TRO locates on X chromosome Xp11.21-22 (Pack et al., 1997). It contains 14 exons spanning 10,8 kb of genomic DNA (Aoyama et al., 2005).

Although TRO produces a transcript with an open reading frame encoding a 138-kDa protein, previous studies did not detect this protein in vivo and in vitro (Nadano et al., 2002). Instead, TRO produces two distinct proteins, magphinin (MAGE D3) and trophinin (Aoyama et al., 2008; Aoyama et al., 2005; Saburi et al., 2001). Translation for magphinin starts at AUG codon in exon 2, whereas translation for TRO starts at AUG codon in exon 12. Entire trophinin protein is encoded by exon 12. There are no sequence overlaps between magphinin and trophinin proteins (figure 2).

None.

Human trophinin is a 69 kDa protein composed of 749 amino acid residues. Trophinin protein contains many decapeptide repeats (figure 3). Although trophinin is not a typical membrane protein and does not contain clear membrane spanning domain, experimental data indicated that trophinin is transmembrane protein. N-terminal region of trophinin is in the cytoplasm, and other region composed of decapeptide repeats is extracellular. Trophinin was identified by functional cloning as the molecule potentially mediating the initial adhesion of the human embryo to the uterine epithelia between trophoblastic and endometrial cells (Fukuda et al., 1995; Nakayama et al., 2003; Sugihara et al., 2007; Suzuki et al., 1998; Tamura et al., 2011). Trophinin associates with two cytoplasmic proteins bystin and tastin, of which complex functions as cell adhesion machinery through trophinin-trophinin binding between trophinin-expressing cells. Trophinin, bystin and tastin also function as molecular switch in signal transduction. Thus in trophoblastic cells trophinin-medited cell adhesion promotes cell growth and invasion (Sugihara et al., 2007), whereas in endometrial epithelial cells trophinin-mediated cell adhesion triggers apoptosis to accept invading embryo (Tamura et al., 2011).

TRO expressed strongly in trophectoderm cells and endometrial epithelial cells during embryo implantation (Fukuda et al., 1995; Nakayama et al., 2003; Suzuki et al., 1999). In early pregnancy human placenta (6 weeks), TRO transcripts and protein were found in syncytiotrophoblast in the chorionic villi, and in endometrial decidual cells at the utero placental interface. Trophinin disappears from placenta in later stage of pregnancy (more than 10 weeks), trophinin disappears from human placenta (Suzuki et al., 1999).
In the pregnant mouse uterus, trophinin transcripts are expressed during the time coinciding with those of blastocyst implantation (Suzuki et al., 2000).
In addition, TRO is strongly expressed in post mitotic neurons. In adult rat brain, trophinin is expressed in the subventricular zone, one of the region where active neurogenesis occurs (Ma et al., 2006).
Trophinin is expressed in brain and mature sperm in testis. Low levels TRO expression is detected in a variety of tissues, including liver, kidney, stomach, muscle and skin.
Trophinin is expressed in testicular cancer (Hatakeyama et al., 2004), colon cancer (Harada et al., 2007), lung cancer (Chen et al., 2007).

Trophinin protein was found in the pinopodes, unique structure found in endometrial epithelial cells at embryo implantation site, which was mimicked by primary culture of human endometrial epithelial cells in medium containing human chorionic gonadotropin (hCG) (Sugihara et al., 2008). In cultured cells, trophinin was found in the nuclear membranes and cytoplasm (Aoyama et al., 2005).
In both human and mouse, trophinin protein was found in the tail of matured sperm cells.

Trophinin function in human embryo implantation: Trophinin expressed on an apical cell membranes of trophectoderm cells of blastocyst and endometrial epithelial cells mediates cell adhesion between these two cell types through hemophilic trophinin-trophinin binding (Fukuda et al., 1995). The signal of trophinin-mediated cell adhesion is transmitted to the cytoplasm, leading into trophectoderm cell activation for proliferation and invasion (Fukuda and Sugihara, 2007; Fukuda and Sugihara, 2008; Fukuda and Sugihara, 2012; Sugihara et al., 2007). Thus trophinin functions as a molecular switch in signal transduction. The mechanism underlying this trophinin-ligation triggered cell activation involves receptor tyrosine kinase ErbB4 and bystin as shown in figure 4. In endometrial epithelial cells, trophinin-mediated cell adhesion triggered signal transduction through PKC-γ, leading into apoptosis to accept invading embryo (Tamura et al., 2011).
In ectopic tubal pregnancies, high levels of trophinin was found in both trophoblasts and tubal epithelia. Trophinin expression in maternal cells was particularly high in the area adjacent to the trophoblasts, whereas trophinin was barely detectable in intact fallopian tubes from women with in utero pregnancies or without pregnancies. The human chorionic gonadotrophin (hCG), trophinin expression was enhanced in epithelial cells. As both beta-subunit of hCG and trophinin genes have diverged in mammals, the study suggests a unique role of hCG and trophinin in human embryo implantation (Nakayama et al., 2003).
Tro gene knock-out mouse did not show infertility (Nadano et al., 2002). This evidence in mouse, and induction of TRO expression by hCG (Nakayama et al., 2003; Sugihara et al., 2008), and significant genetic diversion CGβ between non-human primate and human (Maston and Ruvolo, 2002) suggest that function of trophinin in embryo implantation is unique to humans.
Trophinin function in sperm motility: Trophinin suppresses sperm tail motility in human and mouse, which can be activated by trophinin-binding peptide (Hatakeyama et al., 2008; Park et al., 2012).
Trophinin function in neuronal cells: In adult rat brain, trophinin is expressed in the subventricular zone together with bystin, implicated to active neurogenesis in this region (Ma et al., 2006).

None.

No mutation of TRO in human is known.