cells at the time of trophoblast penetration of the uterine epithelium.

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.