Retinol-binding proteins (RBP) are a family of proteins with diverse functions. They are carrier proteins that bind retinol. Assessment of retinol-binding protein is used to determine visceral protein mass in health-related nutritional studies.
Retinol and retinoic acid play crucial roles in the modulation of gene expression and overall development of an embryo. However, deficit or excess of either one of these substances can cause early embryo mortality or developmental malformations. Regulation of transport and metabolism of retinol necessary for a successful pregnancy is accomplished via RBP. Retinol binding proteins have been identified within the uterus, embryo, and extraembryonic tissue of the bovine, ovine, and porcine, clearly indicating that RBP plays a role in proper retinol exposure to the embryo and successful transport at the maternal-fetal interface. Further research is necessary to determine the exact effects of poor RBP expression on pregnancy and threshold levels for said expression.
Retinol plays a crucial role in the growth and differentiation of various body tissues, and it has been previously characterized that embryos are extremely sensitive to alterations in retinol concentration that can lead to spontaneous abortion and malformations occurring during development (Wolf, 1984; Liu et al., 1993). Within a mature animal, retinol is transported from the liver via the circulatory system while bound to RBP to the desired target tissue. RBP is also bound to a carrier protein, transthyretin (Hendricks et al., 1987). The process by which RBP releases retinol for cellular availability is still unknown and not concisely determined (Hodam et al., 1991; MacDonald et al., 1990; van Bennekum et al., 1993). Sites of Synthesis Traditionally, RBP is synthesized within the liver with secretion being dependent upon retinol concentrations. However, the concentrations levels do not appear to have an effect upon transcription of RBP messenger RNA (mRNA) which remains constant (Blaner et al., 1986; Soprano et al., 1986). Literature reveals that the bovine endometrium has also been identified as a location of RBP synthesis, as well as, the conceptus and extraembryonic tissues of various livestock species (Harney et al., 1994; Liu et al., 1992; Liu et al., 1992 (2); Thomas et al., 1991).
Plasma retinol binding protein, the retinol transport vehicle in serum (Soprano et al., 1994).
CRBP I/II, cellular binding proteins involved in transport of retinol and metabolites into retinyl esters for storage or into retinoic acid (Johansson et al., 2001).
CRABPs, cellular retinoic acid binding proteins that are capable of binding retinol and retinoic acid with high affinity (Li et al., 1996; Napoli et al., 1995; Napoli, 1996). It has also been characterized that CRABPs are involved in many aspects of the retinoic acid signaling pathway such as the regulation and availability of retinoic acid to nuclear receptors (Gustafson et al., 2004).
Presence in Livestock Species during Gestation
RBP, identical to that found in plasma has been identified in the placental tissues of both the ovine and the bovine, suggesting that RBP may be highly involved in retinol transport and metabolism during pregnancy (Liu et al., 1993; Liu et al., 1992). However, exact timing of expression had been yet to be identified. Liu et al. (1993) utilized an antiserum specific for bovine conceptus RBP and immunohistochemistry to identify the presence of RBP at different stages of early pregnancy. Strong immunostaining and hybridization were observed in the trophectoderm of tubular, but not spherical blastocysts at d 13. RBP mRNA was localized to epithelial cells of the chorion, allantois, and amnion at d 45 of pregnancy (Liu et al., 1993). Lastly, RBP mRNA was detected in the cotyledons, the fetal contribution to the placenta and the site of attachment to the uterine epithelium for fetal/maternal exchange (Liu et al., 1993). Expression of RBP in developing conceptuses, extraembryonic membranes, and at the fetal-maternal interface indicate that there may be some regulation of retinol transport and metabolism that occurs due to RBP by the extraembryonic membranes (Liu et al., 1992). Within the uterus of pregnant bovines, it has been found that RBP synthesis in the luminal and glandular epithelium is quite similar to that of a cyclic animal’s; however upon reaching d 17 of the estrous cycle, levels of RBP remain constant and continue to gradually rise throughout gestation (MacKenzie et al., 1997). It has also been suggested that ovarian steroids may play a role in regulating uterine RBP expression (MacKenzie et al., 1997).
All three previously mentioned types of retinol binding proteins (RBP, CRBP, CRABP) have been identified within the porcine placenta during pregnancy via immunohistochemistry (Johansson et al., 2001). As previously mentioned, retinol and retinoic acid are modulators of gene expression and are necessary for the proper development and growth of a conceptus (Johansson et al., 2001). Porcine exhibit a diffuse type placenta that has areolar-gland subunits which allows for transport of larger molecules between dam and fetus. RBP and CRBP have been identified in the endometrial glands and areolar trophoblasts, suggesting that RBP is crucial in transport of retinol from the gland to the trophectoderm of the conceptus (Johannson et al., 2001). RBP expression has also been identified within the yolk sac, myometrium, oviduct, and numerous other fetal tissues (Harney et al., 1994).
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